NIS5102

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11

300 k

OVLO

200 k

Ground

Source

Figure 19. Equivalent Overvoltage Lockout Circuit

2 M

7 V

The theoretical equation for the OVLO turn−on voltage is:

ROVLO (KW) +

300 Vin * 2100

21.8

* Vin

The OVLO trip point voltage calculated through the

theoretical formula may show small variations with respect

to Figure 4, therefore it is recommended to use the formulas

gotten from the OVLO characterization, which are shown

below:

ROVLO (kW) + e [(OVLO ) 5.2) 3.46]; for TJ + 25°C

where “OVLO” is the desired overvoltage lockout value,

To reduce nuisance tripping due to transients and noise

spikes, a capacitor may be added from the OVLO pin to

ground. This will create a low pass filter with a cutoff

frequency of f. The required capacitance on this pin is:

COVLO +

[1

) (8.83 E−6 ·ROVLO)]

2

p ·f ·ROVLO

This pin is also used as a common shutdown pin. In this

mode, if this pin is pulled to ground, it will shutdown the chip

and all chips connected to its OVLO pin.

The OVLO pin has an internal switch to ground that will

pull it low, whenever the device is disabled due to any fault

other than an Overvoltage condition. An enable pin

shutdown is not considered a fault and will not cause a

common shutdown. This feature allows multiple units to

turn on and off simultaneously by tying the OVLO pins

together in parallel. This can be used for operating several

hot plug devices in parallel, or for use with separate loads,

when

all

devices

need

to

startup

and

shutdown

simultaneously.

Temperature Limit

The temperature limit circuit senses the temperature of the

Power FET and removes the gate drive if the maximum level

is exceeded. For the auto−retry device, there is a nominal

hysteresis of 40

°C for this circuit. After a thermal shutdown,

the device will automatically restart when the temperature

drops to a safe level as determined by the hysteresis. The

latching thermal circuit can be reset either by recycling the

input power, or by toggling the enable signal.

Current Limit

An external resistor from the current limit pin to the source

pins set the level at which the device will limit the current.

The plot of resistance vs. current limit includes two curves,

one for short circuit and one for overload.

A short circuit condition is one in which the SENSEFET

is not fully enhanced, and is therefore in a high impedance

mode of operation. In this case there are many hundreds of

millivolts across the drain to source pins of the SENSEFET.

This occurs when the output sees a very low impedance short

as well as when the capacitor is charging at turn on. In both

cases there are several volts or more across the FET.

An overload condition is one in which the SENSEFET is

still fully enhanced and the drain to source voltage is the

product of the drain current and the on resistance of the FET.

The sense voltage out of the SENSEFET has a different

relation to the drain current in these two conditions. The

difference in current limit levels for these two cases is called

DI, where:

DI + Vref RDSon