ADP2121 datasheet(14/20 Pages) AD

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ADP2121

Rev. B | Page 14 of 20

THEORY OF OPERATION

PWM

COMP

BANDGAP

THERMAL

SHUTDOWN

SOFT START

PGND

AGND

THRESHOLD

DETECT

GND

VOUT

470nH

VBAT

2.3V TO 5.5V

B2 EN

VIN

AGND

VOUT

REFPFM

AGND

EAMP

PFM

COMPARATOR

THRESHOLD

DETECT

A1 MODE

LOGIC

AND

PWM/PFM

CONTROL

PFM

DETECT

SHOOT-

THROUGH

CONTROL

PREF

NREF

PILIM

ZXCOMP

PVIN

VIN

AVIN

2.2µF

X5R

6.3V

PDRIVE

NDRIVE

4.7µF

X5R

6.3V

6MHz

OSCILLATOR

RAMP

V(VIN)

COMPENSATION

OFF

PWM

AUTO

Figure 51. Internal Block Diagram

OVERVIEW

The ADP2121 is a high efficiency, synchronous step-down

dc-to-dc converter that provides up to 600 mA of continuous

output current. It operates from a 2.3 V to 5.5 V input voltage

for the 1.8 V, 1.82 V, 1.85 V, and 1.875 V (typical) fixed-output

voltages, and from a 2.9 V to 5.5 V input voltage for the 2.3 V

(typical) output voltage. The 6 MHz operating frequency

enables the use of tiny external components. The internal

control schemes of the ADP2121 give excellent stability and

transient response. External control for mode selection and

device enable provide power-saving options that are aided by

internal features such as synchronous rectification and compen-

sation. Other internal features, such as cycle-by-cycle peak current

limit, soft start, undervoltage lockout, output-to-ground short-

circuit protection, and thermal shutdown, protect the internal

and external circuit components.

MODE SELECTION

The ADP2121 has two modes of operation (PWM mode and

auto mode), determined by the state of the MODE pin.

Pull the MODE pin high to force the converter to operate in

PWM mode regardless of the output current. Otherwise, set

MODE low to allow the converter to automatically enter the

power-saving PFM mode at light load currents. Do not leave

this pin floating. The MODE pin is not designed for dynamic

control and should not be changed after the ADP2121 is enabled.

Pulse-Width Modulation (PWM) Mode

The PWM mode forces the part to maintain a fixed frequency

of 6 MHz (typical) over all load conditions. The ADP2121 uses

a hybrid proprietary voltage mode control scheme to control

the duty cycle over load current and line voltage variation. This

control provides excellent stability, transient response, and output

regulation but results in lower efficiencies at light load currents.

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