MAX15023
Wide 4.5V to 28V Input, Dual-Output
Synchronous Buck Controller
12
Maxim Integrated
Detailed Description
The MAX15023 dual, synchronous, step-down con-
troller operates from a 5.5V to 28V or 5V ±10% input
voltage range and generates two independent output
voltages. As long as the controller’s input bias voltage
is within the specified range, the input power bus can
also be lower than 4.5V and step-down conversion
from a 3.3V rail is also possible. Both output voltages
can be set from 0.6V to 85% of regulator’s input volt-
age. Each output can support loads of 12A or higher.
The switching sequence of the regulators is interleaved
with 180° out-of-phase operation, so that input voltage
ripple and total RMS input ripple current are reduced.
Enable inputs with precise turn-on/off threshold
(±4.2%) allow accurate external UVLO settings. Power-
good (PGOOD) open-drain outputs can be used for
supply sequencing.
The MAX15023’s capability to provide low output volt-
ages (down to 0.6V) and high output current (in excess
of 12A) makes it ideal for applications where a 5V or
12V bus is postregulated to deliver low voltages and
high currents, such as in set-top boxes.
The switching frequency is adjustable from 200kHz to
1MHz using an external resistor. The MAX15023’s
adaptive synchronous rectification eliminates the need
for external freewheeling Schottky diodes.
The MAX15023 utilizes voltage-mode control and exter-
nal compensation. The device also utilizes cycle-by-
cycle low-side source peak current limit for overcurrent
protection, where the external low-side MOSFET’s on-
resistance is used as a current-sense element during
the inductor freewheeling time, eliminating the need for
a current-sense resistor. The current-limit threshold
voltage is resistor adjustable independently on each
regulator from 30mV to 300mV and is temperature
compensated, so that the effects of the MOSFET’s
R
DS(ON)
variation over temperature are reduced.
Hiccup-mode current limit reduces average current
and power dissipation during a prolonged short-circuit
condition.
The MAX15023 also features a proprietary adaptive
internal digital soft-start and allows prebias startup
without discharging the output. Adaptive digital soft-
start, by acting on the loop voltage reference, automati-
cally prolongs the soft-start time, if the current-limit
threshold is reached during the soft-start sequence.
This increases the ability to smoothly bring up a large,
unknown amount of output capacitance. Also, since
soft-start is invoked during hiccup-mode short-circuit
protection, the same voltage reference rollback algo-
rithm achieves good control of the peak inductor cur-
rent during steady short-circuit or overload conditions.
An additional protection feature (cycle-by-cycle low-
side sink peak current limit) prevents the regulators from
sinking excessive amount of current if the prebias volt-
age exceeds the programmed steady-state regulation
level, or if another voltage source is trying to force the
output above that. This way, the synchronous rectifier
MOSFET and the body diode of the high-side MOSFET
do not experience dangerous levels of current stress
while the regulator is sinking current from the output.
Thermal shutdown protects the MAX15023 from exces-
sive power dissipation.
DC-DC PWM Controller
The MAX15023 step-down controller uses a PWM volt-
age-mode control scheme (see the
Functional
Diagram
) for each channel. Control loop compensation
is external for providing maximum flexibility in choosing
the operating frequency and output LC filter compo-
nents. An internal transconductance error amplifier pro-
duces an integrated error voltage at COMP_ that helps
provide higher DC accuracy. The voltage at COMP_
sets the duty cycle using a PWM comparator and a
ramp generator. On the rising edge of its internal clock,
the high-side n-channel MOSFET of each regulator
turns on and remains on until either the appropriate
duty cycle or the maximum duty cycle is reached.
During the high-side MOSFET’s on-time, the inductor
current ramps up. During the second-half of the switch-
ing cycle, the high-side MOSFET turns off and the low-
side n-channel MOSFET turns on. Now the inductor
releases the stored energy as its current ramps down,
providing current to the output. Under overload condi-
tions, when the inductor current exceeds the selected
cycle-by-cycle low-side source peak current-limit
threshold (see the
Current-Limit Circuit (LIM_)
section),
the high-side MOSFET does not turn on at the subse-
quent clock rising edge and the low-side MOSFET
remains on to let the inductor current ramp down.
Interleaved Out-of-Phase Operation
The two independent regulators in the MAX15023 oper-
ate 180° out-of-phase to reduce input filtering require-
ments, reduce electromagnetic interference (EMI), and
improve efficiency. This effectively lowers component
cost and saves board space, making the MAX15023
ideal for cost-sensitive applications.
