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

Down Regulator with POR and RSI/PFO

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13

the PWM comparator’s inverting input. The PWM com-

parator turns off the internal high-side MOSFET when

this sum exceeds the integrated feedback voltage.

Current Limit

The internal MOSFET has a current limit of 1.2A (typ). If

the current flowing out of LX_ exceeds this maximum,

the high-side MOSFET turns off and the synchronous

rectifier MOSFET turns on. This lowers the duty cycle

and causes the output voltage to droop until the current

limit is no longer exceeded. There is also a synchro-

nous rectifier current limit of -0.85A. This is to protect

the device from current flowing into LX_. If the negative

current limit is exceeded, the synchronous rectifier is

turned off, and the inductor current continues to flow

through the high-side MOSFET body diode back to the

input until the beginning of the next cycle or until the

inductor current drops to zero.

Due to the high-switching frequency and tight output

recommended. Connect a 10

Ω resistor between IN and

possible.

Startup

To reduce the supply inrush current, soft-start circuitry

ramps up the output voltage during startup. This is

done by charging the REF capacitor with a current

source of 25µA. Once REF reaches 1.2V, the output is

in full regulation. The soft-start time is determined from:

Soft-start occurs when power is first applied, and when

EN is pulled high with power already present. The part

also goes through soft-start when coming out of under-

voltage lockout (UVLO) or thermal shutdown. The range

Undervoltage Lockout

MAX1972 assume that the supply voltage is too low to

provide a valid output voltage, and the UVLO circuit

UVLO is disabled and the soft-start sequence initiates.

Enable

A logic-enable input (EN) is provided. For normal oper-

ation, drive EN logic high. Driving EN low turns off both

outputs, and reduces the input supply current to

approximately 1µA.

Power-Fail Output

The input voltage is sensed for 5V (typical USB applica-

put (PFO) goes high. The time from PFO going high to

the outputs going out of regulation depends on the oper-

ating output voltage and currents, and the upstream 5V

bus storage capacitor value, which is 120µF minimum

(per USB specification, version 2.0). The lower the oper-

ating voltages and currents, and the higher the storage

capacitor, the longer the elapsed time. PFO is an open-

either output, is recommended.

Power-On Reset

Power-on reset (POR) provides a system reset signal.

During power-up, POR is held low until both outputs

reach 92% of their regulated voltages, POR continues

to be held low for a delayed period, and then goes

MAX1971 and MAX1972 have a delay of 175ms. Figure

2 is an example of a timing diagram.

The POR comparator is designed to be relatively

immune to short-duration negative-going output glitch-

es.The Typical Operating Characteristics gives a plot of

maximum transient duration vs. POR comparator over-

drive. The graph was generated using a negative-going

pulse applied to an output, starting at 100mV above the

actual POR threshold, dropping below the POR thresh-

old by the percentage indicated as comparator over-

drive, and then returning to 100mV above the

threshold. The graph indicates the maximum pulse

width the output transient can have without causing

POR to trip low.

Reset Input

Reset input (RSI) is an input on the MAX1971 that,

when driven high, forces the POR to go low. When RSI

goes low, POR goes through a delay time identical to a

power-up event. See Figure 2 for timing diagram. RSI

allows software to command a system reset. RSI must

be high for a minimum period of 1µs in order to initiate

the POR.

Thermal-Overload Protection

Thermal-overload protection limits total power dissipa-

+170°C, a thermal sensor shuts down the device,

allowing the IC to cool. The thermal sensor turns the

part on again after the junction temperature cools by

20°C. This results in a pulsed output during continuous

overload conditions.

During a thermal event, POR goes low, PFO goes high,

and soft-start is reset.

t

V

I

CC

SS

REF

REF

REF

REF

==

×

×

48

10

4

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