LM3524DM National Semiconductor, LM3524DM Datasheet - Page 14

• Manufacturer Part Number: LM3524DM
• Description: IC, REGULATING PWM, 16-SOIC
• Manufacturer: National Semiconductor

Specifications of LM3524DM

• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Voltage: 18V
• Output Current: 200mA
• No. Of Pins: 16
• Operating Temperature Range: 0°C To +125°C
• Power Dissipation Pd: 1W
• Termination Type: SMD
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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
to the load and C
between I
From Figure 13 it can be seen that current will be flowing into
C
time, t
The resulting ΔV
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
o
for the second half of t
times (V
ON
/2 + t
L
, and I
A
OFF
is at the collector of Q1). This curent must flow
c
o
/2. The current flowing for this time is ΔI
.
o
or ΔV
. C
o
's current will then be the difference
o
Ic
ON
is described by:
o
= I
through the first half of t
L
− I
o
o
, and thus in-
o
OFF
:
ON
, or a
and
L
/4.
14
A complete step-down switching regulator schematic, using
the LM3524D, is illustrated in Figure 15. Transistors Q1 and
Q2 have been added to boost the output to 1A. The 5V reg-
ulator 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:
where V
input.
Resistor R3 sets the current limit to:
Figures 16, 17 and show a PC board layout and stuffing dia-
gram for the 5V, 1A regulator of Figure 15. The regulator's
performance is listed in Table 1.
FIGURE 14. Inductor Current Slope in Step-Down
NI
is the voltage at the error amplifier's non-inverting
Regulator
865019