RT1720 Datasheet, PDF (14/19 Pages) Richtek Technology Corporation – 60V Hot Swap Controller with Fault Timer

RT1720
F^̅̅ÌL^̅ÌT^Ì Output
The RT1720 includes an open-drain fault output that
indicates the state of the TMR pin voltage. Typically,
F^̅̅ÌL^̅ÌT^Ì is externally pulled up to some positive voltage
(such as VIN or a system logic supply) through a
resistor such as 100k.
When an over-voltage or over-current condition occurs,
the TMR pin begins charging CTMR. When VTMR is less
than the fault threshold (1.2V typical) the F^̅̅ÌL^̅ÌT^Ì output
remains unconnected allowing the external resistor to
pull it high. When VTMR exceeds the threshold, the
F^̅̅ÌL^̅ÌT^Ì output is internally pulled to GND, signaling that
VTMR is nearing the GATE latch threshold (1.4V typical)
where the external MOSFET is turned completely off.
When GATE turns off or the fault ends, the F^̅̅ÌL^̅ÌT^Ì output
remains low while GATE is off, until VTMR reaches the
GATE unlatch threshold (0.5V typical) and GATE turns
the external MOSFET back on. The F^̅̅ÌL^̅ÌT^Ì output rises
when TMR reaches the threshold, indicating that GATE
is beginning to turn on.
PGOOD Output
The RT1720’s includes an open-drain PGOOD output.
The PGOOD output’s state relates to the VOUT voltage
relative to VIN, rather than the absolute level of VOUT.
Since it is open drain, PGOOD only shows a voltage
level if it is externally pulled up to some positive voltage
(such as VIN, VOUT, or a system logic supply) through
a resistor such as 100k.
When GATE begins charging and VOUT rises toward
VIN, PGOOD rises when VOUT is within the VOUT
high threshold (VIN-1V typical). Once high, PGOOD
stays high even if GATE turns off and VOUT falls, until
VOUT discharges to about 2V (typical). Once low,
PGOOD only rises again when GATE turns on and
VOUT again approaches VIN. PGOOD is always high
when SHDN is low and the IC is in its low-power
shutdown state, unless PGOOD is pulled up to a
voltage (like VOUT) that turns off in shutdown. PGOOD
is not designed as a traditional power-good indicator. A
traditional power-good indicator usually has a fixed
threshold voltage and indicates if VOUT is above or
below that threshold. The RT1720’s PGOOD output is
intended to indicate to downstream load devices that
GATE has fully turned on the external MOSFET load
switch and full output current is available. Enabling the
load before GATE has fully turned on is poor practice
because the load current causes high dissipation in a
partially-enhanced MOSFET. Also, drawing a large
load current through the partially-on MOSFET might
cause the output voltage to collapse, possibly leaving
system components in an unreliable logic state.
PGOOD goes high (open-circuit) only when VOUT is
reaching VIN and the MOSFET switch is nearly fully on.
On MOSFET turn off (if SHDN falls or if there is a
voltage or current fault) PGOOD stays high, allowing
the load to operate as long as possible, until VOUT falls
to about 2V (typical). Once PGOOD falls, it only rises
again if the output voltage nears VIN.
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DS1720-00 November 2014