www.austriamicrosystems.com/DC-DC_Step-Down/AS1335

Revision 1.03

10 - 18

AS1335

Datasheet - D e t a i l e d D e s c r i p t i o n

Short-Circuit Protection

This frequency reduction ensures that the inductor current has more time to decay, thus preventing runaway condi-

Dropout Operation

The AS1335 is working with a low input-to-output voltage difference by operating at 100% duty cycle. In this state, the

PMOS is always on. This is particularly useful in battery-powered applications with a 3.3V output.

The AS1335 allows the output to follow the input battery voltage as it drops below the regulation voltage. The quies-

cent current in this state rises minimally to only 400µA (max), which aids in extending battery life. This dropout (100%

duty-cycle) operation achieves long battery life by taking full advantage of the entire battery range.

The input voltage requires maintaining regulation and is a function of the output voltage and the load. The difference

between the minimum input voltage and the output voltage is called the dropout voltage. The dropout voltage is there-

proportional to the load current.

Therefore, power dissipation should be taken in consideration.

Shutdown

Connecting EN to GND or logic low places the AS1335 in shutdown mode and reduces the supply current to 0.1µA. In

shutdown the control circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance discon-

necting the input from the output. The output capacitance and load current determine the voltage decay rate. For nor-

Note: Pin EN should not be left floating.

Power-OK Functionality

The AS1335’s power-ok circuitry offers a 215ms delayed power-ok signal. As long as the output voltage is outside of

the power-ok regulation window the POK pin drives an open-drain low signal. As soon as the output voltage is within

the regulation window, the internal open-drain MOSFET is turned off and the POK pin can be externally pulled to high.

The output of the power-ok signal is delayed by 215ms.

RSI Signal

With the RSI signal the internal power-ok timer can be reseted or delayed. As long as the input to RSI is high the POK

signal remains low, regardless of the output voltage condition.

Thermal Shutdown

Due to its high-efficiency design, the AS1335 will not dissipate much heat in most applications. However, in applica-

tions where the AS1335 is running at high ambient temperature, uses a low supply voltage, and runs with high duty

cycles (such as in dropout) the heat dissipated may exceed the maximum junction temperature of the device.

As soon as the junction temperature reaches approximately 150ºC the AS1335 goes in thermal shutdown. In this mode

the internal PMOS & NMOS switch are turned off. The device will power up again, as soon as the temperature falls

below +125°C again.