Datasheet

LM2524D, LM3524D
SNVS766E –JUNE 2009–REVISED MAY 2013 www.ti.com
(10)
where: L1 is in Henrys
f is switching frequency in Hz
Also, see LM1578 data sheet for graphical methods of inductor selection.
Calculating Output Filter Capacitor C
o
Figure 23 shows L1's current with respect to Q1's t
ON
and t
OFF
times (V
A
is at the collector of Q1). This curent
must flow to the load and C
o
. C
o
's current will then be the difference between I
L
, and I
o
.
Ic
o
= I
L
− I
o
(11)
From Figure 23 it can be seen that current will be flowing into C
o
for the second half of t
ON
through the first half of
t
OFF
, or a time, t
ON
/2 + t
OFF
/2. The current flowing for this time is ΔI
L
/4. The resulting ΔV
c
or ΔV
o
is described by:
(12)
For best regulation, the inductor's current cannot be allowed to fall to zero. Some minimum load current I
o
, and
thus inductor current, is required as shown below:
(13)
Figure 24. Inductor Current Slope in Step-Down Regulator
A complete step-down switching regulator schematic, using the LM3524D, is illustrated in Figure 25. Transistors
Q1 and Q2 have been added to boost the output to 1A. The 5V regulator of the LM3524D has been divided in
half to bias the error amplifier's non-inverting input to within its common-mode range. Since each output
transistor is on for half the period, actually 45%, they have been paralleled to allow longer possible duty cycle, up
to 90%. This makes a lower possible input voltage. The output voltage is set by:
(14)
where V
NI
is the voltage at the error amplifier's non-inverting input.
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