AS1367
Datasheet - Detailed Description
Power-OK
The AS1367’s power-ok is built around an N-channel MOSFET. The circuitry monitors the voltage on pin SET and if the
voltage goes out of regulation (e.g. during dropout, current limit, or thermal shutdown) the pin POK goes low. If the pin
SET 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 = 2.0V. 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).
Current Limiting
The AS1367 include current limiting circuitry to protect against short-circuit conditions. The circuitry monitors and
controls the gate voltage of the P-channel MOSFET, typically limiting the output current to 180mA. 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 tool to prevent die overheating since the power transistor is the principle heat source in the
device.
If the junction temperature exceeds 160ºC with 20ºC 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 20ºC, 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.
Operating Region and Power Dissipation
Maximum power dissipation is determined by the thermal resistance of the package and circuit board, the temperature
difference between the die junction and the ambient air and the rate of the air flow. The power dissipation of the device
is calculated by:
P = IOUT × (VIN – VOUT
)
(EQ 1)
Maximum power dissipation is calculated by:
TJ – TAMB
------------------------
=
PMAX
(EQ 2)
θJA
Where:
TJ - TAMB is the temperature difference between the device die junction and the surrounding air.
JA is the thermal resistance through the circuit board, copper traces, and other materials to the surrounding.
θ
Note: Pin GND is a multi-function pin providing a connection to the system ground and acting as a heat sink. This pin
should be connected to the system ground using a large pad or a ground plane.
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