LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION FEATURES Wide Input Voltage Range: 3.6V to 25V 2.2A Maximum Output Current Adjustable Switching Frequency: 300kHz to 2.8MHz Low Shutdown Current: IQ < 1A Integrated Boost Diode Power Good Flag Saturating Switch Design: 0.18 On-Resistance 1.265V Feedback Reference Voltage Output Voltage: 1.265V to 20V Soft-Start Capability Small 10-Pin Thermally Enhanced (3mm x 3mm) DFN Package APPLICATIONS Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation The LT(R)1938 is an adjustable frequency (300kHz to 2.8MHz) monolithic buck switching regulator that accepts input voltages up to 25V. A high efficiency 0.18 switch is included on the die along with a boost Schottky diode and the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability. The LT1938's high operating frequency allows the use of small, low cost inductors and ceramic capacitors resulting in low output ripple while keeping total solution size to a minimum. The low current shutdown mode reduces input supply current to less than 1A while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT1938 is available in a 3mm x 3mm DFN package with Exposed Pad for low thermal resistance. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION 3.3V Step-Down Converter Efficiency (VOUT = 3.3V) VOUT 3.3V 2.2A VIN 4.5V TO 25V 16.2k 4.7F BD RUN/SS BOOST 0.47F VC RT 680pF LT1938 SW BIAS PG 60.4k 80 4.7H GND 324k FB VIN = 7V 85 EFFICIENCY (%) OFF ON VIN 90 VIN = 24V VIN = 12V 75 70 65 60 200k 22F 1938 TA01 55 50 L: NEC PLC-0745-4R7 f = 800kHz 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 LOAD CURRENT (A) 1938 G02 1938fa 1 LT1938 Absolute Maximum Ratings (Note 1) pIN CONFIGURATION VIN, RUN/SS Voltage..................................................25V BOOST Pin Voltage....................................................50V BOOST Pin Above SW Pin..........................................25V FB, RT, VC Voltage........................................................5V BIAS, PG, BD Voltage.................................................25V Operating Junction Temperature Range (Note 2) LT1938E.............................................. -40C to 125C LT1938I............................................... -40C to 125C Storage Temperature Range.................... -65C to 150C TOP VIEW 10 RT BD 1 BOOST 2 SW 3 VIN 4 7 BIAS RUN/SS 5 6 PG 11 9 VC 8 FB DD PACKAGE 10-LEAD (3mm x 3mm) PLASTIC DFN TJMAX = 125C, JA = 45C/W, JC = 10C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB order information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT1938EDD#PBF LT1938EDD#TRPBF LDFT 10-Lead (3mm x 3mm) Plastic DFN -40C to 125C LT1938IDD#PBF LT1938IDD#TRPBF LDFT 10-Lead (3mm x 3mm) Plastic DFN -40C to 125C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ Electrical Characteristics The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS Minimum Input Voltage Quiescent Current from VIN VRUN/SS = 0.2V VBIAS = 3V, Not Switching Quiescent Current from BIAS MIN MAX 3 3.6 UNITS V 0.01 0.5 A 0.4 0.8 mA VBIAS = 0, Not Switching 1.2 2.0 mA VRUN/SS = 0.2V 0.01 0.5 A 0.85 1.5 mA 0 0.1 mA 2.7 3 V 1.265 1.265 1.28 1.29 V V 30 100 nA 0.002 0.02 %/V VBIAS = 3V, Not Switching VBIAS = 0, Not Switching Minimum Bias Voltage Feedback Voltage FB Pin Bias Current (Note 3) FB Voltage Line Regulation TYP 4V < VIN < 25V 1.25 1.24 1938fa 2 LT1938 Electrical Characteristics The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V VBOOST = 15V, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN TYP Error Amp gm 330 Error Amp Gain 1000 MAX UNITS Mho VC Source Current 75 A VC Sink Current 100 A VC Pin to Switch Current Gain 3.5 A/V VC Clamp Voltage Switching Frequency 2 RT = 8.66k RT = 29.4k RT = 187k Minimum Switch Off-Time 2.7 1.25 250 Switch Current Limit Duty Cycle = 5% Switch VCESAT ISW = 2A Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V Minimum Boost Voltage (Note 4) 3.1 V 3.0 1.4 300 3.3 1.55 350 MHz MHz kHz 100 150 nS 3.6 4.0 A 360 mV 0.02 2 A 1.6 2.1 V BOOST Pin Current ISW = 1A 18 30 mA RUN/SS Pin Current VRUN/SS = 2.5V 5 10 A RUN/SS Input Voltage High 2.5 V RUN/SS Input Voltage Low 0.2 VFB Rising 100 PG Leakage VPG = 5V 0.1 PG Sink Current VPG = 0.4V PG Threshold Offset from Feedback Voltage PG Hysteresis 10 Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT1938E is guaranteed to meet performance specifications from 0C to 125C. Specifications over the -40C to 125C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT1938I specifications are guaranteed over the -40C to 125C temperature range. 