Overvoltage-Protection Controllers with

Status

FLAG

6

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Detailed Description

The MAX4838A/MAX4840A/MAX4842A provide up to

+28V overvoltage protection for low-voltage systems.

When the input voltage exceeds the overvoltage trip level,

the MAX4838A/MAX4840A/MAX4842A turn off a low-cost

external n-channel FET(s) to prevent damage to the pro-

tected components. An internal charge pump (Figure 5)

drives the FET gate for a simple, robust solution.

Undervoltage Lockout (UVLO)

The MAX4838A/MAX4840A have a fixed 3.25V typical

undervoltage-lockout level (UVLO) while the MAX4842A

UVLO, the GATE driver is held low and FLAG is asserted.

Overvoltage Lockout (OVLO)

The MAX4838A has a 7.4V typical overvoltage threshold

(OVLO), and the MAX4840A has a 5.8V typical overvolt-

age threshold. The MAX4842A has a 4.7V typical over-

GATE driver is held low and FLAG is asserted.

FLAG Output

The FLAG output is used to signal the host system

there is a fault with the input voltage. FLAG asserts

immediately to an overvoltage fault. FLAG is held low

for 50ms after GATE turns on before deasserting.

All devices have an open-drain FLAG output. Connect

a pullup resistor from FLAG to the logic I/O voltage of

the host system.

EN Enable Input

EN is an active-low enable input. Drive EN low or con-

nect to ground to enable normal device operation.

Drive EN high to force the external MOSFET(s) off. EN

does not override an OVLO or UVLO fault.

GATE Driver

An on-chip charge pump is used to drive GATE above

IN, allowing the use of low-cost n-channel MOSFETS.

The charge pump operates from the internal 5.5V

regulator.

The actual GATE output voltage tracks approximately

level is exceeded, whichever comes first. The

MAX4838A has a 7.4V typical OVLO; therefore GATE

remains relatively constant at approximately 10.5V for

OVLO, but this can be as low as 5.5V. The MAX4840A

in practice may never actually achieve the full 10.5V

GATE output. The MAX4842A has a 4.7V (typ) OVLO,

and the GATE output voltage is 2x the input voltage.

The GATE output voltage as a function of input voltage

is shown in the Typical Operating Characteristics.

Device Operation

The MAX4838A/MAX4840A/MAX4842A have an on-

board state machine to control device operation. A

flowchart is shown in Figure 6. On initial power-up, if

FLAG is low.

startup after a 50ms internal delay. The internal charge

pump is enabled, and GATE begins to be driven above

ing startup until the FLAG blanking period expires, typi-

cally 50ms after the GATE starts going high. At this

point the device is in its on state.

low and GATE is driven to ground.

IN

GATE

GND

EN

5.5V

REGULATOR

2x CHARGE

PUMP

GATE DRIVER

CONTROL

LOGIC AND TIMER

UVLO AND

OVLO

DETECTOR

FLAG

MAX4838A

MAX4840A

MAX4842A

Figure 5. Functional Diagram