Datasheet

MAX1954A
Low-Cost, Current-Mode PWM Buck
Controller with Foldback Current Limit
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Detailed Description
The MAX1954A single-output, current-mode, PWM, step-
down DC-DC controller features foldback current limit
and switches at 300kHz for high efficiency. The
MAX1954A is designed to drive a pair of external N-
channel power MOSFETs in a synchronous buck topolo-
gy to improve efficiency and cost compared with a
P-channel power-MOSFET topology. The on-resistance
of the low-side MOSFET is used for short-circuit current-
limit sensing, while the high-side MOSFET’s on-resis-
tance is used for current-mode feedback, thus
eliminating the need for current-sense resistors. The
short-circuit current limit is fixed at 135mV. The foldback
current scheme reduces the input current during short-
circuit and severe-overload conditions. The MAX1954A
is configured with a high-side drain input (HSD) allowing
an extended input voltage range of 3V to 13.2V that is
independent of the IC input supply (Figure 1).
DC-DC Converter Control Architecture
The MAX1954A step-down converter uses a PWM, cur-
rent-mode control scheme. An internal transconductance
amplifier establishes an integrated error voltage. An
open-loop comparator compares the integrated voltage-
feedback signal against the amplified current-sense sig-
nal plus the slope compensation ramp, which is summed
into the main PWM comparator to preserve inner-loop sta-
bility and eliminate inductor staircasing. At each rising
edge of the internal clock, the high-side MOSFET turns on
until the PWM comparator trips or the maximum duty
cycle is reached. During this on-time, current ramps up
through the inductor, storing energy in a magnetic field
and sourcing current to the output. The current-mode
feedback system regulates the peak inductor current as a
function of the output-voltage error signal. The circuit acts
as a switch-mode transconductance amplifier because
the average inductor current is close to the peak inductor
current (assuming the inductor is large enough to provide
a reasonably small ripple current). This pushes the output
inductance-capacitance filter pole normally found in a
voltage-mode PWM to a higher frequency.
PIN NAME FUNCTION
1 HSD
High-Side Drain Current-Sense Input. HSD senses the voltage at the drain of the high-side, N-channel MOSFET.
Connect to the high-side MOSFET drain using a Kelvin connection.
2 COMP
Compensation and Shutdown Control Pin. Connect appropriate RC networks to compensate the control loop.
Pull to GND to shut down the IC. See the Compensation Design section for instructions on calculating the RC
values.
3 FB
Feedback Input. Regulates at V
FB
= 0.8V. Connect FB to the center tap of a resistor-divider from the output to
GND to set the output voltage.
4 GND Ground
5 IN
IC Supply Voltage. Provides power for the IC. Connect to a 3V to 5.5V power supply. Bypass to GND with a
0.22µF ceramic capacitor and to PGND with a 1µF ceramic capacitor.
6 DL
Low-Side Gate-Drive Output. Drives the synchronous-rectifier MOSFET. Swings from 0 to V
IN
. DL is low in
shutdown and UVLO.
7 PGND Power Ground
8 DH
High-Side Gate-Drive Output. Drives the high-side N-channel MOSFET. DH is a floating driver output that swings
from V
LX
to V
BST
. DH is low in shutdown and UVLO.
9 LX
Controller Current-Sense Input. Connect LX to the junction of the MOSFETs and inductor. LX is the reference
point for the current limit.
10 BST
High-Side MOSFET Supply Input. Connect a 0.1µF ceramic capacitor from BST to LX to supply the necessary
gate drive for the high-side N-channel MOSFET.
Pin Description