100 300 V mV mV 1 A A Note 3: Bias current measured in regulation. Bias current flows into the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. 1938fa 3 LT1938 typical performance characteristics 90 VIN = 24V 75 70 65 55 L: NEC PLC-0745-4R7 f = 800kHz 50 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 LOAD CURRENT (A) Maximum Load Current 2.0 VOUT = 3.3V TA = 25C L = 4.7H f = 800kHz 10 5 3.0 MINIMUM 2.5 2.0 VOUT = 5V TA = 25C L = 4.7H f = 800kHz 1.0 10 5 20 15 INPUT VOLTAGE (V) 4.5 1.5 1.0 0 25 50 75 TEMPERATURE (C) 1.5 100 125 1938 G07 0 20 60 40 DUTY CYCLE (%) 100 80 500 400 300 200 0 80 90 100 0.5 -25 2.0 Boost Pin Current BOOST PIN CURRENT (mA) DUTY CYCLE = 90 % 2.0 0 -50 2.5 1938 G06 600 VOLTAGE DROP (mV) 2.5 3.0 1.0 25 700 DUTY CYCLE = 10 % 3 3.5 Switch Voltage Drop Switch Current Limit 3.0 1 2 2.5 1.5 SWITCHING FREQUENCY (MHz) 1938 G05 1938 G04 4.0 0.5 Switch Current Limit TYPICAL 1.5 25 20 15 INPUT VOLTAGE (V) 0 1938 G03 SWITCH CURRENT LIMIT (A) LOAD CURRENT (A) LOAD CURRENT (A) MINIMUM VOUT = 3.3V L = 10H LOAD = 1A 4.0 3.5 1.5 SWITCH CURRENT LIMIT (A) 50 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 LOAD CURRENT (A) Maximum Load Current 3.5 2.5 65 55 L: NEC PLC-0745-4R7 f = 800kHz 4.0 3.0 70 1938 G02 4.0 TYPICAL VIN = 24V 75 60 1938 G01 1.0 80 VIN = 12V 60 60 VIN = 12V 85 EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%) VIN = 7V 80 VIN = 24V 70 50 90 85 VIN = 12V 80 Efficiency Efficiency (VOUT = 3.3V) Efficiency (VOUT = 5.0V) 100 90 (TA = 25C unless otherwise noted) 70 60 50 40 30 20 10 0 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 1938 G08 0 0 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 1938 G09 1938fa 4 LT1938 typical performance characteristics Switching Frequency 1.20 1.285 1.15 1.280 1.10 1.275 1.270 1.265 1.00 0.95 0.90 1.255 0.85 -25 0 25 50 75 TEMPERATURE (C) 100 RT = 45.3k 1.05 1.260 1.250 -50 0.80 -50 125 Frequency Foldback 1200 SWITCHING FREQUENCY (kHz) 1.290 FREQUENCY (MHz) FEEDBACK VOLTAGE (V) Feedback Voltage (TA = 25C unless otherwise noted) -25 0 25 50 75 TEMPERATURE (C) 100 1938 G10 400 200 0 125 3.5 80 60 40 20 0 50 75 25 TEMPERATURE (C) 100 RUN/SS Pin Current 3.0 2.5 2.0 1.5 1.0 0 125 10 8 6 4 2 0 0.5 2.5 2 1.5 RUN/SS PIN VOLTAGE (V) 1 3 0 3.5 Error Amp Output Current 1.4 80 1.2 60 VC PIN CURRENT (A) 100 0.6 0.4 1.0 0.5 1.5 BOOST DIODE CURRENT (A) 2.0 4.0 40 20 0 -20 1938 G16 -80 1.065 25 Minimum Input Voltage 3.5 3.0 2.5 -60 0 20 15 10 RUN/SS PIN VOLTAGE (V) 4.5 -40 0.2 5 1938 G15 INPUT VOLTAGE (V) Boost Diode 1.6 0.8 0 1938 G14 1938 G13 1.0 400 600 800 1000 1200 1400 FB PIN VOLTAGE (mV) 200 12 0.5 -25 0 1938 G12 RUN/SS PIN CURRENT (A) 120 0 -50 BOOST DIODE Vf (V) 600 Soft-Start 4.0 SWITCH CURRENT LIMIT (A) MINIMUM SWITCH ON-TIME (ns) Minimum Switch On-Time 0 800 1938 G11 140 100 RT = 45.3k 1000 1.165 1.365 1.265 FB PIN VOLTAGE (V) 1.465 1938 G17 2.0 0.001 VOUT = 3.3V TA = 25C L = 4.7H f = 800kHz 0.1 0.01 1 LOAD CURRENT (A) 10 1938 G18 1938fa 5 LT1938 typical performance characteristics 6.0 2.00 5.5 5.0 VOUT = 5V TA = 25C L = 4.7H f = 800kHz 4.5 4.0 0.001 0.1 0.01 1 LOAD CURRENT (A) Power Good Threshold VC Voltages 1.200 THRESHOLD VOLTAGE (V) 2.50 THRESHOLD VOLTAGE (V) INPUT VOLTAGE (V) Minimum Input Voltage 6.5 (TA = 25C unless otherwise noted) CURRENT LIMIT CLAMP 1.50 1.00 SWITCHING THRESHOLD 0.50 10 0 -50 -25 25 75 0 50 TEMPERATURE (C) 100 125 PG RISING 1.180 1.160 1.140 1.120 1.100 -50 -25 25 75 0 50 TEMPERATURE (C) 1938 G20 1938 G19 Switching Waveforms (Discontinuous Operation) 100 125 1938 G21 Switching Waveforms (Continuous Operation) IL 0.5A/DIV IL 0.5A/DIV VSW 5V/DIV VSW 5V/DIV VOUT 10mV/DIV VOUT 10mV/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 140mA 1s/DIV 1938 G22 VIN = 12V, FRONT PAGE APPLICATION ILOAD = 1A 1s/DIV 1938 G23 1938fa 6 LT1938 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. VIN (Pin 4): The VIN pin supplies current to the LT1938's internal regulator and to the internal power switch. This pin must be locally bypassed. RUN/SS (Pin 5): The RUN/SS pin is used to put the LT1938 in shutdown mode. Tie to ground to shut down the LT1938. Tie to 2.3V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. RUN/SS also provides a soft-start function; see the Applications Information section. PG (Pin 6): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG output is valid when VIN is above 3.5V and RUN/SS is high. BIAS (Pin 7): The BIAS pin supplies the current to the LT1938's internal regulator. Tie this pin to the lowest available voltage source above 3V (typically VOUT). This architecture increases efficiency especially when the input voltage is much higher than the output. FB (Pin 8): The LT1938 regulates the FB pin to 1.265V. Connect the feedback resistor divider tap to this pin. VC (Pin 9): The VC pin is the output of the internal error amplifier. