MAX1761
Small, Dual, High-Efficiency
Buck Controller for Notebooks
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Typical Application Circuit
The typical application circuit in Figure 1 generates two
low-voltage rails for general-purpose use in notebook
and subnotebook computers (I/O supply, fixed CPU
core supply, DRAM supply). This DC-DC converter
steps down a battery or AC adapter voltage to voltages
from 1.0V to 5.5V with high efficiency and accuracy.
See Table 1 for a list of components for common appli-
cations. Table 2 lists component manufacturers.
Detailed Description
The MAX1761 dual buck controller is designed for low-
voltage power supplies in notebook and subnotebook
computers. Maxim’s proprietary Quick-PWM pulse-
width modulation circuit (Figure 2) is specifically
designed for handling fast load steps while maintaining
a relatively constant operating frequency over a wide
range of input voltages. The Quick-PWM architecture
circumvents the poor load-transient timing problems of
fixed-frequency current-mode PWMs while preventing
problems caused by widely varying switching frequen-
cies in conventional constant-on-time and constant-off-
time PWM schemes.
This MAX1761 controls two synchronously rectified out-
puts with complementary N- and P-channel MOSFETs.
Using the P-channel for the high-side MOSFET elimi-
nates external boost capacitors and diodes, reducing
PC board area and cost. The MAX1761 can step down
input voltages from 5V to 20V, to outputs ranging from
1V to 5.5V on either output. Dual Mode feedback inputs
allow fixed output voltages of 2.5V and 1.8V for OUT1
and OUT2, respectively; or, a resistive voltage-divider
can be used to adjust the output voltages from 1V to
5.5V. Other appropriate applications for this device are
digital cameras, large PDAs, and handy-terminals.
V+ Input and VL +5V Logic Supplies
The MAX1761 has a 5V to 20V input voltage supply
range. A linear regulator powers the control logic and
other internal circuitry from the input supply pin (V+).
The linear regulator’s 4.65V output is available at VL
and can supply 25mA to external circuitry. When used
as an external supply, bypass VL to GND with a 4.7µF
capacitor. VL is turned off when the device is in shut-
down, and drops to approximately 4V when the device
experiences an output voltage fault.
The MAX1761 includes an input undervoltage lockout
(UVLO) circuit that prevents the device from switching
until VL > 4.25V (max). UVLO ensures there is sufficient
drive for the external MOSFETs, prevents the high-side
MOSFET from being turned on for near 100% duty
cycle, and keeps the output in regulation. The UVLO
