ISL6141 Intersil Corporation, ISL6141 Datasheet - Page 13

• Manufacturer Part Number: ISL6141
• Description: Negative Voltage Hot Plug Controller
• Manufacturer: Intersil Corporation

In addition to the above current limit and 600 s time-out,
there is a Hard Fault comparator to respond to short circuits
with an immediate GATE shutdown (typically 10 s) and a
single retry. The trip point of this comparator is set ~4 times
(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 600 s 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 for 600 s before latching the GATE pin low.
Note: Since the 600 s 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 27 illustrates
the Hard Fault response with a zero ohm short circuit at the
output.
As in the Over-Current response discussed previously the
supply is set at -48V and the current limit is set at 2.5A. After
the initial gate shutdown (10 s) 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. The
fault remains for the duration of the time-out period and the
GATE pin is quickly pulled low and latched off requiring a
UVLO or UV reset to resume normal operation (assuming
the fault has gone away).
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. Each pin is tied to an
FIGURE 27. HARD FAULT SHUTDOWN AND RETRY
13
ISL6141, ISL6151
internal comparator with a nominal reference of 1.255V. A
resistor divider between the V
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
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
transients need to be filtered. (Note that even some
transients which will 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
V
Similarly, for OV:
1.255 = V
V
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 draws 100 A), and then the other two will be in the
10k range. Solve the two equations for two unknowns. Note
that some iteration may be necessary to select values that
UV
OV
= 1.255 (R4 + R5 + R6)/(R5 + R6)
= 1.255 (R4 + R5 + R6)/(R6)
UV
OV
EE
* (R5 + R6)/(R4 + R5 + R6) or
* (R6)/(R4 + R5 + R6) or
or OV to V
EE
DD
is a possibility, if certain
(gnd) and V
EE
is typically