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop. RT (Pin 10): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to the PCB. 1938fa 7 LT1938 BLOCK DIAGRAM VIN 4 C1 7 5 10 VIN BIAS RUN/SS RT SLOPE COMP OSCILLATOR 300kHz-2.8MHz RT SOFT-START 6 - + INTERNAL 1.265V REF BD SWITCH LATCH BOOST 1 2 C3 R S Q SW L1 VOUT 3 VC CLAMP C2 D1 PG ERROR AMP + - + - 1.12V GND 11 FB VC 9 CC RC CF 8 R2 R1 1938 BD 1938fa 8 LT1938 Operation The LT1938 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BIAS pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The RUN/SS pin is used to place the LT1938 in shutdown, disconnecting the output and reducing the input current to less than 1A. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. The oscillator reduces the LT1938's operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload. The LT1938 contains a power good comparator which trips when the FB pin is at 90% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT1938 is enabled and VIN is above 3.6V. 1938fa 9 LT1938 Applications Information FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: V R1= R2 OUT - 1 1.265 Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT1938 uses a constant frequency PWM architecture that can be programmed to switch from 300kHz to 2.8MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Figure 1. SWITCHING FREQUENCY (MHz) RT VALUE (k) 0.2 0.3 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 267 187 133 84.5 60.4 45.3 36.5 29.4 23.7 20.5 16.9 14.3 12.1 10.2 8.66 Figure 1. Switching Frequency vs RT Value Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX ) = VD + VOUT tON(MIN) ( VD + VIN - VSW ) where VIN is the typical input voltage, VOUT is the output voltage, is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the LT1938 switch has finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. This means that the minimum and maximum duty cycles are: DCMIN = fSW tON(MIN) DCMAX = 1- fSW tOFF(MIN) where fSW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the tOFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased. A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range The maximum input voltage for LT1938 applications depends on switching frequency, the Absolute Maximum Ratings on VIN and BOOST pins, and on operating mode. If the output is in start-up or short-circuit operating modes, then VIN must be below 25V and below the result of the following equation: VIN(MAX ) = VOUT + VD -V +V fSW tON(MIN) D SW where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on time (~150ns). Note that a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching 1938fa 10 LT1938 Applications Information frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 25V are acceptable regardless of the switching frequency. In this mode, the LT1938 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT1938's minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD -V +V 1- fSW tOFF(MIN) D SW where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current IL increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A reasonable starting point for selecting the ripple current is: IL = 0.4(IOUT(MAX)) where IOUT(MAX) is the maximum output load current. To guarantee sufficient output current, peak inductor current must be lower than the LT1938's switch current limit (ILIM). The peak inductor current is: IL(PEAK) = IOUT(MAX) + IL/2 where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and IL is the inductor ripple current. The LT1938's switch current limit (ILIM) is at least 3.5A at low duty cycles and decreases linearly to 2.