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LT3754

3754f

The power rating of RS should be selected to exceed

the I2R losses in the resistor. The peak inductor current

should be recalculated for the chosen RS value to ensure

the chosen inductor will not saturate.

Power MOSFET: Overcurrent and Hiccup Mode

For severe external faults which may cause the external

MOSFET to reach currents greater than the peak current

deﬁned by RS and the 52mV sense pin threshold described

above, the LT3754 has an overcurrent comparator which

triggers soft start and turns off the MOSFET driver for

currents exceeding,

D OVERCURRENT

() =

100

In this fault mode the LT3754 only allows MOSFET turn

on for approximately 100ns every 2ms. This hiccup mode

signiﬁcantly reduces the power rating required for the

MOSFET.

APPLICATIONS INFORMATION

Soft Start

To limit inductor inrush current and output voltage

during startup or recovery from a fault condition, the

LT3754 provides a soft start function. The LT3754

when entering these faults will discharge an internal

soft start node and prevent switching at the GATE

pin for any of the following faults: VIN, SHDN/UVLO

or INTVCC voltages too low or MOSFET current

too high (see the timing diagram in Figure 2). When exiting

these faults the LT3754 ramps up an internal soft start node

at approximately 0.5V/ms to control VC pin voltage rise and

hence control MOSFET switch current rise. In addition the

soft start period gradually ramps up switching frequency

from approximately 33% to 100% of full scale.

The conditions required to exit all faults and allow a soft

start ramp of the VC pin are listed in Figure 2. An added

feature of the LT3754 is that it waits for the ﬁrst PWM pin

active high (minimum 200ns pulse width) before it allows

Figure 2. LT3754 Fault Detection and Soft Start Timing for VC Pin and Internal SS Node

GATE

3754 F02

0.4V

VBE (VC SWITCHING

THRESHOLD)

0.1V

VBE

0.5V/ms

(INTERNAL)

0.4V

0.1V

ANY OF THE FOLLOWING FAULTS

TRIGGERS SOFT START LATCH

WITH GATE TURNED OFF

IMMEDIATELY:

VIN < 3.7V, SHDN < 1.476V,

INTVCC < 3.4V

IDSS (EXTERNAL MOSFET) > 100mV/RS

SOFT-START LATCH RESET REQUIRES

ALL CONDITIONS SATISFIED:

SS (INTERNAL) < 0.2V, VIN 4.2V,

SHDN > 1.476V, INTVCC > 3.8V,

IDSS (EXTERNAL MOSFET) < 100mV/RS,

PWM > 1.4V (FOR AT LEAST 200ns)

SOFT-START

LATCH SET:

VC MIN

CLAMP

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