turns off and the internal low-side n-channel MOSFET

turns on. Now 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 dis-

charges 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 is not

turned on at the rising edge of the clock and the low-

side MOSFET remains on to let the inductor current

ramp down.

Current Sense

The current-sense circuit amplifies the current-sense

voltage generated by the high-side MOSFET’s on-resis-

This amplified current-sense signal and the internal

slope compensation signal are summed together into

Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-

Down Regulator with POR and RSI/PFO

12

______________________________________________________________________________________

Figure 1. Functional Diagram

FB

SELECT

PWM CONTROL

LX1

PGND

RSI

PFO

POR

MAX1970/MAX1972 ONLY

MAX1971 ONLY

CLAMP

ERROR SIGNAL

REGULATOR 1

REGULATOR 2

FB1

IN

FBSEL1

COMP1

REF

EN

COMP2

FB2

FBSEL2

GND

SLOPE COMP

CURRENT SENSE

SOFT-START

THERMAL SHUTDOWN

VOK

REFERENCE

VOLTAGE

1.2V

÷2(MAX1970/

MAX1972)/

÷4(MAX1971)

2.8MHz

OSCILLATOR

POR

PFO

MAX1970

MAX1971

MAX1972