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Evaluates: MAX6397

MAX6397 Evaluation Kit

Detailed Description

The MAX6397 EV kit demonstrates a high-voltage

overvoltage-protection circuit for applications that must

survive load dump and high-voltage transient conditions.

This EV kit can be configured in overvoltage mode or

overvoltage limiter mode by setting jumper J2 (see Table 1

for the jumper settings), and can supply up to 5A of output

current.

The MAX6397 EV kit has three positive power-supply

inputs: VIN1, VIN2, and VIN3. Inputs VIN2 and VIN3

have diode-based and p-channel MOSFET-based reverse-

battery protections, respectively, and VIN1 bypasses all

reverse-battery protections.

Overvoltage Mode

In overvoltage mode, the MAX6397 monitors the input

voltage and turns off the series-pass n-channel MOSFET

(M1) when the input voltage exceeds the programmed

threshold voltage. As soon as the input voltage drops

below the overvoltage threshold, the MAX6397 charge

pump fully enhances MOSFET M1 to turn the output back

on. The voltage-divider formed by R1 and R2 sets the

threshold voltage. The resistors provided in the MAX6397

EV kit set the threshold at 17V. If inputs VIN2 or VIN3 are

used, this threshold is higher due to the voltage drop in

D1 or M2.

The overvoltage threshold can be adjusted by varying R1

or R2 using the equation below:

OV

V

R1

1 R2

1.215



=

−×





tain threshold accuracy, R2 must be less than 250kΩ.

Since the EV kit ships with R2 set at 49.9kΩ, use the for-

mula above to change the threshold by changing R1 only.

Overvoltage-Limiter Mode

In overvoltage-limiter mode, the MAX6397 monitors

the output voltage instead of the input voltage. The

output voltage is sensed through the same voltage-

divider formed by R1 and R2, so the equation given for

overvoltage mode also applies to the threshold voltage

in overvoltage limiter mode. During an input overvoltage

transient in this mode, the MOSFET switches off until the

output voltage falls to 95% of the threshold voltage, and

then the MOSFET switches back on. This cycle repeats,

generating a sawtooth waveform on the output.

The minimum output voltage in overvoltage-limiter mode

depends on load current, output capacitance, and the

MOSFET’s switching period. The MAX6397 EV kit comes

with one 22μF capacitor at the output to supply the

load during the time when the MOSFET is off. Connect

the optional electrolytic capacitor C13 (150μF, 100V) to

support load currents higher than 0.5A when the EV kit

operates in overvoltage limiter mode.

Add capacitor C3 on the gate of MOSFET M1 to decrease

the frequency of the sawtooth waveform. This process

helps limit the device’s power dissipation.

Linear Regulator Output and Power-Good

Signal

Connections are also included for the linear regulator

output and the power-good (POK) signal. The linear

regulator supplies up to 100mA at 5V, limited by the

ambient temperature, the input/output voltages, and the

package power dissipation. The POK signal has a 100kΩ

resistor (R3) to the regulator output. The linear regulator is

always on regardless of the state of SHDN.

Jumper Selection

To filter fast transients that may be present at the input

from reaching the MAX6397, place a small resistor, R4,

(10Ω, for example) on the board, and cut jumper J1.

Three-pin jumper J2 selects between overvoltage mode

and overvoltage limiter mode; do not leave this jumper

unconnected. Three-pin jumper J3 controls the gate drive

of p-channel MOSFET M3 used as a reverse- battery

protection. Use J3 to disconnect resistor R5 when M3 is

not used to avoid supply leakage through R5. Three-pin

jumper J4 controls the SHDN pin of the MAX6397 and

can enable or disable the MOSFET M1 enhancement.

Table 1 lists the jumper options.

*Default position.

JUMPER

SHUNT POSITION AND FUNCTION

1 and 2

2 and 3

J1

Shorted: RC input filter disabled*

J2

Overvoltage mode*

Overvoltage limiter

mode

J3

M2 gate drive is

disabled*

M2 gate drive is

enabled

J4

U1 is enabled*

U1 is disabled

Table 1. Jumper Function