Datasheet

UC2577-ADJ
3. Compensation Network (R
C
, C
C
) and Output
Capacitor (C
OUT
) Selection
The compensation network consists of resistor R
C
and
capacitor C
C
which form a simple pole-zero network and
stabilize the regulator. The values of R
C
and C
C
depend
upon the voltage gain of the regulator, I
LOADmax
, the in-
ductor L, and output capacitance C
OUT
. A procedure to
calculate and select the values for R
C
, C
C,
and C
OUT
which ensures stability is described below. It should be
noted, however, that this may not result in optimum com-
pensation. To guarantee optimum compensation a stand-
ard procedure for testing loop stability is recommended,
such as measuring V
OUT
transient responses to pulsing
I
LOAD
.
A. Calculate the maximum value for R
C
.
R
C
≤
750
•
I
LOADmax
•
V
OUT
2
V
INmin
2
Select a resistor less than or equal to this value, not to
exceed 3k
Ω
.
B. Calculate the minimum value for C
OUT
using the fol-
lowing two equations.
C
OUT
≥
0.19
•
L
•
R
C
•
I
LOADmax
V
INmin
•
V
OUT
and
C
OUT
≥
V
INmin
•
R
C
•
(
V
INmin
+
(
3.74
•
10
5
•
L
))
487,800
•
V
OUT
3
The larger of these two values is the minimum value that
ensures stability.
C. Calculate the minimum value of C
C
.
C
C
≥
58.5
•
V
OUT
2
•
C
OUT
R
C
2
•
V
INmin
The compensation capacitor is also used in the soft start
function of the regulator. When the input voltage is ap-
plied to the part, the switch duty cycle 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
instantly rise to about 90% and draw large currents from
the input supply. For proper soft starting, the value for C
C
should be equal or greater than 0.22
µ
F.
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 that charges the capacitor during each switching cy-
cle. For step-up and flyback regulators, the formula for
ripple current is:
I
RIPPLErms
=
I
LOADmax
•
D
max
1
−
D
max
Choose a capacitor that is rated at least 50% higher than
this value at 52kHz.
Equivalent Series Resistance (ESR): This is the primary
cause of output ripple voltage, and it also affects the val-
ues of R
C
and C
C
needed to stabilize the regulator. As a
result, the preceding calculations for C
C
and R
C
are only
valid if the ESR does not exceed the maximum value
specified by the following equations.
ESR
≤
0.01
•
15V
I
RIPPLE
(
P
−
P
)
and
≤
8.7
•
10
−
3
•
V
IN
I
LOADmax
where
I
RIPPLE
(
P
−
P
)
=
1.15
•
I
LOADmax
1
−
D
max
Select a capacitor with an ESR, at 52kHz, that is less
than or equal to the lower value 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
≥
470
µ
F), 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.,
[phone]
2801 72nd Street North, St. Petersburg, FL 33710
Pulse Engineering,
[phone]
12220 World Trade Drive, San Diego, CA 92128
Renco Electronics, Inc.,
[phone]
60 Jeffryn Blvd. East, Deer Park, NY 11729
Figure 5. Table of Standardized Inductors and
Manufacturer’s Part Numbers
APPLICATIONS INFORMATION (cont.)
6