LM2577N-12 National Semiconductor, LM2577N-12 Datasheet - Page 16

• Manufacturer Part Number: LM2577N-12
• Description: SIMPLE SWITCHER Step-Up Voltage Regulator
• Manufacturer: National Semiconductor

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Application Hints
Schott Corp., [phone]
1000 Parkers Lake Rd., Wayzata, MN 55391
Pulse Engineering, [phone]
P.O. Box 12235, San Diego, CA 92112
Renco Electronics Inc., [phone]
60 Jeffryn Blvd. East, Deer Park, NY 11729
2. Compensation Network (R
(C
R
bilizes the regulator. The values of R
pendant on the regulator voltage gain, I
C
and C
procedure doesn’t necessarily result in R
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
sient response when pulsing I
A. First, calculate the maximum value for R
Select a resistor less than or equal to this value, and it
should also be no greater than 3 k .
B. Calculate the minimum value for C
two equations.
The larger of these two values is the minimum value that en-
sures stability.
C. Calculate the minimum value of C
C
OUT
Inductor
OUT
and C
H1000
H1500
H2200
FIGURE 10. Table of Standardized Inductors and
Code
H150
H220
H330
H470
H680
L100
L150
L220
L330
L470
L680
L47
L68
. The following procedure calculates values for R
) Selection
OUT
C
that ensure regulator stability. Be aware that this
form a pole-zero compensation network that sta-
Manufacturer’s Part Numbers
67126980
67126990
67127000
67127010
67127020
67127030
67127040
67127050
67127060
67127070
67127080
67127090
67127100
67127110
67127120
67127130
Schott
Manufacturer’s Part Number
LOAD
C
(Continued)
, C
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
C
Pulse
) and Output Capacitor
(see Figure 15 ).
C
OUT
C
and C
.
C
using the following
LOAD(max)
and C
C
C
.
are mainly de-
RL2442
RL2443
RL2444
RL1954
RL1953
RL1952
RL1951
RL1950
RL2445
RL2446
RL2447
RL1961
RL1960
RL1959
RL1958
RL2448
C
Renco
OUT
that pro-
, L and
C
, C
tran-
C
,
16
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 C
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
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
the preceding calculations for C
ESR doesn’t exceed the maximum value specified by the fol-
lowing 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
WVDC, or by paralleling smaller-value capacitors.
C
and C
C
needed to stabilize the regulator. As a result,
C
0.22 µF.
470 µF), and capacitors with high
C
and R
C
are only valid if