LTM4602 datasheet(12/24 Pages) LINER

12 / 24 page

LTM4602

4602fa

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, shutting down the converter

permanently. The RUN/SS pin must be actively pulled

down to ground in order to restart operation.

The overcurrent protection timer requires the soft-start

timing capacitor CSS be made large enough to guarantee

that the output regulation by the time CSS has reached the

4V threshold. In general, this will depend upon the size of

the output capacitance, output voltage and load current

characteristic. A minimum external soft-start capacitor

can be estimated from:

CSS_EXT +1000pF >COUT •VOUT(10–3[F / VS])

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 short circuit,

overcurrent latchoff operation is NOT always needed or

desired, especially if the output has large capacitance or

the load draws high 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. Using a resistor from

RUN/SS pin to VINisasimplesolutiontodefeatlatchoff.Any

pull-up network must be able to maintain RUN/SS above

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

4V maximum latchoff threshold and overcome the 4μA

maximum discharge current. Figure 3 shows a conceptual

drawing of VRUN during start-up and short circuit.

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

4602 F03

VOUT

VIN

RRUN/SS

RUN/SS

4602 F04

LTM4602

PGND SGND

VIN

4.5V TO 5.5V

10.8V TO 13.8V

16V TO 20V

RRUN/SS

50k

150k

330k

RECOMMENDED VALUES FOR RRUN/SS

APPLICATIONS INFORMATION

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