Datasheet

ADP1821
Rev. C | Page 11 of 24
COMPENSATION
The control loop is compensated by an external series RC
network from COMP to FB and sometimes requires a series
RC in parallel with the top voltage divider resistor. COMP is
the output of the internal error amplifier.
The internal error amplifier compares the voltage at FB to the
internal 0.6 V reference voltage. The difference between the two
(the feedback voltage error) is amplified by the error amplifier.
To optimize the ADP1821 for stability and transient response
for a given set of external components and input/output voltage
conditions, choose the compensation components carefully. For
more information on choosing the compensation components,
see the
Compensating the Voltage Mode Buck Regulator section.
POWER-GOOD INDICATOR
The ADP1821 features an open-drain, power-good output
(PWGD) that sinks current when the output voltage drops 8.3%
below or 25% above the nominal regulation voltage. Two com-
parators measure the voltage at FB to set these thresholds. The
PWGD comparator directly monitors FB, and the threshold is
fixed at 0.55 V for undervoltage and 0.75 V for overvoltage. The
PWGD output also sinks current if an overtemperature or input
undervoltage condition is detected and is operational with VCC
voltage as low as 1 V.
Use this output as a logical power-good signal by connecting a
pull-up resistor from PWGD to an appropriate supply voltage.
THERMAL SHUTDOWN
The ADP1821 controller does not generate much heat under
normal conditions, even when driving a relatively large MOSFET.
However, the surrounding power components or other circuits
on the same PCB could heat up the PCB to an unsafe operating
temperature. The ADP1821 controller goes into shutdown and
shuts off the gate drivers when its junction temperature reaches
about 145°C. When the junction temperature drops below
about 135°C, the ADP1821 resumes normal operation in a soft
start mode.
SHUTDOWN CONTROL
The ADP1821 dc-to-dc converter features a low power shut-
down mode that reduces quiescent supply current to 1 A. To
shut down the ADP1821, drive
SHDN
low. To turn it on, drive
SHDN
high. For automatic startup, connect
SHDN
to VCC.