LTM4600 查看數據表（PDF） - Linear Technology

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LTM4600

10A High Efficiency DC/DC µModule

Linear Technology 

LTM4600

applications information

Since the load current is already limited by the current mode

control and current foldback circuitry during a short circuit,

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. Us-

ing 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 discharge current. With a

pull-up resistor, the delay before starting is approximately:

( ) tDELAY = –RRUN/SS • CSS _EXT + 1000pF

•

⎛

ln⎝⎜⎜1−

VIN

+

1.5V

(1.2µA • RRUN/SS

)

⎞

⎠⎟⎟

Figure 3 shows a conceptual drawing of VRUN during

startup and short circuit.

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.

VRUN/SS

4V

3.5V

3V

1.5V

SHORT-CIRCUIT

LATCH ARMED

SOFT-START

CLAMPING

OF IL RELEASED

t

OUTPUT

OVERLOAD

SHORT-CIRCUIT

LATCHOFF

HAPPENS

VO

75%VO

SWITCHING

STARTS

t

4600 F03

Figure 3. RUN/SS Pin Voltage During Startup and

Short-Circuit Protection

VIN

RRUN/SS

VIN

LTM4600

RUN/SS

PGND SGND

4600 F04

RECOMMENDED VALUES FOR RRUN/SS

VIN

RRUN/SS

4.5V TO 5.5V

50k

10.8V TO 13.8V 150k

16V TO 20V

330k

RUN/SS

ON

LTM4600

2N7002

PGND SGND

4600 F05

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

Resistor to VIN

Figure 5. Enable Circuit with External Logic

4600fd

For more information www.linear.com/LTM4600

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