Overvoltage Protection Controllers

with Reverse Polarity Protection

6

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Detailed Description

The MAX4864L/MAX4865L/MAX4866L/MAX4867 provide

up to +28V overvoltage and negative voltage protection

for low voltage systems. When the input voltage exceeds

the overvoltage trip level, the MAX4864L/MAX4865L/

MAX4866L/MAX4867 turn off a low-cost external n-channel

MOSFET to prevent damage to the protected compo-

nents. The devices also drive an external p-channel

MOSFET to protect against negative voltage inputs. An

internal charge-pump (see the Functional Diagram),

drives the MOSFET GATEN for a simple, robust solution.

On power-up, the device waits for 50ms before driving

GATEN high. The open-drain FLAG output is kept at a

high impedance for an additional 50ms after GATEN goes

high before deasserting. The FLAG output asserts high

immediately to an overvoltage fault.

Undervoltage Lockout (UVLO)

The MAX4864L/MAX4865L/MAX4866L/MAX4867 have

a fixed +2.85V typical UVLO level, and the MAX4867

the GATEN driver is held low and FLAG is asserted.

Overvoltage Lockout (OVLO)

The MAX4864L has a +7.4V typical OVLO threshold;

the MAX4865L has +6.35V typical OVLO threshold; the

MAX4866L has a +5.8V typical OVLO threshold; and

the MAX4867 has a +4.65V typical OVLO threshold.

held low and FLAG is asserted.

FLAG Output

The open-drain FLAG output is used to signal to the

host system when there is a fault with the input voltage.

On power-up, FLAG is held high for 50ms after GATEN

turns on, before deasserting. FLAG asserts immediately

to overvoltage and undervoltage faults. When the fault

condition is removed, FLAG deasserts 50ms after

GATEN turns on. Connect a pullup resistor from FLAG

to the logic I/O voltage of the host system.

GATEN Driver

An on-chip charge pump is used to drive GATEN

above IN, allowing the use of a low-cost n-channel

MOSFET. The charge pump operates from the internal

+5.5V regulator.

The actual GATEN output voltage tracks approximately

level is exceeded, whichever comes first. The

MAX4864L has a +7.4V typical OVLO, therefore GATEN

remains relatively constant at approximately +10.5V for

cal OVLO, but this can be as low as +5.5V. The GATEN

output voltage is a function of input voltage, as shown

in the Typical Operating Characteristics.

GATEP Driver

When the input voltage drops below ground, GATEP

goes high turning the external p-channel MOSFET off.

When the input voltage goes above ground, GATEP

pulls low and turns on the p-channel MOSFET. An inter-

nal clamp protects the p-channel MOSFET by insuring

that the GATEP-to-IN voltage does not exceed +16V

when the input (IN) rises to +28V.

Device Operation

The MAX4864L/MAX4865L/MAX4866L/MAX4867 have

an on-board state machine to control device operation.

FLAG is high.

50ms internal delay. The internal charge pump is

the internal charge pump. FLAG is held high during

startup until the FLAG blanking period expires, typically

50ms after the GATEN starts going high. At this point,

the device is in its on-state.

high and GATEN is driven to ground.

STANDBY

GATEN = 0

FLAG = HIGH

TIMER STARTS

COUNTING

OVLO CHECK

GATEN = 0

FLAG = HIGH

STARTUP

GATEN DRIVEN HIGH

FLAG = HIGH

ON

GATEN HIGH

FLAG = LOW

t = 50ms

t = 50ms

Figure 5. State Diagram