UC2577-ADJ
3. Compensation Network (RC, CC) and Output
Capacitor (COUT) Selection
The compensation network consists of resistor RC and
capacitor CC which form a simple pole-zero network and
stabilize the regulator. The values of RC and CC depend
upon the voltage gain of the regulator, ILOADmax, the in-
ductor L, and output capacitance COUT. A procedure to
calculate and select the values for RC, CC, and COUT
which ensures stability is described below. It should be
noted , however, that this may not result in optimum c om-
pens ation . To g uarante e optimum compens ation a stand-
ard procedure for testing loop stability is recommended,
such as measuring VOUT transient responses to pulsing
ILOAD.
A. Calculate the maximum value for R
C
.
RC ≤ 750 • ILOADmax • VOUT2
VINmin2
Select a resistor less than or equal to this value, not to
exceed 3kΩ.
B. Calculate the minimum value for C
OUT
using the fol-
lowing two equations.
COUT ≥ 0.19 • L • RC • ILOADmax
VINmin • VOUT and
COUT ≥ VINmin • RC • (VINmin + (3.74 • 105 • L))
487,800 • VOUT3
The larger of these two val ues is the minimum value that
ensures stability.
C. Calculate the minimum value of C
C
.
CC ≥ 58.5 • VOUT2 • COUT
RC2 • VINmin
The compensation capacitor is also used in the soft start
function of the regulator. When the input voltage is ap-
pli ed t o the pa rt, the switch duty cycl e is increased slowly
at a rate defined by the compensation capacitor and the
soft start current, thus eliminating high input currents.
Without the soft start circuitry, the switch duty cycle would
instantl y rise to about 90% and draw large currents from
the i nput s up pl y. For p r op er s o ft s tarti ng, the val ue for CC
should be equal or greater than 0.22µF.
Figure 6 lists several types of aluminum electrolytic ca-
pacitors which could be used for the output filter. Use the
following parameters to select the capacitor.
Working Voltage (WVDC):
Choose a capacitor with a
working voltage at least 20% higher than the regulator
output voltage.
Ripple Current:
This is the maximum RMS value of cur-
rent tha t cha rges t he ca paci tor dur ing each swi tching cy-
cle. For step-up and flyback regulators, the formula for
ripple current is:
IRIPPLErms = ILOADmax
• Dmax
1 − Dmax
Cho ose a capac itor that is rated at least 50% hi gher than
this value at 52kHz.
Equivalent Series Resistance (ESR):
This is the primary
caus e of output rippl e vo ltage , and it als o affects the val-
ues of RC and CC neede d to stabilize the regulator. As a
res ult, the preceding calculations for CC and RC ar e only
valid if the ESR does not exceed the maximum value
specified by the following equations.
ESR ≤ 0.01 • 15V
IRIPPLE(P−P) and ≤ 8.7 • 10−3 • VIN
ILOADmax where
IRIPPLE(P−P) = 1.15 • ILOADmax
1 − Dmax
Select a capacitor with an ESR, at 52kHz, that is less
than or eq ual to the lower val ue calculated. Most electro-
lytic capacitors specify ESR at 120kHz which is 15% to
30% higher than at 52kHz. Also, note that ESR increases
by a factor of 2 when operating at −20°C.
In general, low values of ESR are achieved by using
large value capacitors (C ≥ 470µF), and capacitors with
high WVDC, or by paralleling smaller value capacitors.
Inductor
Code Manufacturer’s Part Number
AIE Pulse Renco
L47 415 - 0932 PE - 53112 RL2442
L68 415 - 0931 PE - 92114 RL2443
L100 415 - 0930 PE - 92108 RL2444
L150 415 - 0953 PE - 53113 RL1954
L220 415 - 0922 PE - 52626 RL1953
L330 415 - 0926 PE - 52627 RL1952
L470 415 - 0927 PE - 53114 RL1951
L680 415 - 0928 PE - 52629 RL1950
H150 415 - 0936 PE - 53115 RL2445
H220 430 - 0636 PE - 53116 RL2446
H330 430 - 0635 PE - 53117 RL2447
H470 430 - 0634 PE - 53118 RL1961
H680 415 - 0935 PE - 53119 RL1960
H1000 415 - 0934 PE - 53120 RL1959
H1500 415 - 0933 PE - 53121 RL1958
H2200 415 - 0945 PE - 53122 RL2448
AIE Magnetics, Div. Vernitron Corp., (813)3 47 -21 81
2801 72 nd Stree t Nort h, St. Pet ers bu rg, FL 33710
Pulse Eng ineerin g, (619)674-8100
12220 W o rld Trade Driv e, San Di eg o, CA 9 21 28
Renco Ele ctro ni cs, In c. , (51 6)5 86 -55 66
60 Jef fry n Blv d. East, Dee r Park, NY 11729
Figu re 5. Table of Standardized Indu cto r s an d
Manufacturer’s Part Numbers
APPLICATIONS INFORMATION (cont.)
6