MAX1953/MAX1954/MAX1957
Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
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Detailed Description
The MAX1953/MAX1954/MAX1957 are single-output,
fixed-frequency, current-mode, step-down, PWM, DC-
DC converter controllers. The MAX1953 switches at
1MHz, allowing the use of small external components for
small applications. Table 1 lists suggested components.
The MAX1954 switches at 300kHz for higher efficiency
and operates from a wider range of input voltages.
Figure 1 is the MAX1953 typical application circuit. The
MAX1953/MAX1954/MAX1957 are designed to drive a
pair of external N-channel power MOSFETs in a syn-
chronous buck topology 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 on-resistance is used for current-mode feed-
back and current-limit sensing, thus eliminating the
need for current-sense resistors. The MAX1953 has
three selectable short-circuit current-limit thresholds:
105mV, 210mV, and 320mV. The MAX1954 and
MAX1957 have 210mV fixed short-circuit current-limit
thresholds. The MAX1953/MAX1954/MAX1957 accept
input voltages from 3V to 5.5V. The MAX1954 is config-
ured with a high-side drain input (HSD) allowing an
extended input voltage range of 3V to 13.2V that is
independent of the input supply (Figure 2). The
MAX1957 is tailored for tracking output voltage applica-
tions such as DDR bus termination supplies, referred to
as V
TT
. It utilizes a resistor-divider network connected
to REFIN to keep the 1/2 ratio tracking between V
TT
and V
DDQ
(Figure 3). The MAX1957 can source and
sink up to 3A. Figure 4 shows the MAX1954 20A circuit.
DC-DC Converter Control Architecture
The MAX1953/MAX1954/MAX1957 step-down convert-
ers use a PWM, current-mode control scheme. An inter-
nal transconductance amplifier establishes an integrated
error voltage. The heart of the PWM controller is an open-
loop comparator that compares the integrated voltage-
feedback signal against the amplified current-sense
signal plus the slope compensation ramp, which are
summed into the main PWM comparator to preserve
inner-loop stability 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, cur-
rent 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 transconduc-
tance amplifier and pushes the output LC filter pole nor-
mally found in a voltage-mode PWM to a higher
frequency.
During the second half of the cycle, the high-side MOS-
FET turns off and the low-side MOSFET turns on. The
inductor releases the stored energy as the current ramps
down, providing current to the output. The output capaci-
tor stores charge when the inductor current exceeds the
required load current and discharges when the inductor
current is lower, smoothing the voltage across the load.
Under overload conditions, when the inductor current
exceeds the selected current-limit (see the Current Limit
Circuit section), the high-side MOSFET is not turned on
at the rising clock edge and the low-side MOSFET
remains on to let the inductor current ramp down.
The MAX1953/MAX1954/MAX1957 operate in a forced-
PWM mode. As a result, the controller maintains a con-
stant switching frequency, regardless of load, to allow for
easier postfiltering of the switching noise.
