RT9181
Application Information
EN
The RT9181 is enabled by driving the EN input high, and
shutdown by pulling the input low. If this feature is not to
be used, the EN input should be tied to VIN to keep the
regulator enabled at all times (the EN input
must
not
be
left floating).
Internal P-Channel Pass Transistor
The RT9181 features a P-Channel MOSFET pass
transistors. It provides several 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
RT9181 does not suffer from these problems and
consume only 160μA of quiescent current whether in
dropout, light-load, or heavy-load applications.
Power Good
The power good output is an open-drain output. It is
designed essentially to work as a power-on reset
generator once the regulated voltage was up or a fault
condition. The output of the power good drives low when
a fault condition occurs. The power good output come
back up once the output has reached 90% of its nominal
value and a 2ms (typ.) delay has passed. See Timing
Diagram. The output voltage
level will be drooped at the
fault conditions including current limit, thermal shutdown,
or shutdown and triggers the PGOOD detector to alarm
a fault condition. This output is fed into an on-board delay
circuitry that drives the open drain transistor to indicate
a fault.
Because at shutdown mode, a fault condition occurs by
pulling the PGOOD output low, it will sink a current from
the open drain and the external
power. Selecting a
suitable pulling resistance will be well to control this
dissipated power.
Current Limit and Thermal Protection
The RT9181 includes a current limit structure which
monitor and control the pass transistor's gate voltage
limiting the guaranteed maximum output current to
160mA minimum.
Thermal-overload protection limits total power dissipation
in the RT9181. When the junction temperature exceeds
T
J
= +140°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 20°C,
resulting in a pulsed output during continuous thermal-
overload conditions. Thermal-overloaded protection is
designed to protect the RT9181 in the event of fault
conditions. Do not exceed the recommended maximum
junction-temperature rating of T
J
= +110°C for continuous
operation. The output can be shorted to ground for and
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 RT9181 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 air flow. The power dissipation
across the device is
P = I
OUT
(V
IN
−
V
OUT
)
The GND pin of the RT9181 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.
Capacitor Selection and Regulator Stability
Like any low-dropout regulator, the external capacitors used
with the RT9181 must be carefully selected for regulator
stability and performance. Using a capacitor whose value
is
≥
1μF on the RT9181 input and the amount of capacitance
can be increased without limit. The input capacitor must
be located a distance of not more than 0.5" from the input
pin of the IC and returned to a clean analog ground.
www.richtek.com
8
DS9181-14 March 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
