LTC3772_15 Datasheet, PDF(8/20 Page) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

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LTC3772_15 Datasheet, PDF (8/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

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LTC3772

OPERATIO (Refer to the Functional Diagram)

Main Control Loop (Normal Operation)

The LTC3772 is a constant frequency current mode step-

down switching regulator controller. During normal op-

eration, the external P-channel MOSFET is turned on each

cycle when the oscillator sets the RS latch and turned off

when the current comparator resets the latch. The peak

inductor current at which the current comparator trips is

controlled by the voltage on the ITH/RUN pin, which is the

output of the error amplifier. The negative input to the error

amplifier is the output feedback voltage VFB, which is

generated by an external resistor divider connected be-

tween VOUT and ground. When the load current increases,

it causes a slight decrease in VFB relative to the 0.8V

reference, which in turn causes the ITH/RUN voltage to

increase until the average inductor current matches the

new load current.

The main control loop is shut down by pulling the ITH/RUN

pin to ground. Releasing the ITH/RUN pin allows an

internal 1ÂµA current source to charge up the external

compensation network. When the ITH/RUN pin voltage

reaches approximately 0.6V, the main control loop is

enabled and the ITH/RUN voltage is pulled up by a clamp

to its zero current level of approximately one diode

voltage drop (0.7V). As the external compensation net-

work continues to charge up, the corresponding peak

inductor current level follows, allowing normal operation.

The maximum peak inductor current attainable is set by a

clamp on the ITH/RUN pin at 1.2V above the zero current

level (approximately 1.9V).

Burst Mode Operation

The LTC3772 incorporates Burst Mode operation at low

load currents (<10% of IMAX). In this mode, an internal

clamp sets the peak current of the inductor at a level cor-

responding to an ITH/RUN voltage 0.925V, even though the

actual ITH/RUN voltage is lower. When the inductorâs av-

erage current is greater than the load requirement, the

voltage at the ITH/RUN pin will drop. When the ITH/RUN

voltage falls to 0.85V, the sleep comparator will trip, turn-

ing off the external MOSFET. In sleep, the input DC supply

current to the IC is reduced to 40ÂµA from 250ÂµA in normal

operation. With the switch held off, average inductor cur-

rent will decay to zero and the load will eventually cause the

error amplifier output to start drifting higher. When the error

amplifier output rises to 0.87V, the sleep comparator will

untrip and normal operation will resume. The next oscilla-

tor cycle will turn the external MOSFET on and the switch-

ing cycle will repeat.

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

at some input-output differential, the external P-channel

MOSFET will remain on for more than one oscillator cycle

(start dropping off-cycles) since the inductor current has

not ramped up to the threshold set by the error amplifier.

Further reduction in input supply voltage will eventually

cause the external 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

sense resistor, the MOSFET and the inductor.

Undervoltage Lockout Protection

To prevent operation of the external P-channel MOSFET

with insufficient gate drive, an undervoltage lockout cir-

cuit is incorporated into the LTC3772. When the input

supply voltage drops below approximately 2V, the

P-channel MOSFET and all internal circuitry other than the

undervoltage block itself are turned off. Input supply

current in undervoltage is approximately 1ÂµA.

Short-Circuit Protection

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

oscillator is folded back from 550kHz to approximately

200kHz while maintaining the same minimum on time.

This lower frequency allows the inductor current to safely

discharge, thereby preventing current runaway. After the

short is removed, the oscillator frequency will gradually

increase back to 550kHz as VFB rises through 0.3V on its

way back to 0.8V.

3772f

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