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current: IOUT(MAX) = ILIM - IL/2 Be sure to pick an inductor ripple current that provides sufficient maximum output current (IOUT(MAX)). The largest inductor ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: V +V V +V L = OUT D 1- OUT D VIN(MAX ) fIL where VD is the voltage drop of the catch diode (~0.4V), VIN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. The inductor's RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types. Table 1. Inductor Vendors VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF7045 SLF10145 Shielded Shielded Toko www.toko.com D62CB Shielded D63CB Shielded D75C Shielded D75F Open CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open Sumida www.sumida.com 1938fa 11 LT1938 Applications Information Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 2A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See AN19. Input Capacitor Bypass the input of the LT1938 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7F to 10F ceramic capacitor is adequate to bypass the LT1938 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT1938 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7F capacitor is capable of this task, but only if it is placed close to the LT1938 and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT1938. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT1938 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT1938's voltage rating. This situation is easily avoided (see the Hot Plugging Safety section). For space sensitive applications, a 2.2F ceramic capacitor can be used for local bypassing of the LT1938 input. However, the lower input capacitance will result in increased input current ripple and input voltage ripple, and may couple noise into other circuitry. Also, the increased voltage ripple will raise the minimum operating voltage of the LT1938 to ~3.7V. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT1938 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT1938's control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 VOUT fSW where fSW is in MHz, and COUT is the recommended output capacitance in F. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. See the Frequency Compensation section to choose an appropriate compensation network. 1938fa 12 LT1938 Applications Information Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, COMMANDS Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum Sanyo (408) 749-9714 www.sanyovideo.com T494, T495 Ceramic, Polymer, POSCAP Tantalum Murata (408) 436-1300 AVX www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier, and should be 0.05 or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Catch Diode The catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT (VIN - VOUT)/VIN where IOUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a diode with a reverse voltage rating greater than the input voltage. Table 3 lists several Schottky diodes and their manufacturers. TPS Series Ceramic Table 3. Diode Vendors PART NUMBER VR (V) IAVE (A) VF AT 1A (mV) VF AT 2A (mV) On Semiconductor MBRM120E 20 1 530 595 Diodes Inc. B120 B130 B220 B230 DFLS230L 20 30 20 30 30 1 1 2 2 2 500 500 International Rectifier 10BQ030 20BQ030 30 30 1 2 420 500 500 500 470 470 Frequency Compensation The LT1938 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT1938 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 2. Generally a capacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. 1938fa 13 LT1938 Applications Information VOUT = 12V, FRONT PAGE APPLICATION LT1938 CURRENT MODE POWER STAGE gm = 3.5mho SW ERROR AMPLIFIER OUTPUT R1 - + CF ESR 1.265V C1 RC C1 + POLYMER OR TANTALUM GND VC IL 1A/DIV FB gm = 330mho 3M CPL VOUT 100mV/DIV CERAMIC 10s/DIV R2 1938 F03 Figure 3. Transient Load Response of the LT1938 Front Page Application as the Load Current is Stepped from 500mA to 1500mA. VOUT = 3.3V CC 1938 F02 Figure 2. Model for Loop Response Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. Figure 2 shows an equivalent circuit for the LT1938 control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. Figure 3 shows the transient response when the load current is stepped from 500mA to 1500mA and back to 500mA. BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22F capacitor will work well. Figure 2 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 4a) is best. For outputs between 2.8V and 3V, use a 1F boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1F boost capacitor (see Figure 4b). For lower output voltages the boost diode can be tied to the input (Figure 4c), or to another supply greater than 2.8V. The circuit in Figure 4a is more efficient because the BOOST pin current and BIAS pin quiescent current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BIAS pins are not exceeded. The minimum operating voltage of an LT1938 application is limited by the minimum input voltage (3.6V) and by the maximum duty cycle as outlined in a previous section. For 1938fa 14 LT1938 Applications Information 6.0 VOUT BD BOOST VIN LT1938 GND 4.7F C3 INPUT VOLTAGE (V) VIN TO START 5.5 SW 5.0 4.5 4.0 3.5 TO RUN 3.0 (4a) For VOUT > 2.8V 2.5 2.0 0.001 VOUT D2 BD BOOST VIN LT1938 GND 4.7F 8.0 C3 VOUT BD 4.7F LT1938 GND TO START 6.0 5.0 TO RUN 4.0 3.0 2.0 0.001 BOOST VIN 10 7.0 (4b) For 2.5V < VOUT < 2.8V VIN 0.1 0.01 1 LOAD CURRENT (A) SW INPUT VOLTAGE (V) VIN VOUT = 3.3V TA = 25C L = 4.7H f = 800kHz C3 VOUT = 5V TA = 25C L = 4.7H f = 800kHz 0.1 0.01 1 LOAD CURRENT (A) 10 1938 F05 SW Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit 1938 F04 (4c) For VOUT < 2.5V Figure 4. Three Circuits For Generating The Boost Voltage proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT1938 is turned on with its RUN/SS pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 5 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher and the minimum input to start will be the same as the minimum input to run. This occurs, for example, if RUN/SS is asserted after VIN is applied. The plots show the worst-case situation where VIN is ramping very slowly. For lower start-up voltage, the boost diode can be tied to VIN; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT1938, requiring a higher input voltage to maintain regulation. 1938fa 15 LT1938 Applications Information Soft-Start The RUN/SS pin can be used to soft-start the LT1938, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 7 shows the startup and shut-down waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20A when the RUN/SS pin reaches 2.3V. IL 1A/DIV RUN GND 0.22F 2ms/DIV VIN VIN BOOST LT1938 RUN/SS VOUT SW VC GND FB BACKUP 1938 F07 Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT1938 Runs Only When the Input is Present VRUN/SS 2V/DIV LT1938 can pull large currents from the output through the SW pin and the VIN pin. Figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. VOUT 2V/DIV PCB Layout 15k RUN/SS D4 MBRS140 1938 F06 Figure 6. To Soft-Start the LT1938, Add a Resisitor and Capacitor to the RUN/SS Pin Shorted and Reversed Input Protection If the inductor is chosen so that it won't saturate excessively, an LT1938 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT1938 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT1938's output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LT1938's internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the RUN/SS pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 8 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT1938's VIN and SW pins, the catch diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and VC nodes small so that the ground traces will shield them from the SW and BOOST nodes. The Exposed Pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT1938 to additional ground planes within the circuit board and on the bottom side. 1938fa 16 LT1938 Applications Information L1 C2 VOUT RRT CC RC R2 R1 D1 C1 RPG GND 1938 F08 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VOUT VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1938 circuits. However, these capacitors can cause problems if the LT1938 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the VIN pin of the LT1938 can ring to twice the nominal input voltage, possibly exceeding the LT1938's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT1938 into an energized supply, the input network should be designed to prevent this overshoot. Figure 9 shows the waveforms that result when an LT1938 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 4.7F ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 9b. A 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1F capacitor improves high frequency filtering. For high input voltages its impact on efficiency is minor, reducing efficiency by 1.5 percent for a 5V output at full load operating from 24V. High Temperature Considerations The PCB must provide heat sinking to keep the LT1938 cool. