11

FN8150.0

March 15, 2005

Figure 9. Overvoltage/Undervoltage for Primary and

Battery Backup

Overcurrent Protection (Circuit Breaker Function)

The X80070 overcurrent circuit provides the following functions:

– Overcurrent shut-down of the power FET and external

power good indicators.

– Noise filtering of the current monitor input.

– Relaxed overcurrent limits for initial board insertion.

– Overcurrent recovery retry operation.

Overcurrent shut-down

SENSE (see Figure 6) generates a voltage internal to the

X80070. When this voltage exceeds 50mV, an overcurrent

condition exists and an internal “circuit breaker” trips, turning

off the gate drive to the external FET. The actual overcurrent

level is dependent on the value of the current sense resistor.

For example a 20m

Ω sense resistor sets the overcurrent level

to 2.5A.

As shown in Figure 10, this overcurrent circuit block contains a

resistor ladder, a comparator, a noise filter and a voltage

reference to monitor for overcurrent conditions.

factory set, by special order, to any setting between 30mV and

100mV.

If an overcurrent condition is detected, the GATE output is shut

down and the power good indicator goes inactive.

Overcurrent during insertion

Insertion is defined as the first plug-in of the board to the

backplane. In this case, the X80070 is initially fully powered off

prior to the hot plug connection to the main supply. This

condition is different from a situation where the main supply has

temporarily failed resulting in a partial recycle of the power. This

second condition will be referred to as a power cycle.

During insertion, the board can experience high levels of

current for short periods of time as power supply capacitors

charge up on the power bus. To prevent the overcurrent sensor

from turning off the FET inadvertently, the X80070 has the

ability to allow more current to flow through the power FET and

the sense resistor for a short period of time until the FET turns

on and the PWRGD signal goes active.

In the X80070, 150mV is allowed across sense resistor during

insertion (7.5A assuming a 20mW resistor). This provides a

mechanism to reduce insertion issues associated with huge

current surges. Insertion currents of 1X, 2X, or 4X are also

available. Please contact Intersil for these factory options.

After the PWRGD signal is asserted, the X80070 switches back

to the normal overcurrent setting.

Hardshort Protection - (Retry)

In the event on an overcurrent or hard short condition, the

X80070 includes a retry circuit. This circuit waits for 100ms,

then attempts to again turn on the FET. If the fault condition still

exists, the FET turns off and the sequence repeats. For

versions X80070 and X80072, this process continues indefinely

until the overcurrent condition does not exist. For the X80071

and X80073, this process repeats five times only, then will keep

the FET off and set the FAR pin active. After FAR is asserted, it

can be cleared using the master reset pin, MR, or cycling the

cleared upon MR assertion.

If an overcurrent condition does not occur on any retry, the gate

pin will proceed to open at the user defined slew rate.

Figure 10. Overcurrent Detection/Short Circuit

Protection.

Overcurrent noise filter

The X80070 has a noise (low pass) filter built into the

overcurrent comparator. The comparator will thus require

the current spikes to exceed the overcurrent limit for more

than 5µs.

Voltage

Voltage

2:1

Mux

Voltage

-48V

BATT-ON

R1

R2

To Gate

To Gate

+

-

+

-

+

-

Reference

Reference

Reference

Control

Control

25K

450K

Overcurrent/

Gate

Short-Circuit

Retry Logic

RETRY

Voltage

5us

noise

38R

3R

+

–

-48V

Overcurrent Event

Reference

Delay

Control

Block

filtering

X80070, X80071, X80072, X80073