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LT4351

sn4351 4351fs

APPLICATIO S I FOR ATIO

Setting Fault Thresholds

The gate drive amplifier implements the ideal diode func-

tion. The fault comparators (UV and OV) prevent out of

range input voltages from affecting the output by disabling

the amplifier during these conditions. Think of the UV and

OV as gating the ideal diode function, something a regular

diode can’t do.

A resistive divider from VIN to UV and one from VIN to OV

are the usual way of setting the FAULT thresholds. For UV

the resistor values are set by:

HYST

UVHYST

FAULT

–

•

where UVHYST is the desired undervoltage hysteresis at

the input. UVFAULT is the desired undervoltage trip voltage

at the input. VUV is the part undervoltage trip point (0.3V)

and IHYSTUV is the undervoltage hysteresis current (10µA).

See Figure 1.

The divider on the OV pin is a straightforward resistive

divider (Figure 2):

R R Divider Current

FAULT

= 







–

where OVFAULT is the desired overvoltage trip point at the

input and VOV is the OV pin threshold (0.3V). The OV pin

has 7mV of voltage hysteresis at room.

It is possible to do both dividers together using only three

resistors though with more interdependence in compo-

nents (Figure 3). The input bias current for UV and OV is

less than 200nA, so keep resistor values less than 10k.

IHYS

µA

VUV

300mV

VIN

UV TURNING ON

UV TURNING OFF

IHYS

µA

VUV

300mV

4351 F01

VIN

Figure 1

VOV

300mV

4351 F02

VIN

Figure 2

4351 F03

VIN

Figure 3

R2A

R2B

4351 F04

VIN

Figure 4

actively low. In the event of VDD collapse there still is an

active pull-down (though of lesser strength) of GATE

powered from OUT, guaranteeing turn off.

The on-chip boost regulator uses a constant off-time

control scheme. When VDD is below the regulation trip

voltage, the switch turns on after a 600ns off-time. When

the switch turns on current ramps up in the inductor until

the current limit is reached (450mA). The switch turns off

and the inductor’s current flows through the external

diode to charge up the VDD capacitor. If VDD is still too low,

the switch turns on again after a fixed off-time of 600ns.

The boost regulator regulates VDD to approximately 10.7V

above VIN. When VDD is above this level, the SW transistor

turn-on is disabled. When VDD falls below this level by the

hysteresis level, the SW transistor is allowed to turn on.

There is approximately 0.15V of hysteresis.

OPERATIO

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