RT9168/A
To ensure proper operation, the signal source used to
drive the SHDN input must be able to swing above and
below the specified turn-on/turn-off voltage thresholds
which guarantee an ON or OFF state (see Electrical
Characteristics). The ON/OFF signal may come from
either CMOS output, or an open-collector output with
pull-up resistor to the RT9168/A input voltage or
another logic supply. The high-level voltage may
exceed the RT9168/A input voltage, but must remain
within the absolute maximum ratings for the SHDN pin.
Internal P-Channel Pass Transistor
The RT9168/A features a typical 1.1Ω P-channel
MOSFET
pass
transistor.
It
provides
several
Fig. 2
advantages over similar designs using PNP pass
transistors, including longer battery life. The P-channel
MOSFET requires no base drive, which reduces
quiescent current considerably. PNP-based regulators
waste considerable current in dropout when the pass
transistor saturates. They also use high base-drive
currents under large loads. The RT9168/A does not
suffer from these problems and consume only 80µA of
quiescent current whether in dropout, light-load, or
heavy-load applications.
Input-Output (Dropout) Voltage
A
regulator’s
minimum
input-output
voltage
differential (or dropout voltage) determines the
lowest usable supply voltage. In battery-powered
systems, this will determine the useful end-of-life
battery voltage. Because the RT9168/A uses a P-
channel MOSFET pass transistor, the dropout
voltage is a function of drain-to-source on-resistance
[R
DS(ON)]
multiplied by the load current.
Reverse Current Path
The power transistor used in the RT9168/A has an
inherent diode connected between the regulator
input and output (see Fig.2). If the output is forced
above the input by more than a diode-drop, this
diode will become forward biased and current will
flow from the V
OUT
terminal to V
IN
. This diode will
also be turned on by abruptly stepping the input
voltage to a value below the output voltage. To
www.richtek-ic.com.tw
V
IN
V
OUT
V
IN
V
OUT
prevent regulator mis-operation, a Schottky diode
should
be
used
in
any
applications
where
input/output voltage conditions can cause the
internal diode to be turned on (see Fig.3). As shown,
the Schottky diode is connected in parallel with the
internal parasitic diode and prevents it from being
turned on by limiting the voltage drop across it to
about 0.3V. < 100 mA to prevent damage to the part.
Fig. 3
Current Limit and Thermal Protection
The RT9168 includes a current limit which monitors
and controls the pass transistor’s gate voltage limiting
the output current to 300mA Typ. (700mA Typ. for
RT9168A). Thermal-overload protection limits total
power dissipation in the RT9168/A. When the junction
temperature exceeds T
J
= +155°C, the thermal sensor
signals the shutdown logic turning off the pass
transistor and allowing the IC to cool. The thermal
sensor will turn the pass transistor on again after the
IC’s junction temperature cools by 10°C, resulting in a
pulsed output during continuous thermal-overload
conditions. Thermal-overloaded protection is designed
to protect the RT9168/A in the event of fault conditions.
Do not exceed the absolute maximum junction-
temperature rating of T
J
= +150°C for continuous
operation. The output can be shorted to ground for an
DS9168/A-03 May 2001
8
ETC [ ETC ]
