Datasheet

MAX1954A
Low-Cost, Current-Mode PWM Buck
Controller with Foldback Current Limit
8 _______________________________________________________________________________________
During the second half of the cycle, the high-side
MOSFET 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 current limit (see the Current-Limit Circuit
section), the high-side MOSFET is not turned on at the ris-
ing clock edge and the low-side MOSFET remains on to
let the inductor current ramp down.
The MAX1954A operates in a forced-PWM mode; there-
fore, the controller maintains a constant switching fre-
quency, regardless of load, to allow for easier post-
filtering of the switching noise.
Current-Sense Amplifier
The current-sense circuit amplifies the current-sense
voltage (the high-side MOSFET’s on-resistance
(R
DS(ON)
) multiplied by the inductor current). This ampli-
fied current-sense signal and the internal slope-compen-
sation signal are summed (V
SUM
) together and fed into
the PWM comparator’s inverting input. The PWM com-
parator shuts off the high-side MOSFET when V
SUM
exceeds the integrated feedback voltage (V
COMP
).
REFERENCE
AND
SOFT-START
DIGITAL-TO-ANALOG
CONVERTER
FOLDBACK CURRENT-
LIMIT CIRCUITRY
FB
COMP
0.5V
SHUTDOWN
COMPARATOR
ERROR
AMPLIFIER
SLOPE
COMPENSATION
THERMAL
LIMIT
UVLO
PWM
CONTROL
CIRCUITRY
CLOCK
SHORT-CIRCUIT
CURRENT-LIMIT
CIRCUITRY
CURRENT-LIMIT
COMPARATOR
PGND
2.36V
CLAMP
GND
DL
IN
LX
CURRENT-
SENSE
CIRCUITRY
DH
BST
HSD
IN
MAX1954A
Functional Diagram