GATE cycles on-off-on-off-on in a sawtooth waveform

until OUT remains below the overvoltage fault threshold

voltage limiter’s sawtooth GATE output operates the

MOSFET in a switched-linear mode while the input volt-

age remains above the overvoltage fault threshold. The

sawtooth frequency depends on the load capacitance,

load current, and MOSFET turn-on time (GATE charge

current and GATE capacitance).

GATE goes high when the following startup conditions

high, an overvoltage fault is not present and the device

is not in thermal shutdown.

Overvoltage Monitoring

When operating in overvoltage mode, the MAX6397/

MAX6398 feedback path (Figure 3) consists of IN,

SET’s internal comparator, the internal gate charge

pump, and the external n-channel MOSFET resulting in

a switch-on/off function. When the programmed over-

voltage threshold is tripped, the internal fast compara-

tor turns off the external MOSFET, pulling GATE to OUT

the load. When IN decreases below the adjusted over-

voltage threshold, the MAX6397/MAX6398 slowly

enhance GATE above OUT, reconnecting the load to

the power source.

Overvoltage Limiter

When operating in overvoltage limiter mode, the

MAX6397/MAX6398 feedback path (Figure 4) consists

of OUT, SET’s internal comparator, the internal gate

charge pump and the external n-channel MOSFET,

which results in the external MOSFET operating as a

voltage regulator.

During normal operation, GATE is enhanced 10V above

OUT. The external MOSFET source voltage is monitored

through a resistor-divider between OUT and SET. When

OUT rises above the adjusted overvoltage threshold, an

internal comparator sinks the charge-pump current, dis-

charging the external GATE, regulating OUT at the set

overvoltage threshold. OUT remains active during the

overvoltage transients and the MOSFET continues to con-

duct during the overvoltage event, operating in switched-

linear mode.

Overvoltage Protection Switch/Limiter

Controllers Operate Up to 72V

_______________________________________________________________________________________

9

10V/div

10V/div

10V/div

10ms/div

Figure 2a. MAX6397/MAX6398 GATE Waveform During Over-

voltage Switch Mode

10V/div

10V/div

10V/div

4ms/div

Figure 2b. MAX6397/MAX6398 GATE Waveform During Over-

voltage Limit Mode

MAX6397

MAX6398

IN

SET

GATE

OUT

GND

R1

R2

Figure 3. Overvoltage Switch Protection Configuration