MAX6397_ATA-T 查看數據表（PDF） - Maxim Integrated

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MAX6397_ATA-T

Overvoltage Protection Switch/Limiter Controllers Operate Up to 72V

Maxim Integrated 

Overvoltage Protection Switch/Limiter

Controllers Operate Up to 72V

VBATT

GATE

IN

OUT

MAX6397

MAX6398 SET

GND

COUT

R1

R2

VPEAK

VBATT

tRISE > 5ms

100ms

200ms

300ms

400ms

Figure 4. Overvoltage Limiter Protection Switch Configuration

As the transient begins decreasing, OUT fall time will

depend on the MOSFET’s GATE charge, the internal

charge-pump current, the output load, and the tank

capacitor at OUT.

For fast-rising transients and very large-sized MOSFETs,

add an additional external bypass capacitor from GATE

to GND to reduce the effect of the fast-rising voltages at

IN. The external capacitor acts as a voltage-divider

working against the MOSFETs’ drain-to-gate capaci-

tance. For a 6000pF Cgd, a 0.1µF capacitor at GATE will

reduce the impact of the fast-rising VIN input.

Caution must be exercised when operating the

MAX6397/MAX6398 in voltage-limiting mode for long

durations. If the VIN is a DC voltage greater than the

MOSFET’s maximum gate voltage, the FET will dissipate

power continuously. To prevent damage to the external

MOSFET, proper heatsinking should be implemented.

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. Power in the alternator (essen-

tially an inductor) flows into the distributed power sys-

tem 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

Figure 5. Load Dump Voltage Profile

characteristics of the charging system (Figure 5).

These transients are capable of destroying semicon-

ductors on the first ‘fault event.’

Setting Overvoltage Thresholds

SET provides an accurate means to set the overvoltage

level for the MAX6397/MAX6398. Use a resistor-divider to

set the desired overvoltage condition (Figure 6). SET has

a rising 1.215V threshold with a 5% falling hysteresis.

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

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

rent equivalent to a minimum 100 x ISET (SET’s input

bias current) at the desired overvoltage threshold.

For example:

With an overvoltage threshold set to 20V:

RTOTAL < 20V/(100 x ISET)

where ISET is SET’s 50nA input bias current.

RTOTAL < 4MΩ

Use the following formula to calculate R2:

R2

=

VTH ×

R TOTAL

VOV

where VTH is the 1.215V SET rising threshold and VOV

is the overvoltage threshold.

R2 = 243kΩ, use a 240kΩ standard resistor.

RTOTAL = R2 + R1, where R1 = 3.76MΩ.

Use a 3.79MΩ standard resistor.

A lower value for total resistance dissipates more

power but provides slightly better accuracy.

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