LM3524 National Semiconductor, LM3524 Datasheet - Page 11

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

Typical Applications
Neglecting V
where T = Total Period
The above shows the relation between V
cycle.
as Q1 only conducts during t
The efficiency, , of the circuit is:
Q1. For this reason Q1 should be selected to have the maxi-
mum possible f
CALCULATING INDUCTOR L1
Since I
Solving the above for L1
MAX will be further decreased due to switching losses in
L
+ = I
SAT
T
L
, V
, which implies very fast rise and fall times.
−
= 0.4I
FIGURE 13. Relation of Switch Timing to Inductor Current in Step-Down Regulator
D
, and settling I
o
ON
.
(Continued)
L
+
= I
L
IN
−
;
, V
o
and duty
11
where: L1 is in Henrys
Also, see LM1578 data sheet for graphical methods of induc-
tor selection.
CALCULATING OUTPUT FILTER CAPACITOR C
Figure 13 shows L1’s current with respect to Q1’s t
t
flow to the load and C
ence between I
From Figure 13 it can be seen that current will be flowing into
C
a time, t
For best regulation, the inductor’s current cannot be allowed
to fall to zero. Some minimum load current I
ductor current, is required as shown below:
OFF
I
o
L
f is switching frequency in Hz
/4. The resulting V
for the second half of t
times (V
ON
/2 + t
A
L
is at the collector of Q1). This curent must
, and I
OFF
/2. The current flowing for this time is
o
c
o
. C
DS008650-17
.
Ic
or V
o
ON
o
’s current will then be the differ-
= I
through the first half of t
o
L
is described by:
− I
o
o
, and thus in-
www.national.com
o
:
ON
OFF
and
, or