LM3524DN National Semiconductor, LM3524DN Datasheet - Page 13

• Manufacturer Part Number: LM3524DN
• Description: ENHANCED PWM CONTROLLER, 3524, DIP
• Manufacturer: National Semiconductor

Specifications of LM3524DN

• Frequency: 350kHz
• Power Dissipation Pd: 1W
• Digital Ic Case Style: DIP
• No. Of Pins: 16
• Operating Temperature Range: 0°C To +125°C
• Svhc: No SVHC (15-Dec-2010)
• Operating Temperature Max: 125°C
• Base Number: 3524
• Controller Type: PWM
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Typical Applications
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
cycle.
as Q1 only conducts during t
The efficiency, η, of the circuit is:
FIGURE 12. Basic Step-Down Switching Regulator
SAT
, V
D
FIGURE 13. Relation of Switch Timing to Inductor Current in Step-Down Regulator
, and settling ∆I
ON
.
L
(Continued)
+
= ∆I
L
IN
−
;
, V
o
00865016
and duty
13
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
reverse 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
through L1 is equal to the nominal DC load current plus
some ∆I
good rule of thumb is to set ∆I
ηMAX will be further decreased due to switching losses in
Q1. For this reason Q1 should be selected to have the
maximum possible f
times.
CALCULATING INDUCTOR L1
Since ∆I
Solving the above for L1
o
filter giving a clean DC output. The current flowing
L
L
+ = ∆I
which is due to the changing voltage across it. A
L
−
= 0.4I
T
, which implies very fast rise and fall
o
A
is charging. When Q1 turns OFF the
o
negative to keep the current flowing
00865017
A
is at approximately V
LP-P
. 40% x I
A
is smoothed by the L1,
IN
o
.
and supplied
www.national.com
IN
, D1 is