ISL6142 Datasheet, PDF (18/23 Pages) Intersil Corporation – Negative Voltage Hot Plug Controller

ISL6142, ISL6152
(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.
FIGURE 33. HARD FAULT SHUTDOWN AND RETRY
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 (10µs) 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 V_DD (gnd) and -V_IN 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 -V_IN or OV to -V_IN 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 = V_UV [(R5 + R6)/(R4 + R5 + R6)] or
V_UV = 1.255 [(R4 + R5 + R6)/(R5 + R6)]
Similarly, for OV:
1.255 = V_OV [(R6)/(R4 + R5 + R6)] or
V_OV = 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 /
500kΩ draws 100µA), and then the other two will be in the
10kΩ 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
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