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AS1331

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

8 Detailed Description

The synchronous buck-boost converter AS1331 uses a Power Save Mode control technique to reach a high efficiency

over a wide dynamic range of load currents. The output voltage is monitored by a comparator with 3% accuracy. The

Power Save Mode puts the device into “sleep mode” when VOUT is above its programmed reference threshold.

Meaning, the switching is stopped and only quiescent current is drawn from the power source. The switching is started

again when VOUT drops below the reference threshold and the output capacitor is charged again.

The numbers of current pulses which are necessary to load the output capacitor are set by the value of the output

capacitor, the load current, and the comparator hysteresis (~1%).

Figure 23. AS1331 - Block Diagram - Fixed Output Voltage

Modes of Operation

When VOUT drops below the reference threshold, the AS1331 switches on the transistors SW A and SW C until the

inductor current reaches approximately 400mA. In the next step SW A and SW D are closed and depending on the

difference between VIN and VOUT the inductor current raises, falls or stays constant.

VIN > VOUT: The inductor current is going up to 600mA.

VIN ~ VOUT: The device stops after 2µs.

VIN < VOUT: The inductor current falls down to 0mA.

If the inductor current is not 0mA, the transistors SW B and SW D are closed to ramp down the current to zero. If VOUT

is still below the threshold voltage the next cycle is started. If IMAX (600mA) wasn’t reached in the previous cycle, SW A

and SW D are closed until the inductor current is 600mA.

Note:

The 4-switch-mode (SW A+SW C => SW B+SW D => SW A + SW C...) and also the buck-mode (SW A+SW D

=> SW B+SW D => SW A+SW D...) are never used.

Start-Up Mode

At start-up the switch SW D is disabled and its diode is used to transfer current to the output capacitor until VOUT

reaches approximately 2.15V. The inductor current is controlled by an alternate algorithm during start-up.

Note:

Do not apply loads >1mA until VOUT = 2.3V is reached.

VIN

SW1

SW2

SW A

SW B

SW C

SW D

VIN

1.6V

600mA

VOUT

PGND

GND

Peak

Current

Limit

UVLO

State

Machine

and

Logic

VBEST

Gate Drivers

and

Anticross

Conduction

IZERO

Detect

1.25V

Thermal Shutdown

Shutdown

VOUT

VBEST

VOUT

Comp

AS1331

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