Datasheet

MAX1951A
1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC
Step-Down Regulator with Enable
6 _______________________________________________________________________________________
Detailed Description
The MAX1951A high-efficiency switching regulator is a
small, simple, current-mode DC-DC step-down convert-
er capable of delivering up to 2A of output current. The
device operates in pulse-width modulation (PWM) at a
fixed frequency of 1MHz from a 2.6V to 5.5V input volt-
age and provides an output voltage from 0.8V to V
IN
,
making the MAX1951A ideal for on-board postregula-
tion applications. The high switching frequency allows
for the use of smaller external components, and an
internal synchronous rectifier improves efficiency and
eliminates the typical Schottky free-wheeling diode.
Using the on-resistance of the internal high-side MOS-
FET to sense switching currents eliminates current-
sense resistors, further improving efficiency and cost.
The MAX1951A total output error over load, line, and
temperature (-40°C to +85°C) is less than 1.5%.
Controller Block Function
The MAX1951A step-down converter uses a PWM cur-
rent-mode control scheme. An open-loop comparator
compares the integrated voltage-feedback signal against
the sum of the amplified current-sense signal and the
slope compensation ramp. At each rising edge of the
internal clock, the internal high-side MOSFET turns on
until the PWM comparator trips. During this on-time, cur-
rent ramps up through the inductor, sourcing current to
the output and storing energy in the inductor. The current-
mode feedback system regulates the peak inductor cur-
rent as a function of the output-voltage error signal. Since
the average inductor current is nearly the same as the
peak inductor current (< 30% ripple current), the circuit
acts as a switch-mode transconductance amplifier. To
preserve inner-loop stability and eliminate inductor stair-
casing, a slope-compensation ramp is summed into the
main PWM comparator. During the second half of the
cycle, the internal high-side p-channel MOSFET turns off,
and the internal low-side n-channel MOSFET turns on.
The inductor releases the stored energy as its current
ramps down while still providing current to the output. The
output capacitor stores charge when the inductor current
exceeds the 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
section), the high-side MOSFET does not turn on at the
rising edge of the clock and the low-side MOSFET
remains on to let the inductor current ramp down.
Current Sense
An internal current-sense amplifier produces a current
signal proportional to the voltage generated by the
high-side MOSFET on-resistance and the inductor cur-
rent (R
DS(ON)
x I
LX
). The amplified current-sense signal
and the internal slope compensation signal are
summed together into the comparator’s inverting input.
The PWM comparator turns off the internal high-side
MOSFET when this sum exceeds the output from the
voltage-error amplifier.
Current Limit
The internal high-side MOSFET has a current limit of 3.1A
(typ). If the current flowing out of LX exceeds this limit, the
high-side MOSFET turns off and the synchronous rectifier
turns on. This lowers the duty cycle and causes the output
voltage to droop until the current limit is no longer exceed-
ed. A synchronous rectifier current limit of -0.6A (typ) pro-
tects the device from current flowing into LX. If the
negative current limit is exceeded, the synchronous recti-
fier turns off, forcing the inductor current to flow through
the high-side MOSFET body diode, back to the input, until
the beginning of the next cycle or until the inductor cur-
rent drops to zero. The MAX1951A utilizes a pulse-skip
mode to prevent overheating during short-circuit output
conditions. The device enters pulse-skip mode when the
FB voltage drops below 300mV, limiting the current to 3A
(typ) and reducing power dissipation. Normal operation
resumes upon removal of the short-circuit condition.
Pin Description
PIN NAME FUNCTION
1V
CC
Supply Voltage. Bypass with a 0.1µF capacitor
to ground and a 10Ω resistor to IN.
2EN
Enable Input. Connect to V
CC
for normal
operation. Connect to GND to disable the
MAX1951A.
3 GND Signal Ground
4FB
Feedback Input. Connect an external resistor-
divider from the output to FB and GND to set
the output to a voltage between 0.8V and V
IN
.
5 COMP
Regulator Compensation. Connect series RC
network to GND.
6 PGND
Power Ground. Internally connected to GND.
Keep power ground and signal ground planes
separate.
7LX
Inductor Connection. Connect an inductor
between LX and the regulator output.
8IN
Power-Supply Voltage. Input voltage range
from 2.6V to 5.5V. Bypass with a 10µF (min)
ceramic capacitor to GND and a 10Ω resistor
to V
CC
.