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AS1339

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

8 Detailed Description

The AS1339 is designed to dynamically power the PA in WCDMA and NCDMA handsets. The device is empowered

with a high-frequency, high-efficiency step-down converter, and two LDOs. The step-down converters are capable of

delivering 650mA. The PWM control scheme provides fast transient response, while 2MHz switching frequency allows

the trade-off between efficiency and small external components. A 110m

Ω bypass FET connects the PA directly to the

battery during high-power transmission.

Figure 49. Block Diagram

Operating the AS1339

The AS1339’s control block turns on the internal PFET (P-channel MOSFET) switch during the first part of each

switching cycle, thus allowing current to flow from the input through the inductor to the output filter capacitor and load.

During the second part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and

then turns the NFET (N-channel MOSFET) synchronous rectifier on. As a result, the inductor’s magnetic field

collapses, generating a voltage that forces current from ground through the synchronous rectifier to the output filter

capacitor and load.

While the stored energy is transferred back into the circuit and depleted, the inductor current ramps down with a slope

smoothing the voltage across the load. The output voltage is regulated by modulating the PFET switch on-time to

control the average current sent to the load.

The output voltage is equal to the average voltage at the LX pin.

While in operation, the output voltage is regulated by switching at a constant frequency and then modulating the

energy per cycle to control the power to the load. Energy per cycle is set by modulating the PFET switch on-time pulse

width to control the peak inductor current. This is done by comparing the signal from the current-sense amplifier with a

slope compensated error signal from the voltage-feedback error amplifier. At the beginning of each cycle, the clock

turns on the PFET switch, causing the inductor current to ramp up. When the current sense signal ramps past the error

amplifier signal, the PWM comparator turns off the PFET switch and turns on the NFET synchronous rectifier, ending

the first part of the cycle.

If an increase in load pulls the output down, the error amplifier output increases, which allows the inductor current to

ramp higher before the comparator turns off the PFET. This increases the average current sent to the output and

AS1339

LDO2

REF

2MHz BUCK

Control

Logic

LDO1

BASEBAND

PROCESSOR

GPIO

GPIO

GPIO

DAC

+

PA_EN

EN1

EN2

TEST

IN2

REFIN

IN1B

IN1A

Li+ Battery

1µF

LDO2

LDO1

AGND

NC

PGND

LX

PAB

PAA

2.2µH

4.7µF

0.1µF

0.1µF

PFET

NFET

Bypass FET

ROFF

ROFF

2.5x REFIN

2.85V

2.85V

Not

Connected