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT1938. Place additional vias can reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to JA = 35C/W or less. With 100 LFPM airflow, this resistance can fall by another 25%. Further increases in airflow will lead to lower thermal resistance. Because of the large output current capability of the LT1938, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125C. When operating at high ambient temperatures, the 1938fa 17 LT1938 Applications Information CLOSING SWITCH SIMULATES HOT PLUG IIN VIN + LOW IMPEDANCE ENERGIZED 24V SUPPLY DANGER RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT1938 4.7F IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 0.7 + VIN 20V/DIV 0.1F 20s/DIV (9a) LT1938 VIN 20V/DIV 4.7F IIN 10A/DIV (9b) + 22F 35V AI.EI. + LT1938 20s/DIV VIN 20V/DIV 4.7F IIN 10A/DIV (9c) 20s/DIV 1938 F09 Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT1938 is Connected to a Live Supply maximum load current should be derated as the ambient temperature approaches 125C. Power dissipation within the LT1938 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT1938 power dissipation by the thermal resistance from junction to ambient. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator. 1938fa 18 LT1938 Typical Applications 5V Step-Down Converter VOUT 5V 2.2A VIN 6.3V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47F VC 4.7F SW LT1938 D RT 20k BIAS PG 60.4k 680pF L 6.8H 590k FB GND 22F 200k f = 800kHz 1938 TA02 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VOUT 3.3V 2.2A VIN 4.4V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47F VC 4.7F LT1938 SW D RT 16.2k 680pF BIAS PG 60.4k f = 800kHz L 4.7H GND 324k FB 22F 200k 1938 TA03 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB4R7M 1938fa 19 LT1938 Typical Applications 2.5V Step-Down Converter VOUT 2.5V 2.2A VIN 4V TO 25V BD VIN RUN/SS ON OFF D2 BOOST L 4.7H 1F VC 4.7F SW LT1938 D1 RT 22.1k BIAS PG 84.5k 680pF 196k FB GND 47F 200k f = 600kHz 3684 TA04 D1: DIODES INC. DFLS230L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M 5V, 2MHz Step-Down Converter VOUT 5V 2A VIN 8.6V TO 22V BD VIN RUN/SS ON OFF BOOST 0.47F VC 2.2F LT1938 SW D RT 20k 680pF BIAS PG 16.9k f = 2MHz L 2.2H GND 590k FB 10F 200k 1938 TA05 D: DIODES INC. DFLS230L L: SUMIDA CDRH4D22/HP-2R2 1938fa 20 LT1938 Typical Applications 12V Step-Down Converter VOUT 12V 2.2A VIN 15V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47F VC 10F LT1938 SW D RT 30k 680pF BIAS PG 60.4k f = 800kHz L 10H GND 845k FB 22F 100k 3684 TA06 D: DIODES INC. DFLS230L L: NEC/TOKIN PLC-0755-100 1938fa 21 LT1938 Typical Applications 1.8V Step-Down Converter VOUT 1.8V 2.2A VIN 3.5V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47F VC 4.7F LT1938 SW D RT 15.4k 680pF BIAS PG 105k f = 500kHz L 3.3H GND 84.5k FB 47F 200k 1938 TA07 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB3R3M 1938fa 22 LT1938 Package Description DD Package 10-Lead Plastic DFN (3mm x 3mm) (Reference LTC DWG # 05-08-1699) 0.675 0.05 3.50 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 0.10 (4 SIDES) R = 0.115 TYP 6 0.38 0.10 10 1.65 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 0.05 0.00 - 0.05 5 1 (DD) DFN 1103 0.25 0.05 0.50 BSC 2.38 0.10 (2 SIDES) BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 1938fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23 LT1938 Typical Application 1.265V Step-Down Converter VOUT 1.265V 2.2A VIN 3.6V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47F VC 4.7F LT1938 SW D RT 13k BIAS PG 105k 680pF L 3.3H GND FB 47F f = 500kHz 1938 TA08 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 VIN: 3.6V to 36V, VOUT(MIN) = 12V, IQ = 1.6mA, ISD < 1A, ThinSOTTM Package LT3437 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode Operation VIN: 3.3V to 80V, VOUT(MIN) = 1.25V, IQ = 100A, ISD < 1A, DFN Package LT1936 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1A, MS8E Package LT3493 36V, 1.2A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 40V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1A, DFN Package LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency StepDown DC/DC Converter with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100A, ISD < 1A, TSSOP16E Package LT1767 25V, 1.2A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6A, MS8E Package LT1940 Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 25V, VOUT(MIN) = 1.20V, IQ = 3.8mA, ISD < 30A, TSSOP16E Package LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD < 25A, TSSOP16E Package LT3434/LT3435 60V, 2.4A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100A, ISD < 1A, TSSOP16E Package LT3481 36V, 2A (IOUT), Micropower 2.8MHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 1.265V, IQ = 5A, ISD < 1A, 3mm x 3mm DFN and MS10E Packages 1938fa 24 Linear Technology Corporation LT 1107 REV A * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LINEAR TECHNOLOGY CORPORATION 2007