Application Hints (Continued)
2. Compensation Network (R
C
,C
C
) and Output Capacitor
(C
OUT
) Selection
R
C
and C
C
form a pole-zero compensation network that
stabilizes the regulator. The values of R
C
and C
C
are mainly
dependant on the regulator voltage gain, I
LOAD(max)
, L and
C
OUT
. The following procedure calculates values for R
C
,C
C
,
and C
OUT
that ensure regulator stability. Be aware that this
procedure doesn’t necessarily result in R
C
and C
C
that pro-
vide optimum compensation. In order to guarantee optimum
compensation, one of the standard procedures for testing
loop stability must be used, such as measuring V
OUT
tran-
sient response when pulsing I
LOAD
(see Figure 15).
A. First, calculate the maximum value for R
C
.
Select a resistor less than or equal to this value, and it
should also be no greater than 3 kΩ.
B. Calculate the minimum value for C
OUT
using the following
two equations.
The larger of these two values is the minimum value that
ensures stability.
C. Calculate the minimum value of C
C
.
The compensation capacitor is also part of the soft start
circuitry. When power to the regulator is turned on, the
switch duty cycle is allowed to rise at a rate controlled by this
capacitor (with no control on the duty cycle, it would imme-
diately rise to 90%, drawing huge currents from the input
power supply). In order to operate properly, the soft start
circuit requires C
C
≥0.22 µF.
The value of the output filter capacitor is normally large
enough to require the use of aluminum electrolytic capaci-
tors. Figure 11 lists several different types that are recom-
mended for switching regulators, and the following param-
eters are used to select the proper capacitor.
Working Voltage (WVDC): Choose a capacitor with a work-
ing voltage at least 20% higher than the regulator output
voltage.
Ripple Current: This is the maximum RMS value of current
that charges the capacitor during each switching cycle. For
step-up and flyback regulators, the formula for ripple current
is
Choose a capacitor that is rated at least 50% higher than this
value at 52 kHz.
Equivalent Series Resistance (ESR) : This is the primary
cause of output ripple voltage, and it also affects the values
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
ESR doesn’t exceed the maximum value specified by the
following equations.
Select a capacitor with ESR, at 52 kHz, that is less than or
equal to the lower value calculated. Most electrolytic capaci-
tors specify ESR at 120 Hz which is 15% to 30% higher than
at 52 kHz. Also, be aware 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 µF), and capacitors with high
WVDC, or by paralleling smaller-value capacitors.
Inductor Manufacturer’s Part Number
Code Schott Pulse Renco
L47 67126980 PE - 53112 RL2442
L68 67126990 PE - 92114 RL2443
L100 67127000 PE - 92108 RL2444
L150 67127010 PE - 53113 RL1954
L220 67127020 PE - 52626 RL1953
L330 67127030 PE - 52627 RL1952
L470 67127040 PE - 53114 RL1951
L680 67127050 PE - 52629 RL1950
H150 67127060 PE - 53115 RL2445
H220 67127070 PE - 53116 RL2446
H330 67127080 PE - 53117 RL2447
H470 67127090 PE - 53118 RL1961
H680 67127100 PE - 53119 RL1960
H1000 67127110 PE - 53120 RL1959
H1500 67127120 PE - 53121 RL1958
H2200 67127130 PE - 53122 RL2448
Schott Corp., (612) 475-1173
1000 Parkers Lake Rd., Wayzata, MN 55391
Pulse Engineering, (619) 268-2400
P.O. Box 12235, San Diego, CA 92112
Renco Electronics Inc., (516) 586-5566
60 Jeffryn Blvd. East, Deer Park, NY 11729
FIGURE 10. Table of Standardized Inductors and
Manufacturer’s Part Numbers
LM1577/LM2577
www.national.com 18