MAX8664
Low-Cost, Dual-Output, Step-Down
Controller with Fast Transient Response
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Detailed Description
The MAX8664 dual-output PWM controller is a low-cost
solution for dual power-supply systems. It provides two
individual outputs that operate 180° out-of-phase to
minimize input capacitance requirements. Built-in dri-
vers are capable of driving external MOSFETs to deliv-
er up to 25A of current from each output. The MAX8664
operates from a 4.5V to a 5.5V or a 7.2V to 28V input
and generates output voltages from 0.6V up to 90% of
the input voltage on each channel. Total output error is
less than ±0.8% over load, line, and temperature.
The MAX8664 operates with a constant switching fre-
quency adjustable from 100kHz to 1MHz. Built-in boost
diodes reduce external component count. Digital soft-
start eliminates input inrush current during startup. The
second output has an optional REFIN2 input that takes
an external reference voltage, facilitating tracking supply
applications. Each output is capable of sourcing and
sinking current. Internal 6.5V and 5V linear regulators
provide power for gate drive and internal IC functions.
The MAX8664 has built-in protection against output over-
voltage, overcurrent, and thermal faults. The MAX8664B
latches off both controllers during a fault condition, while
the MAX8664A allows one controller to continue to func-
tion when there is a fault in the other controller.
The MAX8664 employs Maxim’s proprietary peak-volt-
age mode control architecture that provides superior
transient response during either load or line transients.
This architecture is easily stabilized using two resistors
and one capacitor for any type of output capacitors.
Fast transient response requires less output capaci-
tance, consequently reducing total system cost.
DC-DC Controller Architecture
The peak-voltage mode PWM control scheme ensures
stable operation, simple compensation for any output
capacitor, and fast transient response. An on-chip inte-
grator removes any DC error due to the ripple voltage.
This control scheme is simple: when the output voltage
falls below the regulation threshold, the error compara-
tor begins a switching cycle by turning on the high-side
switch at the rising edge of the following clock cycle.
This switch remains on until the minimum on-time
expires and the output voltage is in regulation or the
current-limit threshold is exceeded. At this point, the
low-side synchronous rectifier turns on and remains on
until the rising edge of the first clock cycle after the out-
put voltage falls below the regulation threshold.
Internal Linear Regulators
The internal VL low-dropout linear regulator of the
MAX8664A and MAX8664B provides the 6.5V supply
used for the gate drive. Connect a 4.7µF ceramic
capacitor from VL to PGND. When using a 4.5V to 5.5V
input supply, connect VL directly to IN.
The 5V supply used to power IC functions (V
CC
) is gen-
erated by an internal 1.5V shunt regulator from VL.
Connect a 2.2µF ceramic capacitor from V
CC
to GND.
When using a 4.5V to 5.5V input supply, connect V
CC
to IN through a 10Ω resistor.
High-Side Gate-Drive Supply (BST_)
The gate-drive voltage for the high-side MOSFETs is
generated using a flying capacitor boost circuit. The
capacitor between BST_ and LX_ is charged to the VL
voltage through the integrated BST_ diode during the
low-side MOSFET on-time. When the low-side MOSFET
is switched off, the BST_ voltage is shifted above the
LX_ voltage to provide the necessary turn-on voltage
(V
GS
) for the high-side MOSFET. The controller closes
a switch between BST_ and DH_ to turn the high-side
MOSFET on.
Voltage Reference
An internal 0.6V reference sets the feedback regulation
voltage. Controller 1 always uses the internal reference.
An external reference input is provided for controller 2.
To use the external reference, connect a 0 to 1.3V sup-
ply to REFIN2. This facilitates tracking applications. To
use the internal 0.6V reference for controller 2, connect
REFIN2 to V
CC
.
Undervoltage Lockout (UVLO)
When the V
CC
supply voltage drops below the UVLO
threshold (3.15V falling typ), the undervoltage lockout
(UVLO) circuitry inhibits the switching of both con-
trollers, and forces the DL and DH gate drivers low.
When V
CC
rises above the UVLO threshold (3.5V rising
typ), the controllers begin the startup sequence and
resume normal operation.
Output Overcurrent Protection
When the MAX8664 detects an overcurrent condition,
DH is immediately pulled low. If the overcurrent condition
persists for four consecutive cycles, the controller latch-
es off and both DH_ and DL_ are pulled low. During soft-
start, when FB_ is less than 300mV, the controller latches
off on the first overcurrent condition. The protection cir-
cuit detects an overcurrent condition by sensing the
drain-source voltage across the high-side MOSFET(s).
