LTM4600IV datasheet(12/24 Pages) LINER

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LTM4600

4600fc

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 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 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 / V

S])

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 if the output has a large

amount of capacitance or the load draws huge currents

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 currents prevents the

discharge of CSS during a fault and also shortens the soft-

start period. Using a resistor from RUN/SS pin to VIN is

a simple solution to defeat latchoff. Any pull-up network

must be able to maintain RUN/SS above 4V maximum

latchoff threshold and overcome the 4μA maximum dis-

charge current. Figure 3 shows a conceptual drawing of

VRUN during startup and short circuit.

Figure 5. Enable Circuit with External Logic

RUN/SS

4600 F05

LTM4600

PGND

2N7002

SGND

VIN

RRUN/SS

RUN/SS

4600 F04

LTM4600

PGND SGND

RECOMMENDED VALUES FOR RRUN/SS

VIN

4.5V TO 5.5V

10.8V TO 13.8V

16V TO 20V

RRUN/SS

50k

150k

330k

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

Enable

The RUN/SS pin can be driven from logic as shown in

Figure 5. This function allows the LTM4600 to be turned

on or off remotely. The ON signal can also control the

sequence of the output voltage.

APPLICATIONS INFORMATION

LTM4600IV Datasheet(PDF) 12 Page - Linear Technology

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