LM3524DM National Semiconductor, LM3524DM Datasheet - Page 13

• 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

BASIC SWITCHING REGULATOR THEORY
AND APPLICATIONS
The basic circuit of a step-down switching regulator circuit is
shown in Figure 12, along with a practical circuit design using
the LM3524D in Figure 15.
Neglecting V
where T = Total Period
The above shows the relation between V
as Q1 only conducts during t
The efficiency, η, of the circuit is:
FIGURE 12. Basic Step-Down Switching Regulator
SAT
, V
D
, and settling ΔI
FIGURE 13. Relation of Switch Timing to Inductor Current in Step-Down Regulator
ON
.
L
+
= ΔI
IN
, V
L
−
;
o
and duty cycle.
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The circuit works as follows: Q1 is used as a switch, which
has ON and OFF times controlled by the pulse width modu-
lator. When Q1 is ON, power is drawn from V
to the load through L1; V
verse biased, and C
inductor L1 will force V
in it, D1 will start conducting and the load current will flow
through D1 and L1. The voltage at V
C
L1 is equal to the nominal DC load current plus some ΔI
which is due to the changing voltage across it. A good rule of
thumb is to set
ηMAX will be further decreased due to switching losses in Q1.
For this reason Q1 should be selected to have the maximum
possible f
CALCULATING INDUCTOR L1
Since ΔI
Solving the above for L1
where: L1 is in Henrys
o
f is switching frequency in Hz
filter giving a clean DC output. The current flowing through
L
+ = ΔI
T
, which implies very fast rise and fall times.
Δ I
L
−
LP-P
= 0.4I
o
is charging. When Q1 turns OFF the
A
40% × I
o
negative to keep the current flowing
A
is at approximately V
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o
.
A
is smoothed by the L1,
IN
and supplied
www.national.com
IN
, D1 is re-
L