LTM4600EVPBF datasheet(12/24 Pages) LINER

12 / 24 page

LTM4600

4600fa

APPLICATIO S I FOR ATIO

After the controller has been started and given adequate

time to charge up the output capacitor, CSS is used as a

short-circuit timer. After the RUN/SS pin charges above 4V,

if the output voltage falls below 75% of its regulated value,

then a short-circuit fault is assumed. A 1.8µA current then

begins discharging CSS. If the fault condition persists until

the RUN/SS pin drops to 3.5V, then the controller turns

off both power MOSFETs, shuting down the converter

permanently. The RUN/SS pin must be actively pulled

down to ground in order to restart operation.

The over-current protection timer requires the soft-start

timing capacitor CSS be made large enough to guarantee

that the output is in regulation by the time CSS has reached

the 4V threshold. In general, this will depends upon the

size of the output capacitance, output voltage and load

current characteristic. A minimum external soft-start

capacitor can be estimated from:

CpF

F V

SS EXT

OUT

–

•(

[ /

])

1000

10 3

Generally 0.1µF is more than sufﬁcient.

Since the load current is already limited by the current

mode control and current foldback circuitry during a

shortcircuit, over-current latchoff operation is NOT always

needed or desired, especially the output has large amount

of capacitance or the load draw huge current during start

up. The latchoff feature can be overridden by a pull-up

current greater than 5µA but less than 80µA to the RUN/SS

pin. The additional current prevents the discharge of CSS

during a fault and also shortens the soft-start period. Us-

ing a resistor from RUN/SS pin to VIN is a simple solution

VIN

500k

RUN/SS

4600 F04

LTM4600

PGND

SGND

Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up

Resistor to VIN

Figure 3. RUN/SS Pin Voltage During Startup and

Short-Circuit Protection

VRUN/SS

3.5V

75%VO

SWITCHING

STARTS

SOFT-START

CLAMPING

OF IL RELEASED

SHORT-CIRCUIT

LATCHOFF

OUTPUT

OVERLOAD

HAPPENS

SHORT-CIRCUIT

LATCH ARMED

1.5V

4600 F03

to defeat latchoff. Any pull-up network must be able to

maintain RUN/SS above 4V maximum latchoff threshold

and overcome the 4µA maximum discharge current. Figure

3 shows a conceptual drawing of VRUN during startup and

short circuit.

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