LTC4236 datasheet(11/24 Pages) LINER

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LTC4236

4236f

For more information www.linear.com/LTC4236

operaTion

The LTC4236 functions as an input supply diode-OR with

inrush current limiting and overcurrent protection by

controlling the external N-channel MOSFETs (MD1, MD2

and MH) on a supply path. This allows boards to be safely

insertedandremovedinsystemswithabackplanepowered

byredundantsupplies.TheLTC4236hasasingleHotSwap

controller and two separate ideal diode controllers, each

providing independent control for the two input supplies.

When the LTC4236 is first powered up, the gates of the

external MOSFETs are held low, keeping them off. As the

DGATE2pull-upcanbedisabledbytheD2OFFpin,DGATE2

will pull high only when the D2OFF pin is pulled low. The

gate drive amplifier (GD1, GD2) monitors the voltage be-

tween the IN and SENSE+ pins and drives the respective

DGATE pin. The amplifier quickly pulls up the DGATE pin,

turning on the MOSFET for ideal diode control, when it

senses a large forward voltage drop. With the ideal diode

MOSFETs acting as input supply diode-OR, the SENSE+

pin voltage rises to the highest of the supplies at the IN1

and IN2 pins. An external capacitor connected at the CPO

pin provides the charge needed to quickly turn on the

ideal diode MOSFET. An internal charge pump charges

up this capacitor at device power-up. The DGATE pin

sources current from the CPO pin and sinks current into

the IN1, D2SRC and GND pins. When the DGATE1 to IN1

or DGATE2 to D2SRC voltage exceeds 0.7V, the respec-

tive DSTAT pin pulls low to indicate that the ideal diode

MOSFET is turned on.

Pulling the ON pin high and EN pin low initiates a debounce

timing cycle that can be a fixed 100ms or adjustable delay

as configured at the DTMR pin. After this timing cycle, a

10µA current source from the charge pump ramps up

the HGATE pin. When the Hot Swap MOSFET turns on,

the inrush current is limited at a level set by an external

sense resistor (RS) connected between the SENSE+ and

SENSE– pins. An active current limit amplifier (CL) servos

the gate of the MOSFET to 25mV or less across the cur-

rent sense resistor depending on the voltage at the FB

pin. Inrush current can be further reduced, if desired, by

adding a capacitor from HGATE to GND. When FB voltage

rises above 1.235V and the MOSFET’s gate drive (HGATE

to OUT voltage) exceeds 4.2V, the PWRGD pin pulls low.

The high side current sense amplifier (CM) provides ac-

curate monitoring of current through the current sense

resistor. The sense voltage is amplified by 100 times and

level shifted from the positive rail to a ground-referred

output at the IMON pin. The output signal is analog and

may be used as is or measured with an ADC.

When the ideal diode MOSFET is turned on, the gate drive

amplifier controls DGATE to servo the forward voltage

drop (VIN – VSENSE+) across the MOSFET to 15mV. If the

load current causes more than 15mV of voltage drop,

the gate voltage rises to enhance the MOSFET. For large

output currents, the MOSFET’s gate is driven fully on and

the voltage drop is equal to ILOAD•RDS(ON) of the MOSFET.

In the case of an input supply short-circuit when the

MOSFETs are conducting, a large reverse current starts

flowing from the load towards the input. The gate drive

amplifier detects this failure condition and turns off the

ideal diode MOSFET by pulling down the DGATE pin.

Inthecasewhereanovercurrentfaultoccursonthesupply

output, the current is limited with foldback. After a delay

set by 100µA charging the FTMR pin capacitor, the fault

timer expires and pulls the HGATE pin low, turning off the

Hot Swap MOSFET. The FAULT pin is also latched low. At

this point, the DGATE pin continues to pull high and keeps

the ideal diode MOSFET on.

Internal clamps limit both the DGATE1 and CPO1 to IN1,

and DGATE2 and CPO2 to D2SRC voltages to 12V. The

same clamps also limit the DGATE and CPO pins to a diode

voltage below the IN1 or D2SRC pins. Another internal

clamp limits the HGATE to OUT voltage to 12V and also

clampstheHGATEpintoadiodevoltagebelowtheOUTpin.

Power to the LTC4236 is supplied from either the IN or

OUT pins, through an internal diode-OR circuit to a low

dropout regulator (LDO). That LDO generates a 5V supply

at the INTVCC pin and powers the LTC4236’s internal low

voltage circuitry.

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