Application Hints (Continued)
Inductor
Code
L47
Manufacturer’s Part Number
The compensation capacitor is also part of the soft start cir-
cuitry. When power to the regulator is turned on, the switch
duty cycle is allowed to rise at a rate controlled by this ca-
pacitor (with no control on the duty cycle, it would immedi-
ately rise to 90%, drawing huge currents from the input
power supply). In order to operate properly, the soft start cir-
cuit requires CC ≥ 0.22 µF.
Schott
Pulse
Renco
RL2442
RL2443
RL2444
RL1954
RL1953
RL1952
RL1951
RL1950
RL2445
RL2446
RL2447
RL1961
RL1960
RL1959
RL1958
RL2448
67126980
67126990
67127000
67127010
67127020
67127030
67127040
67127050
67127060
67127070
67127080
67127090
67127100
67127110
67127120
67127130
PE - 53112
PE - 92114
PE - 92108
PE - 53113
PE - 52626
PE - 52627
PE - 53114
PE - 52629
PE - 53115
PE - 53116
PE - 53117
PE - 53118
PE - 53119
PE - 53120
PE - 53121
PE - 53122
L68
L100
L150
L220
L330
L470
L680
H150
H220
H330
H470
H680
H1000
H1500
H2200
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 volt-
age.
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
Schott Corp., (612) 475-1173
1000 Parkers Lake Rd., Wayzata, MN 55391
Pulse Engineering, (619) 268-2400
Choose a capacitor that is rated at least 50% higher than this
value at 52 kHz.
P.O. Box 12235, San Diego, CA 92112
Renco Electronics Inc., (516) 586-5566
60 Jeffryn Blvd. East, Deer Park, NY 11729
Equivalent Series Resistance (ESR) : This is the primary
cause of output ripple voltage, and it also affects the values
of RC and CC needed to stabilize the regulator. As a result,
the preceding calculations for CC and RC are only valid if
ESR doesn’t exceed the maximum value specified by the fol-
lowing equations.
FIGURE 10. Table of Standardized Inductors and
Manufacturer’s Part Numbers
2. Compensation Network (RC, CC) and Output Capacitor
(COUT) Selection
RC and CC form a pole-zero compensation network that sta-
bilizes the regulator. The values of RC and CC are mainly de-
pendant on the regulator voltage gain, ILOAD(max), L and
C
OUT. The following procedure calculates values for RC, CC,
and COUT that ensure regulator stability. Be aware that this
procedure doesn’t necessarily result in RC and CC 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 VOUT tran-
sient response when pulsing ILOAD (see Figure 15).
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.
A. First, calculate the maximum value for RC.
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.
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 COUT using the following
two equations.
The larger of these two values is the minimum value that en-
sures stability.
C. Calculate the minimum value of CC
.
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