AS1341-BTDT(2007) データシートの表示（PDF） - austriamicrosystems AG

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AS1341-BTDT
(Rev.:2007)

20V, 600mA, 100% Duty Cycle, Step-Down Converter

austriamicrosystems AG 

AS1341

Data Sheet - Detailed Description

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-

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

(RDSON) and inductor series resistance (RINDUCTOR), and thus it is proportional to the load current:

VDROPOUT = IOUT x (RDSON + RINDUCTOR)

(EQ 1)

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 TJ = +145ºC, the internal thermal sensor turns off the

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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