LM3524 National Semiconductor, LM3524 Datasheet - Page 15

• Manufacturer Part Number: LM3524
• Description: Regulating Pulse Width Modulator
• Manufacturer: National Semiconductor

Typical Applications
In calculating the output capacitor C
supplies I
time will be some V
lator. Calculation of C
where: C
Calculation of inductor L1 is as follows:
V
IN
V
is applied across L1
o
is the p-p output ripple
o
o
is in farads, f is the switching frequency,
during t
ON
c
. The voltage change on C
o
= V
is:
o
or the output ripple of the regu-
(Continued)
o
it can be seen that C
o
during this
o
15
where: L1 is in henrys, f is the switching frequency in Hz
To apply the above theory, a complete step-up switching
regulator is shown in Figure 20 . Since V
to V
plifier’s inverting input. The output voltage is:
The network D1, C1 forms a slow start circuit.
This holds the output of the error amplifier initially low thus
reducing the duty-cycle to a minimum. Without the slow start
circuit the inductor may saturate at turn-on because it has to
supply high peak currents to charge the output capacitor
from 0V. It should also be noted that this circuit has no sup-
ply rejection. By adding a reference voltage at the
non-inverting input to the error amplifier, see Figure 21 , the
input voltage variations are rejected.
The LM3524D can also be used in inductorless switching
regulators. Figure 22 shows a polarity inverter which if con-
nected to Figure 20 provides a −15V unregulated output.
IN
. The input voltage is divided by 2 to bias the error am-
IN
is 5V, V
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REF
is tied