Preliminary
RT9177
Current Limit and Thermal Protection
The RT9177 is included a current limit circuit used to
monitor and control the pass transistor’s gate voltage
to limit the output current to 300mA in typical and
V
IN
V
OUT
thermal-overload protection circuit used to limit total
power dissipation.
When the junction temperature
exceeds TJ = 150°C, the thermal sensor signals the
Fig. 3
shutdown logic to turn off the pass transistor and allow
the IC to cool down. The thermal sensor will turn on
the pass transistor again after the IC’s junction
temperature cool down to 120°C. This will cause a
pulsing output during continuous thermal-overload
conditions. Thermal-overloaded protection is designed
to protect the RT9177 in the event of fault conditions.
Do not exceed the absolute maximum junction-
temperature rating of TJ = 125°C for continuous
operation. The output can be shorted to ground for an
V
IN
V
OUT
Fig. 4
indefinite amount of time without damaging the part by
cooperation of current limit and thermal protection.
Operating Region and Power Dissipation
The maximum power dissipation of RT9177 depends
on the thermal resistance of the case and circuit
board, the temperature difference between the die
junction and ambient air, and the rate of airflow. The
power dissipation across the device is P = I
OUT
(V
IN
-
V
OUT
). The maximum power dissipation is: PMAX =
(TJ - TA) /θja
where TJ - TA is the temperature difference between
the
RT9177
die
junction
and
the
surrounding
environment,
θja
is the thermal resistance from the
junction to the surrounding environment. The GND pin
of the RT9177 performs the dual function of providing
an electrical connection to ground and channeling heat
away. Connect the GND pin to ground using a large
pad or ground plane.
DS9177-00 Feb. 2001
www.richtek-ic.com.tw
9
ETC [ ETC ]
