LTC4211 datasheet(12/22 Pages) LINER

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LTC4235

4235f

For more information www.linear.com/LTC4235

applicaTions inForMaTion

During a normal power-up, the ideal diode MOSFETs turn

onfirst.Assoonastheinternallygeneratedsupply,INTVCC,

rises above its 2.2V undervoltage lockout threshold, the

internal charge pump is allowed to charge up the CPO

pins. Because the ideal diode MOSFETs are connected in

parallelasadiode-OR,theSENSE+pinvoltageapproaches

the highest of the supplies at the IN1 and IN2 pins. The

MOSFET associated with the lower input supply voltage

willbeturnedoffbythecorrespondinggatedriveamplifier.

Before the Hot Swap MOSFET can be turned on, EN must

remainlowandONmustremainhighfora100msdebounce

timing cycle to ensure that any contact bounces during the

insertion have ceased. At the end of the debounce cycle,

the internal fault latch is cleared. The Hot Swap MOSFET

is then allowed to turn on by charging up HGATE with a

10µA current source from the charge pump. The voltage

at the HGATE pin rises with a slope equal to 10µA/CHG and

the supply inrush current flowing into the load capacitor

CL is limited to:

IINRUSH =

CHG

•10µA

The OUT voltage follows the HGATE voltage when the Hot

Swap MOSFET turns on. If the voltage across the current

sense resistor RS becomes too high based on the OUT pin

voltage, the inrush current will be limited by the internal

current limiting circuitry. Once the MOSFET gate overdrive

exceeds 4.2V and the OUT pin voltage is above 10.5V,

the PWRGD pin pulls low to indicate that the power is

good. Once OUT reaches the input supply voltage, HGATE

continues to ramp up. An internal 12V clamp limits the

HGATE voltage above OUT.

When the ideal diode MOSFET is turned on, the gate

drive amplifier controls the gate of the MOSFET to servo

the forward voltage drop across the MOSFET to 15mV.

If the load current causes more than 15mV of drop, the

MOSFET gate is driven fully on and the voltage drop is

equal to ILOAD•RDS(ON).

Turn-Off Sequence

The external MOSFETs can be turned off by a variety of

conditions. A normal turn-off for the Hot Swap MOSFET is

initiated by pulling the ON pin below its 1.155V threshold

(80mV ON pin hysteresis), or pulling the EN pin above its

1.235V threshold. Additionally, an overcurrent fault that

exceeds the fault timer period also turns off the Hot Swap

MOSFET. Normally, the LTC4235 turns off the MOSFET by

pulling the HGATE pin to ground with a 2mA current sink.

All of the MOSFETs turn off when INTVCC falls below its

undervoltage lockout threshold (2.2V). The DGATE pin is

pulled down with a 100µA current to one diode voltage

below the IN pin, while the HGATE pin is pulled down to

the OUT pin by a 200mA current. When D2OFF is pulled

high above 1.235V, the ideal diode MOSFET in the IN2

power path is turned off with DGATE2 pulled low by a

100µA current.

The gate drive amplifier controls the ideal diode MOSFET

to prevent reverse current when the input supply falls

below SENSE+. If the input supply collapses quickly, the

gate drive amplifier turns off the ideal diode MOSFET with

a fast pull-down circuit. If the input supply falls at a more

modest rate, the gate drive amplifier controls the MOSFET

to maintain SENSE+ at 15mV below IN.

Board Presence Detect with EN

If ON is high when the EN pin goes low, indicating a board

presence, the LTC4235 initiates a debounce timing cycle

for contact debounce. Upon board insertion, any bounces

on the EN pin restart the timing cycle. When the debounce

timing cycle is done, the internal fault latch is cleared. If

the EN pin remains low at the end of the timing cycle,

HGATE is charged up with a 10µA current source to turn

on the Hot Swap MOSFET.

If the EN pin goes high, indicating a board removal, the

HGATE pin is pulled low with a 2mA current sink after a

20µs delay, turning off the Hot Swap MOSFET without

clearing any latched fault.

LTC4211 Datasheet(PDF) 12 Page - Linear Technology

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