ISL6142CB datasheet(18/23 Pages) INTERSIL

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(210mV) higher than the Over-Current threshold of 50mV. If

the hard fault comparator trip point is exceeded, a hard pull

down current (350mA) is enabled to quickly pull down the

GATE and momentarily turn off the FET. The fast shutdown

resets the timer and is followed by a soft start, single retry

event. If the fault is still present after the GATE is slowly

turned on, the current limit regulator will trip (sense pin

voltage > 50mV), turn on the timer, and limit the current to

50mV/Rsense. If the fault remains and the time-out period is

exceeded the GATE pin will be latched low. Note: Since the

timer starts when the SENSE pin exceeds the 50mV

threshold, then depending on the speed of the current

transient exceeding 200mV; it’s possible that the current limit

time-out and shutdown can occur before the hard fault

comparator trips (and thus no retry). Figure 33 illustrates the

hard fault response with a zero ohm short circuit at the output.

As in the Over-Current Time-Out response discussed

previously, the supply is set at -48V and the current limit is

set at 2.5A. After the initial gate shutdown (10

µs) a soft start

is initiated with the short circuit still present. As the GATE

slowly turns on the current ramps up and exceeds the Over-

Current threshold (50mV) enabling the timer and current

limiting (2.5A). The fault remains for the duration of the time-

out period and the GATE pin is quickly pulled low and

latched off.

Applications: OV and UV

The UV and OV pins can be used to detect Over-Voltage and

Under-Voltage conditions on the input supply and quickly

shut down the external FET to protect the system. Each pin

is tied to an internal comparator with a nominal reference of

1.255V. A resistor divider between the VDD (gnd) and -VIN is

typically used to set the trip points on the UV and OV pins. If

the voltage on the UV pin is above its threshold and the

voltage on the OV pin is below its threshold, the supply is

within its expected operating range and the GATE will be

allowed to turn on, or remain on. If the UV pin voltage drops

below its high to low threshold, or the OV pin voltage

increases above its low to high threshold, the GATE pin will

be pulled low, turning off the FET until the supply is back

within tolerance.

The OV and UV inputs are high impedance, so the value of

the external resistor divider is not critical with respect to input

current. Therefore, the next consideration is total current; the

resistors will always draw current, equal to the supply

voltage divided by the total resistance of the divider

(R4+R5+R6) so the values should be chosen high enough to

get an acceptable current. However, to the extent that the

noise on the power supply can be transmitted to the pins, the

resistor values might be chosen to be lower. A filter capacitor

from UV to -VIN or OV to -VIN is a possibility, if certain

transients need to be filtered. (Note that even some

transients which could momentarily shut off the GATE might

recover fast enough such that the GATE or the output current

does not even see the interruption).

Finally, take into account whether the resistor values are

readily available, or need to be custom ordered. Tolerances

of 1% are recommended for accuracy. Note that for a typical

48V system (with a 43V to 72V range), the 43V or 72V is

being divided down to 1.255V, a significant scaling factor. For

UV, the ratio is roughly 35 times; every 3mV change on the

UV pin represents roughly 0.1V change of power supply

voltage. Conversely, an error of 3mV (due to the resistors, for

example) results in an error of 0.1V for the supply trip point.

The OV ratio is around 60. So the accuracy of the resistors

comes into play.

The hysteresis of the comparators is also multiplied by the

scale factor of 35 for the UV pin (35 * 135mV = 4.7V of

hysteresis at the power supply) and 60 for the OV pin (60 *

25mV = 1.5V of hysteresis at the power supply).

With the three resistors, the UV equation is based on the

simple resistor divider:

1.255 = VUV [(R5 + R6)/(R4 + R5 + R6)] or

VUV = 1.255 [(R4 + R5 + R6)/(R5 + R6)]

Similarly, for OV:

1.255 = VOV [(R6)/(R4 + R5 + R6)] or

VOV = 1.255 [(R4 + R5 + R6)/(R6)]

Note that there are two equations, but 3 unknowns. Because

of the scale factor, R4 has to be much bigger than the other

two; chose its value first, to set the current (for example, 50V /

500k

Ω draws 100µA), and then the other two will be in the

10k

Ω range. Solve the two equations for two unknowns. Note

that some iteration may be necessary to select values that

meet the requirement, and are also readily available standard

values.

The three resistor divider (R4, R5, R6) is the recommended

approach for most applications, but if acceptable values can’t

FIGURE 33. HARD FAULT SHUTDOWN AND RETRY

ISL6142, ISL6152

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