Datasheet

ADP2125
Rev. A | Page 10 of 16
THEORY OF OPERATION
SHOOT-
THROUGH
CONTROL
LOGIC
AND
PWM
CONTROL
THERMAL
SHUTDOWN
SOFT START
BANDGAP
BG
FB
BG
AGND
R1
FB
R2
V
OUT
AGND
AGND
FB
6MHz
OSCILLATOR
B2
THRESHOLD
DETECT
V
OUT
DISCHARGE
CLK
DETECT
SHORT-CIRCUIT
PROTECTION
COMPENSATION
EAMP
R
DSCHG
110Ω
RAMP
V(V
IN
)
ZXCOMP
PILIM
PDRIVE
PWM
COMP
PV
IN
C
IN
2.2µF
V
IN
2.1V TO 5.5V
AV
IN
NDRIVE
PREF
C
OUT
4.7µF
V
OUT
1.26
V
L
1.5µH
PGND
VIN
GND
SW
AGND
NREF
EXTCLK
ONOFF
C2
B1
A2
C1
GND TO V
IN
A1
NC
08774-006
Figure 20. Internal Block Diagram
OVERVIEW
The ADP2125 is a high efficiency, synchronous, step-down,
dc-to-dc converter that operates from a 2.1 V to 5.5 V input
voltage. It provides up to 500 mA of continuous output current
at a fixed output voltage. The 6 MHz operating frequency
enables the use of tiny external components. The internal
control schemes of the ADP2125 give excellent stability and
transient response. Other internal features such as cycle-by-
cycle peak current limiting, soft start, undervoltage lockout,
output-to-ground short-circuit protection, and thermal
shutdown provide protection for internal and external circuit
components.
EXTERNAL CLOCK (EXTCLK) ENABLE
The ADP2125 is enabled by a 6 MHz to 27 MHz clock signal
applied to the EXTCLK pin (see Figure 2 and Figure 20). The
ADP2125 internally detects the clock signal and allows the
converter to power up and the output voltage to rise to its
nominal value.
The ADP2125 can detect a nonswitching state and disable the
part whether the EXTCLK gates low or high. If the EXTCLK
signal gates low, the part is shut down, reducing the current
consumption to 0.3 µA (typical).
INTERNAL CONTROL FEATURES
Pulse-Width Modulation (PWM)
PWM forces the part to maintain a fixed frequency of 6 MHz
(maximum) over all load conditions. The ADP2125 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.
Synchronous Rectification
In addition to the P-channel MOSFET switch, the ADP2125
includes an N-channel MOSFET switch to build the synchron-
ous rectifier. The synchronous rectifier improves efficiency,
especially for small load currents, and reduces cost and board
space by eliminating the need for an external rectifier.
Soft Start
To prevent excessive input inrush current at startup, the ADP2125
operates with an internal soft start. When EXTCLK begins to
oscillate, or when the part recovers from a fault (UVLO, TSD,
or SCP), a soft start timer begins. During this time, the peak
current limit is gradually increased to its maximum. The output