ADP5302 Data Sheet
Rev. B | Page 14 of 22
THEORY OF OPERATION
The ADP5302 is a high efficiency, ultralow quiescent current
step-down regulator in a 10-lead LFCSP package, designed to
meet demanding performance and board space requirements.
The device enables direct connection to a wide input voltage range
of 2.15 V to 6.50 V, allowing the use of multiple alkaline/NiMH
or Li-Ion cells and other power sources.
BUCK REGULATOR OPERATIONAL MODES
PWM Mode
In PWM mode, the buck regulator in the ADP5302 operates at
a fixed frequency set by an internal oscillator. At the start of
each oscillator cycle, the high-side MOSFET switch turns on
and sends a positive voltage across the inductor. The inductor
current increases until the current sense signal exceeds the peak
inductor current threshold, which turns off the high-side MOSFET
switch. This threshold is set by the error amplifier output. During
the high-side MOSFET off time, the inductor current decreases
through the low-side MOSFET until the next oscillator clock
pulse starts a new cycle.
Hysteresis Mode
In hysteresis mode, the buck regulator in the ADP5302 charges
the output voltage slightly higher than its nominal output voltage
with PWM pulses by regulating the constant peak inductor current.
When the output voltage increases until the output sense signal
exceeds the hysteresis upper threshold, the regulator enters standby
mode. In standby mode, the high-side and low-side MOSFETs and
a majority of the circuitry are disabled to allow a low quiescent
current, as well as high efficiency performance.
During standby mode, the output capacitor supplies energy into
the load and the output voltage decreases until it falls below the
hysteresis comparator lower threshold. The buck regulator wakes
up and generates the PWM pulses to charge the output again.
Because the output voltage occasionally enters standby mode
and then recovers, the output voltage ripple in hysteresis mode
is larger than the ripple in PWM mode.
Mode Selection
The ADP5302 includes the SYNC/MODE pin to allow flexible
configuration in hysteresis mode or PWM mode.
When a logic high level is applied to the SYNC/MODE pin, the
buck regulator is forced to operate in PWM mode. In PWM mode,
the regulator can supply up to 500 mA of output current. The
regulator can provide lower output ripple and output noise in
PWM mode, which is useful for noise sensitive applications.
When a logic low level is applied to the SYNC/MODE pin, the buck
regulator is forced to operate in hysteresis mode. In hysteresis mode,
the regulator draws only 240 nA of quiescent current (typical) to
regulate the output under zero load, which allows the regulator
to act as a keep-alive power supply in a battery-powered system.
In hysteresis mode, the regulator supplies up to 50 mA of
output current with a relatively large output ripple compared to
PWM mode.
The user can alternate between hysteresis mode and PWM mode
during operation. The flexible configuration capability during
operation of the device enables efficient power management to
meet high efficiency and low output ripple requirements when
the system switches between active mode and standby mode.
OSCILLATOR AND SYNCHRONIZATION
The ADP5302 operates at a typical 2 MHz switching frequency
in PWM operation mode.
The switching frequency of the ADP5302 can be synchronized
to an external clock with a frequency range from 1.5 MHz to
2.5 MHz. The ADP5302 automatically detects the presence of
an external clock applied to the SYNC/MODE pin, and the
switching frequency transitions to the frequency of the external
clock. When the external clock signal stops, the device
automatically switches back to the internal clock.
ADJUSTABLE AND FIXED OUTPUT VOLTAGES
The ADP5302 provides adjustable output voltage settings by
connecting one resistor through the VID pin to AGND. The
VID detection circuitry works in the start-up period, and the
voltage ID code is sampled and held in the internal register and
does not change until the next power recycle. Furthermore, the
ADP5302 provides a fixed output voltage programmed via the
factory fuse. In this condition, connect the VID pin to the
PVIN pin.
For the output voltage settings, the feedback resistor divider is
built into the ADP5302, and the feedback pin (FB) must be tied
directly to the output. An ultralow power voltage reference and
an integrated high impedance feedback divider network contribute
to the low quiescent current. Table 5 lists the output voltage
options by the VID pin configurations. A 1% accuracy resistor
through VID to ground is recommended.
Table 5. Output Voltage (VOUT) Options Using the VID Pin
VID
Configuration
VOUT
Factory Option 0 (V) Factory Option 1 (V)
Short to ground 3.0 3.1
Short to PVIN 2.5 1.3
RVID = 499 kΩ 3.6 5.0
RVID = 316 kΩ 3.3 4.5
RVID = 226 kΩ 2.9 4.2
RVID = 174 kΩ 2.8 3.9
RVID = 127 kΩ 2.7 3.4
RVID = 97.6 kΩ 2.6 3.2
RVID = 76.8 kΩ 2.4 1.9
RVID = 56.2 kΩ 2.3 1.7
RVID = 43 kΩ 2.2 1.6
RVID = 32.4 kΩ 2.1 1.4
RVID = 25.5 kΩ 2.0 1.1
RVID = 19.6 kΩ 1.8 1.0
RVID = 15 kΩ 1.5 0.9
RVID = 11.8 kΩ 1.2 0.8