Datasheet

ADP2121
Rev. B | Page 14 of 20
THEORY OF OPERATION
PWM
COMP
BANDGAP
BG
THERMAL
SHUTDOWN
SOFT START
PGND
AGND
THRESHOLD
DETECT
B1
C2
SW
GND
V
OUT
470nH
A2
V
BAT
2.3V TO 5.5V
B2
EN
V
IN
FB
07407-038
BG
AGND
C1
FB
V
OUT
REF
PFM
AGND
R1
R2
EAMP
PFM
COMPARATOR
THRESHOLD
DETECT
A1
MODE
LOGIC
AND
PWM/PFM
CONTROL
PFM
DETECT
SHOOT-
THROUGH
CONT
ROL
PREF
NREF
PILIM
ZXCOMP
PV
IN
VIN
AV
IN
2.2µF
X5R
6.3V
PDRIVE
NDRIVE
4.7µF
X5R
6.3V
6MHz
OSCILLATOR
RAMP
V(V
IN
)
COMPENSATION
ON
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.