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LTC3732CUHF 查看數據表(PDF) - Linear Technology

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LTC3732CUHF Datasheet PDF : 28 Pages
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LTC3732
APPLICATIO S I FOR ATIO
controller is permitted to start operating. As the voltage on
RUN/SS increases from 1.5V to 3.5V, the internal current
limit is increased from 20mV/RSENSE to 75mV/RSENSE.
The output current limit ramps up slowly, taking an
additional 1s/µF to reach full current. The output current
thus ramps up slowly, eliminating the starting surge
current required from the input power supply. If RUN/SS
has been pulled all the way to ground, there is a delay
before starting of approximately:
( ) tDELAY
=
1.5V
1.5µA
C SS
=
1s/µF
C SS
( ) tIRAMP
=
3V 1.5V
1.5µA
C SS
=
1s/µF
C SS
By pulling the RUN/SS pin below 0.4V the IC is put into low
current shutdown (IQ < 100 µA). The RUN/SS pin can be
driven directly from logic as shown in Figure7. Diode, D1,
in Figure 7 reduces the start delay but allows CSS to ramp
up slowly providing the soft-start function. The RUN/SS
pin has an internal 6V zener clamp (see the Functional
Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor is used initially to turn on and limit the
inrush current of all three output stages. After the control-
lers have been started and been given adequate time to
charge up the output capacitor and provide full load
current, the RUN/SS capacitor is used for a short-circuit
timer. If the output voltage falls to less than 70% of its
nominal value, the RUN/SS capacitor begins discharging
on the assumption that the output is in an overcurrent
condition. If the condition lasts for a long enough period,
as determined by the size of the RUN/SS capacitor, the
discharge current, and the circuit trip point, the controller
will be shut down until the RUN/SS pin voltage is recycled.
If the overload occurs during start-up, the time can be
approximated by:
tLO1 >> (CSS • 0.6V)/(1.5µA) = 4 • 105 (CSS)
If the overload occurs after start-up, the voltage on the
RUN/SS capacitor will continue charging and will provide
18
additional time before latching off:
tLO2 >> (CSS • 3V)/(1.5µA) = 2 • 106 (CSS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin from VCC
as shown in Figure 7. When VCC is 5V, a 200k resistance
will prevent the discharge of the RUN/SS capacitor
during an overcurrent condition but also shortens the
soft-start period, so a larger RUN/SS capacitor value may
be required.
3.3V OR 5V
RUN/SS
PIN
RUN/SS
VCC PIN
5V
D1
RSS
SHDN
CSS SHDN
CSS
3732 F07
Figure 7. RUN/SS Pin Interfacing
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem with
noise pick-up or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
foldback current limiting still remains active, thereby
protecting the power supply system from failure. A deci-
sion can be made after the design is complete whether to
rely solely on foldback current limiting or to enable the
latchoff feature by removing the pull-up resistor.
The value of the soft-start capacitor CSS may need to be
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
CSS > (COUT )(VOUT) (10–4) (RSENSE)
The minimum recommended soft-start capacitor of
CSS = 0.1µF will be sufficient for most applications.
Current Foldback
In certain applications, it may be desirable to defeat the
internal current foldback function. A negative impedance
is experienced when powering a switching regulator.
That is, the input current is higher at a lower VIN and
decreases as VIN is increased. Current foldback is de-
signed to accommodate a normal, resistive load having
3732f

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