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MAX6495

72V, Overvoltage-Protection Switches/Limiter Controllers with an External MOSFET

Maxim Integrated 

MAX6495–MAX6499

Power-OK Output (MAX6497/MAX6498)

POK is an open-drain output that remains low when the voltage

at POKSET is below the internal POKSET threshold

(1.18V). POK goes high impedance when POKSET goes

above the internal POKSET threshold (1.24V). Connect

a resistive divider from OUTFB to GND, and the divider

center node to POKSET, to adjust the desired undervoltage

threshold. Use a resistor in the 100kΩ range from POKSET

to GND to minimize current consumption.

Overvoltage Latch Function

The MAX6497/MAX6499 offers a latch function that prevents

the external MOSFET from turning on until the latch is

cleared. For the MAX6497, the latch can be cleared by

cycling the power on the input IN to a voltage below the

undervoltage lockout or by pulling the shutdown input low

and then back to a logic-high state. The MAX6499 offers a

CLEAR input that latches the nMOSFET off when CLEAR is

high. The latch is removed when the CLEAR input is pulsed

low. Connect CLEAR low to make the latch transparent.

Overvoltage Retry Function

The MAX6498 offers an automatic retry function that tries

to enhance the external nMOSFET after the overvoltage

condition is removed. When the monitored input voltage

detects an overvoltage condition (VSET > VTH+), the

nMOSFET is turned off. The MOSFET stays off until the

voltage at VSET falls below its VTH- (typically 0.13V), at

which point the output tries to turn on again.

Applications Information

Load Dump

Most automotive applications run off a multicell “12V”

lead-acid battery with a nominal voltage that swings

between 9V and 16V (depending on load current,

charging status, temperature, battery age, etc.). The

battery voltage is distributed throughout the automobile

and is locally regulated down to voltages required by

the different system modules. Load dump occurs when

the alternator is charging the battery and the battery

becomes disconnected. The alternator voltage regulator is

temporarily driven out of control. Power from the alternator

flows into the distributed power system and elevates the

voltage seen at each module. The voltage spikes have

rise times typically greater than 5ms and decays within

several hundred milliseconds but can extend out to 1s

or more depending on the characteristics of the charging

system. These transients are capable of destroying sensitive

electronic equipment on the first “fault event.”

72V, Overvoltage-Protection

Switches/Limiter Controllers

with an External MOSFET

Setting Overvoltage Thresholds

OVSET provides an accurate means to set the overvoltage

level for the devices. Use a resistive divider to set the

desired overvoltage condition (see Figure 2). OVSET has a

rising 1.24V threshold with a 5% falling hysteresis (MAX6495/

MAX6496/MAX6499) and a rising 0.505V threshold with a

falling 0.15V threshold (MAX6497/MAX6498).

Begin by selecting the total end-to-end resistance,

RTOTAL = R1 + R2. Choose RTOTAL to yield a total

current equivalent to a minimum 100 x ISET (OVSET’s

input bias current) at the desired overvoltage threshold.

For example:

With an overvoltage threshold (VOV) set to 20V for the

MAX6495/MAX6496/MAX6499, RTOTAL < 20V/(100 x ISET),

where ISET is OVSET’s 50nA (max) input bias current.

RTOTAL < 4MΩ

Use the following formula to calculate R2:

= R2

V TH+

×

R

TOTAL

VOV

where VTH+ is the 1.24V OVSET rising threshold and

VOV is the desired overvoltage threshold.

R2 = 248kΩ. Use a 249kΩ standard resistor.

RTOTAL = R2 + R1, where R1 = 3.751MΩ. Use a 3.74MΩ

standard resistor.

A lower value for total resistance dissipates more power

but provides slightly better accuracy. To improve ESD

protection, keep R2 ≥ 1kΩ.

Reverse-Battery Protection

The MAX6496 is an overvoltage-protection circuit that is

capable of driving a pMOSFET to prevent reverse-battery

conditions. This MOSFET eliminates the need for external

diodes, thus minimizing the input voltage drop (see Figure 8).

Inrush/Slew-Rate Control

Inrush current control can be implemented by placing a

capacitor from GATE to GND to slowly ramp up the GATE,

thus limiting the inrush current and controlling GATE’s

slew rate during initial turn-on. The inrush current can be

approximated using the following equation:

IINRUSH =

C OUT

C GATE

×

I GATE

+

ILOAD

where IGATE is GATE’s 100μA sourcing current, ILOAD

is the load current at startup, and COUT is the output

capacitor.

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