AS1371
Datasheet - Detailed Description
Power-OK and Low-Battery-Detect Functionality
The AS1371’s power-ok or low-battery-detect circuitry is built around an N-channel MOSFET. The circuitry monitors
the voltage on pin SENSE and if the voltage goes out of regulation (e.g. during dropout, current limit or thermal
shutdown) the pin POK goes low. The pin SENSE can be connected to a resistive-divider to monitor a particular
definable voltage and compare it with an internal voltage reference. If the SENSE pin is connected to GND an internal
resistive-divider is activated and connected to the output. Therefore, the Power-OK functionality can be realised with
no additional external components.
The Power-OK feature is not active during shutdown and provides a power-on-reset function that can operate down to
VIN = 1.2V. A capacitor to GND may be added to generate a power-on-reset delay. To obtain a logic-level output,
connect a pull-up resistor from pin POK to pin OUT. Larger values for this resistor will help to minimize current
consumption; a 100kΩ resistor is perfect for most applications (see Figure 1 on page 1).
For the circuit shown in the left of Figure 14 on page 11, the input bias current into SENSE is very low, permitting large-
value resistor-divider networks while maintaining accuracy. Place the resistor-divider network as close to the device as
possible. Use a defined resistor for R2 and then calculate R1 as:
VIN
⎛
⎝
⎞
– 1
------------------
R1 = R2 ×
(EQ 1)
⎠
VSENSE
Where:
VSENSE .... Is the internal sense reference voltage. For values see Table 3 on page 4.
R2 .... Is the predefined resistor in the resistor divider.
In case of the SENSE pin is connected to GND, an internal resistor-divider network is activated and compares the out-
put voltage with a 94% (typ.) voltage threshold. For this particular Power-OK application, no external resistive compo-
nents are necessary.
Current Limiting
The AS1371 include current limiting circuitry to protect against short-circuit conditions. The circuitry monitors and
controls the gate voltage of the P-channel MOSFET, limiting the output current to 400mA. The P-channel MOSFET
output can be shorted to ground for an indefinite period of time without damaging the device.
Thermal-Overload Protection
The devices are protected against thermal runaway conditions by the integrated thermal sensor circuitry. Thermal
shutdown is an effective instrument to prevent die overheating since the power transistor is the principle heat source in
the device.
If the junction temperature exceeds 150ºC with 15ºC hysteresis, the thermal sensor starts the shutdown logic, at which
point the P-channel MOSFET is switched off. After the device temperature has dropped by approximately 15ºC, the
thermal sensor will turn the P-channel MOSFET on again. Note that this will be exhibited as a pulsed output under
continuous thermal-overload conditions.
Note: The absolute maximum junction-temperature of +150ºC should not be exceeding during continual operation.
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