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AS1326

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

8 Detailed Description

The AS1326A/AS1326B are high-efficiency, low-noise DC-DC boost converters suitable as power supplies for portable

devices. Both devices feature integrated boost switching regulator, N-channel power MOSFET, P-channel synchro-

nous rectifier, precision reference, and shutdown control circuitry (see Figure 1 on page 1).

The AS1326A/AS1326B are able to boost a 1- to 3-cell battery voltage input to a fixed 3.3V output, or adjustable output

between 2.5 and 5.0V (an external Schottky diode is required for output voltages greater than 4V).

The devices are guaranteed to startup with an input voltage as low as 1.1V and remain operational down to an input of

as little as 0.7V, and are optimized for use in mobile phones and other RF applications which have low noise and low

quiescent current (extended battery life) requirements.

The integrated shutdown circuitry reduces device quiescent current down to 0.1µA.

Step-Up Converter

During boost operation, the internal N-channel MOSFET switch turns on for the first part of each cycle, allowing current

to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, the MOSFET

turns off and inductor current flows through the synchronous rectifier to the output filter capacitor and the load. As the

energy stored in the inductor is depleted, the current ramps down and the synchronous rectifier turns off.

At light loads, the device operates at fixed-frequency or only as needed to maintain regulation, depending on the set-

ting of pin CLK/SEL (see Table 6).

Operational Modes

The AS1326A/AS1326B are capable of operating in 3 different modes (see Table 6) as controlled by pin CLK/SEL (see

page 2).

Normal Operation

When CLK/SEL is pulled low, the devices are in normal operating mode. In normal mode the devices operate in PWM

when driving medium-to-heavy loads, and automatically switches to automatic powersave mode if the load requires

less power. The use of automatic powersave mode will boost the efficiency futhermore at light-load conditions.

Forced-PWM Operation

Pulling CLK/SEL high, selects the low-noise PWM-only mode. During forced-PWM operation, the devices switch at a

constant frequency (1MHz) and modulates the MOSFET switch pulse width to control the power transferred per cycle

to regulate the output voltage. Switching harmonics generated by fixed-frequency operation are consistent and can be

filtered. See the Noise Spectrum plot in the Typical Operating Characteristics (see Figure 14 on page 7).

Table 5. Typical Output Voltages and Currents

# of NiCd/NiMh Cells

Input Voltage (V)

Output Voltage (V)

Output Current (mA)

1

1.2

3.3

335

2

2.4

3.3

800

2.4

5.0

450

3

3.6

5.0

800

Table 6. Operational Modes

CLK/SEL Setting

Operational Mode

Description

0

Normal

High-efficiency at all loads; Fixed-frequency (1MHz) at heavy and

medium loads.

1

Forced PWM

External 500kHz to

1.2MHz clock

Synchronized PWM