参数资料
型号: LT3503EDCB
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: 1 A SWITCHING REGULATOR, 2400 kHz SWITCHING FREQ-MAX, PDSO6
封装: 2 X 3 MM, PLASTIC, MO-229, DFN-6
文件页数: 6/20页
文件大小: 423K
代理商: LT3503EDCB
14
LT3503
3503f
APPLICATIO S I FOR ATIO
WU
U
Figure 8. To Soft-Start the LT3503, Add a Resistor and Capacitor to the SHDN Pin. VIN = 12V, VOUT = 3.3V, COUT = 10F, RLOAD = 5
Soft-Start
The SHDN pin can be used to soft-start the LT3503,
reducing the maximum input current during start-up. The
SHDN pin is driven through an external RC filter to create
a voltage ramp at this pin. Figure 8 shows the start-up
waveforms with and without the soft-start circuit. By
choosing a large RC time constant, the peak start up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20
A when the SHDN
pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3503 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3503 is absent. This may occur in battery charging
applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3503’s
output. If the VIN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied to
VIN), then the LT3503’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT3503 can
pull large currents from the output through the SW pin and
the VIN pin. Figure 9 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
RUN
VSW
5V/DIV
VIN = 12V
VOUT = 3.3V
L = 3.3
H
COUT = 10F
VOUT
2V/DIV
10
s/DIV
IL
1A/DIV
VSW
5V/DIV
VIN = 12V
VOUT = 3.3V
L = 3.3
H
COUT = 10F
VOUT
2V/DIV
10
s/DIV
IL
1A/DIV
SHDN
GND
RUN
15k
0.068
F
SHDN
GND
3503 F08b
3503 F08a
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