BUCK Regulator
The MAX77826 includes a 3A current-mode BUCK regu-
lator. In normal operation, BUCK consumes only 22µA
quiescent current. In low power mode, the quiescent
current is decreased to 8µA with reduced load capability.
The summary of features is:
●3A of maximum output current rating
●2.6V to 5.5V input voltage range
●Output voltage range from 0.50V to 1.80V in 6.25mV
steps
●±1% (typ) output voltage DC accuracy
●2MHz (typ) switching frequency
●Automatic SKIP/PWM or forced PWM modes
●> 90% peak efficiency
●Programmable slew rate for increasing output voltage
settings
Operating Mode Control
The operating mode bit resides in the top level that con-
trols the enable/disable state of BUCK through the B_EN
register and also controls the operating mode (low power
or normal mode) through the B_LPM register.
SKIP/Forced PWM Operation
In normal operating mode, BUCK automatically transitions
from SKIP mode to fixed frequency operation as load cur-
rent increases. For operating modes where lowest output
ripple is required, forced PWM switching behavior can be
enabled by writing 1 to B_FPWM bit.
Low Power Mode Operation
In low power mode, the quiescent current is reduced from
22µA to 8µA. The output current is limited to 10mA. It
is not recommended to adjust the output voltage in low
power mode. The regulator does not automatically enter/
exit low power mode. The host processor needs to control
low power mode operation in times of known low power
states through the I2C serial interface.
Startup and Soft-Start
When starting up BUCK regulator, the bias circuitry must
be enabled and provided with adequate time to settle. The
bias circuitry is guaranteed to settle within 250µs, at which
time, the BUCK regulators’ power-up sequences can
commence. Note that attempting to implement a power-
up sequence before BIASOK signal is generated results
in all enabled regulators starting up at the same time.
The BUCK regulator supports starting into a prebiased
output. For example, if the output capacitor has an ini-
tial voltage of 0.4V when the regulator is enabled, the
regulator gradually increases the capacitor voltage to the
required target voltage such as 1.0V. This is unlike other
regulators without the start into prebias feature in which
they can force the output capacitor voltage to 0V before
the soft-start ramp begins.
The BUCK regulator has a soft-start rate of 14mV/µs. The
controlled soft-start rate and BUCK regulator current limit
(ILIMP) limit the input inrush current to the output capaci-
tor (IINRUSH). IINRUSH = min (ILIMP and COUT x dv/dt).
Note that the input current of BUCK regulator is lower
than the inrush current to the output capacitor by the ratio
of output to input voltage.
Output Voltage Setting
The output voltage is programmable from 0.50V to 1.80V
in 6.25mV steps to allow fine adjustment to the processor
supply voltage under light load conditions to minimize
power loss within the processor. The default output volt-
age is set by an OTP option at the factory. The default out-
put voltage can be overwritten by changing the contents
in B_VOUT[7:0] register prior to enabling the regulator.
The output voltage can also be adjusted during normal
operation.
Changing Output Voltage While Operating
In a typical smartphone or tablet application, there are
several power domains in which the operating frequency
of the processor increases or decreases (DVFS). When
the operating frequency needs to be changed, it is
expected that BUCK regulator responds to a command to
change the output voltages to new target values quickly.
The high peak current limit, coupled with low inductance
and small output capacitance, allows the BUCK regulator
to respond to a positive step change in output voltage and
settle to the new target value quickly. The BUCK regulator
provides programmable ramp-up slew rates to accommo-
date different requirements.
For a negative step change in output voltage, the settling
time is not critical. In forced PWM mode (either B_FPWM
bit or B_FSRAD bit is enabled), the negative inductor
current through NMOS discharges energy from the out-
put capacitor to help the output voltage decrease to the
new target value faster. In skip mode, negative inductor
current is not allowed so that the output voltage settling
time is dependent on the load current and the output
capacitance.
MAX77826 Power Management IC
www.maximintegrated.com Maxim Integrated
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