ADP5303

Data Sheet

Rev. B | Page 14 of 21

THEORY OF OPERATION

The ADP5303 is a high efficiency, ultralow quiescent current

step-down regulator in a 9-ball WLCSP to meet demanding

performance and board space requirements. The device enables

direct connection to a wide input voltage range of 2.15 V to

6.50 V, allowing the use of multiple alkaline/NiMH or Li-Ion

cells and other power sources.

BUCK REGULATOR OPERATIONAL MODES

PWM Mode

In PWM mode, the buck regulator in the ADP5303 operates at

a fixed frequency set by an internal oscillator. At the start of

each oscillator cycle, the high-side MOSFET switch turns on and

sends a positive voltage across the inductor. The inductor current

increases until the current sense signal exceeds the peak inductor

current threshold, which turns off the high-side MOSFET switch.

This threshold is set by the error amplifier output. During the high-

side MOSFET off time, the inductor current decreases through

the low-side MOSFET until the next oscillator clock pulse starts

a new cycle.

Hysteresis Mode

In hysteresis mode, the buck regulator in the ADP5303 charges

the output voltage slightly higher than its nominal output voltage

with PWM pulses by regulating the constant peak inductor

current. When the output voltage increases until the output

sense signal exceeds the hysteresis upper threshold, the regulator

enters standby mode. In standby mode, the high-side and low-side

MOSFETs and a majority of the circuitry are disabled to allow a

low quiescent current as well as high efficiency performance.

During standby mode, the output capacitor supplies energy into

the load and the output voltage decreases until it falls below the

hysteresis comparator lower threshold. The buck regulator wakes

up and generates the PWM pulses to charge the output again.

Because the output voltage occasionally enters standby mode

and then recovers, the output voltage ripple in hysteresis mode

is larger than the ripple in PWM mode.

Mode Selection

The ADP5303 includes the SYNC/MODE pin to allow flexible

configuration in hysteresis mode or PWM mode.

When a logic high level is applied to the SYNC/MODE pin, the

buck regulator is forced to operate in PWM mode. In PWM mode,

the regulator can supply up to 500 mA of output current. The

regulator can provide lower output ripple and output noise in

PWM mode, which is useful for noise sensitive applications.

When a logic low level is applied to the SYNC/MODE pin, the buck

regulator is forced to operate in hysteresis mode. In hysteresis mode,

the regulator draws only 240 nA of quiescent current typical to

regulate the output under zero load, which allows the regulator to

act as a keep-alive power supply in a battery-powered system. In

hysteresis mode, the regulator supplies up to 50 mA of output cur-

rent with a relatively large output ripple compared to PWM mode.

The user can alternate between hysteresis mode and PWM mode

during operation. The flexible configuration capability during

operation of the device enables efficient power management to

meet high efficiency and low output ripple requirements when

the system switches between active mode and standby mode.

OSCILLATOR AND SYNCHRONIZATION

The ADP5303 operates at a typical 2 MHz switching frequency

in PWM operation mode.

The switching frequency of the ADP5303 can be synchronized

to an external clock with a frequency range from 1.5 MHz to

2.5 MHz. The ADP5303 automatically detects the presence of

an external clock applied to the SYNC/MODE pin, and the

switching frequency transitions to the frequency of the external

clock. When the external clock signal stops, the device

automatically switches back to the internal clock.

ADJUSTABLE AND FIXED OUTPUT VOLTAGES

The ADP5303 provides adjustable output voltage settings by

connecting one resistor through the VID pin to AGND. The

VID detection circuitry works in the start-up, and the voltage

ID code is sampled and held in the internal register and does

not change until the next power cycle. Furthermore, the ADP5303

provides a fixed output voltage programmed via the factory

fuse. In this condition, connect the VID pin to the PVIN pin.

The feedback resistor divider is built into the ADP5303, and the

feedback pin (FB) must be tied directly to the output. An

ultralow power voltage reference and an integrated high

impedance feedback divider network contribute to the low

quiescent current. Table 5 lists the output voltage options by the

VID pin configurations. A 1% accuracy resistor through VID to

ground is recommended.

VID Configuration

Factory Option 0 (V)

Factory Option 1 (V)

Short to ground

3.0

3.1

Short to PVIN

2.5

1.3

3.6

5.0

3.3

4.5

2.9

4.2

2.8

3.9

2.7

3.4

2.6

3.2

2.4

1.9

2.3

1.7

2.2

1.6

2.1

1.4

2.0

1.1

1.8

1.0

1.5

0.9

1.2

0.8