1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC

Step-Down Regulator with Enable

6

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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-

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-

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

Supply Voltage. Bypass with a 0.1µF capacitor

to ground and a 10

Ω resistor to IN.

2EN

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

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 10

Ω resistor