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LTC1772

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OPERATIO

(Refer to Functional Diagram)

oscillator cycle will turn the external MOSFET on and the

switching cycle repeats.

Dropout Operation

When the input supply voltage decreases towards the

output voltage, the rate of change of inductor current

during the ON cycle decreases. This reduction means that

the external P-channel MOSFET will remain on for more

than one oscillator cycle since the inductor current has not

ramped up to the threshold set by EAMP. Further reduc-

tion in input supply voltage will eventually cause the

P-channel MOSFET to be turned on 100%, i.e., DC. The

output voltage will then be determined by the input voltage

minus the voltage drop across the MOSFET, the sense

resistor and the inductor.

Undervoltage Lockout

To prevent operation of the P-channel MOSFET below safe

input voltage levels, an undervoltage lockout is incorpo-

rated into the LTC1772. When the input supply voltage

drops below approximately 2.0V, the P-channel MOSFET

and all circuitry is turned off except the undervoltage block,

which draws only several microamperes.

Short-Circuit Protection

When the output is shorted to ground, the frequency of the

oscillator will be reduced to about 120kHz. This lower

frequency allows the inductor current to safely discharge,

thereby preventing current runaway. The oscillator’s fre-

quency will gradually increase to its designed rate when

the feedback voltage again approaches 0.8V.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1772 will turn the external MOSFET off when the

feedback voltage has risen 7.5% above the reference

voltage of 0.8V. This comparator has a typical hysteresis

of 20mV.

Main Control Loop

The LTC1772 is a constant frequency current mode switch-

ing regulator. During normal operation, the external

P-channel power MOSFET is turned on each cycle when

the oscillator sets the RS latch (RS1) and turned off when

the current comparator (ICMP) resets the latch. The peak

inductor current at which ICMP resets the RS latch is

output of the error amplifier EAMP. An external resistive

relative to the 0.8V reference, which in turn causes the

current matches the new load current.

current source to charge up the external compensation

pulled up to its zero current level of approximately 0.7V.

As the external compensation network continues to charge

up, the corresponding output current trip level follows,

allowing normal operation.

Comparator OVP guards against transient overshoots

> 7.5% by turning off the external P-channel power

MOSFET and keeping it off until the fault is removed.

Burst Mode Operation

The LTC1772 enters Burst Mode operation at low load

currents. In this mode, the peak current of the inductor is

inductor’s average current is greater than the load require-

high, turning off the external MOSFET. The sleep signal

and the LTC1772 resumes normal operation. The next