AS1341 austriamicrosystems AG, AS1341 Datasheet - Page 9

• Manufacturer Part Number: AS1341
• Description: Step-Down Converter
• Manufacturer: austriamicrosystems AG

AS1341
Data Sheet - D e t a i l e d D e s c r i p t i o n
Dropout Voltage
A buck converter’s minimum input-to-output voltage differential (dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this limits the useful end-of-life battery voltage. To maximize battery life, the
AS1341 operates with duty cycles up to 100%, which minimizes the dropout voltage and eliminates switching losses
while in dropout. When the supply voltage approaches the output voltage, the P-channel MOSFET remains on contin-
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uously to supply the load.
Note: Dropout voltage is defined as the difference between the input and output voltages when the input is low
enough for the output to drop out of regulation.
For a step-down converter with 100% duty cycle, dropout is related to the MOSFET drain-to-source on-resistance
(R
) and inductor series resistance (R
), and thus it is proportional to the load current:
DSON
INDUCTOR
V
= I
x (R
+ R
)
(EQ 1)
DROPOUT
OUT
DSON
INDUCTOR
Shutdown
A logic low on pin SHDNN shuts down the AS1341; a logic high on SHDNN powers on the device.
In shutdown mode the supply current drops to 0.8µA to maximize battery life, and the internal P-channel MOSFET
turns off to isolate the output from the input. The output capacitance and load current determine the output voltage
decay rate.
Note: Pin SHDNN should not be left floating. If the shutdown feature is not used, connect SHDNN to IN.
Power-OK Output
The AS1341 provides a Power OK output (POK) that goes high-impedance when the output reaches 92.5% of its reg-
ulation point. POK goes low when the output is below 92.5% of the regulation point and the AS1341 is turned on
(IN ≥ 4.5V and SHDNN ≥ 2.4V). A 12kΩ to 1MΩ pullup resistor between pin POK and pin IN or pin OUT or another volt-
age (≤ IN) can provide a microprocessor logic control signal.
Note: Connect pin POK to GND when the Power-Ok feature is not used.
Thermal-Overload Protection
Integrated thermal-overload protection limits total power dissipation in the AS1341. During continuous thermal-over-
load conditions, when the AS1341 junction temperature exceeds T
= +145ºC, the internal thermal sensor turns off the
J
pass transistor, allowing the AS1341 to cool down. When the AS1341 junction temperature cools by 10ºC, the thermal
sensor turns the pass transistor on again resulting in a pulsed output.
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