LT4239 datasheet(15/24 Pages) AD

15 / 24 page

LT4239

Rev. 0

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Supply Transients

In card-resident applications, an output short-circuits

working against the inductive nature of the supply can

cause the input to collapse almost to ground resulting in

supply brown-out. The architecture of the LT4239 allows

a simple RC filter to be connected at the VCC pin to pre-

serve the supply voltage not falling below its undervoltage

lockout threshold when the event happens. Meanwhile,

the LT4239’s fast comparator begins to engage and turn

off the MOSFETs in less than 1µs once the short-circuit

current through the sense resistor rises above its trip

threshold. When that happens, the input supply voltage

rapidly recovers and overshoots to a very high value due

to inductive kick. With the RC filter connected damping

the transient spikes, the VCC pin is protected from dam-

age. This removes the need for a transient voltage sup-

pressor (TVS) connected from VCC to GND. Without the

help of a TVS at the input, the voltage spikes can rise

to a level causing the MOSFET to breakdown and dis-

charge the inductor energy to the output. The MOSFET

can be damaged if the specified avalanche current or

energy is exceeded. A TVS (Z1) across the MOSFET can

be added from supply input to output for more protection.

See Figure 1.

APPLICATIONS INFORMATION

0.01µF

RIN

200

ROUT

20k

0.1µF

10µF

SENSE2

OUT

OUTK

GATE2

LT4239

GND

12V

GND

REF–

REF+

SCL

SDA

LTC2451

2-WIRE I2C

INTERFACE

IMON

VSENSE

ILOAD

VCC

4239 F04

–

LOAD

VCC

Figure 4. Low Side Current Monitor with LTC2451 ADC

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