Datasheet

ADP5037 Data Sheet
Rev. D | Page 18 of 28
100% Duty Operation
With a drop in input voltage, or with an increase in load current,
the buck may reach a limit where, even with the pFET switch
on 100% of the time, the output voltage drops below the desired
output voltage. At this limit, the buck transitions to a mode where
the pFET switch stays on 100% of the time. When the input
conditions change again and the required duty cycle falls, the
buck immediately restarts PWM regulation without allowing
overshoot on the output voltage.
Active Pull-Downs
All regulators have optional, factory programmable, active pull-
down resistors discharging the respective output capacitors
when the regulators are disabled. The pull-down resistors are
connected between VOUTx and AGND. Active pull-downs are
disabled when the regulators are turned on. The typical value of
the pull-down resistor is 600 Ω for the LDOs and 75 Ω for the
bucks. Figure 46 shows the activation timings for the active
pull-downs during regulator activation and deactivation.
LDO1 AND LDO2
The ADP5037 contains two LDOs with low quiescent current
and low dropout voltage, and provides up to 300 mA of output
current. Drawing a low 10 μA quiescent current (typical) at no load
makes the LDO ideal for battery-operated portable equipment.
Each LDO operates with an input voltage of 1.7 V to 5.5 V. The
wide operating range makes these LDOs suitable for cascading
configurations where the LDO supply voltage is provided from
one of the buck regulators.
Each LDO output voltage is set through external resistor dividers
as shown in Figure 48 for LDO1. The output voltage can optionally
be factory programmed to default values as indicated in the
Ordering Guide section. In this event, Ra and Rb are not needed,
and FB3 must be connected to the top of the capacitor on
VOUT3. FB3 is 0.5 V.
LDO1
FB3
Ra
Rb
VOUT3
VOUT3
VIN3
C7
1µF
V
OUT3
= V
FB3
+ 1
Ra
Rb
09887-009
Figure 48. LDO1 External Output Voltage Setting
The LDOs also provide high power supply rejection ratio (PSRR),
low output noise, and excellent line and load transient response
with only a small 1 µF ceramic input and output capacitor.
LDO1 is optimized to supply analog circuits because it offers
better noise performance compared to LDO2. LDO1 should be
used in applications where noise performance is critical.