LM1577, LM2577
SNOS658D –JUNE 1999–REVISED APRIL 2013
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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 immediately rise to 90%, drawing huge currents from the input power supply). In order to operate properly,
the soft start circuit requires CC ≥ 0.22 μF.
The value of the output filter capacitor is normally large enough to require the use of aluminum electrolytic
capacitors. Table 3 lists several different types that are recommended for switching regulators, and the following
parameters are used to select the proper 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 current that charges the capacitor during each switching
cycle. For step-up and flyback regulators, the formula for ripple current is
(11)
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 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 following equations.
(12)
Select a capacitor with ESR, at 52 kHz, that is less than or equal to the lower value calculated. Most electrolytic
capacitors 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.
3. Output Voltage Selection (R1 and R2)
This section is for applications using the LM1577-ADJ/LM2577-ADJ. Skip this section if the LM1577-12/LM2577-
12 or LM1577-15/LM2577-15 is being used.
With the LM1577-ADJ/LM2577-ADJ, the output voltage is given by
VOUT = 1.23V (1 + R1/R2)
(13)
Resistors R1 and R2 divide the output down so it can be compared with the LM1577-ADJ/LM2577-ADJ internal
1.23V reference. For a given desired output voltage VOUT, select R1 and R2 so that
(14)
4. Input Capacitor Selection (CIN)
The switching action in the step-up regulator causes a triangular ripple current to be drawn from the supply
source. This in turn causes noise to appear on the supply voltage. For proper operation of the LM1577, the input
voltage should be decoupled. Bypassing the Input Voltage pin directly to ground with a good quality, low ESR,
0.1 μF capacitor (leads as short as possible) is normally sufficient.
20
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