LTC3607 datasheet(8/20 Pages) LINER

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LTC3607

3607fb

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operaTion

The LTC3607 uses a constant-frequency, peak current

mode architecture. The operating frequency is set at

2.25MHz and can be synchronized to an external oscillator

between 1MHz and 4MHz. Both channels share the same

clock and run in-phase. To suit a variety of applications,

the selectable MODE/SYNC pin allows the user to trade-

off ripple for efficiency.

The output voltage is set by an external divider returned

to the VFB pins. An error amplifier compares the divided

output voltage with a reference voltage of 0.6V and ad-

justs the peak inductor current accordingly. Overvoltage

and undervoltage comparators will pull the independent

PGOOD outputs low if the output voltage is not within

±8.5%. The PGOOD outputs will go high 64 clock cycles

after achieving regulation and will go low 64 cycles after

falling out of regulation.

Whether in Burst Mode or pulse-skipping operation, the

overvoltage protection circuit is still enabled when the rest

of the regulator is asleep. Hence, if VOUT rises above the

overvoltage threshold, the regulator is forced out of sleep.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle

when the VFB voltage is below the reference voltage. The

current into the inductor and the load increases until the

current limit is reached. The switch turns off and energy

stored in the inductor flows through the bottom switch

(N-channelMOSFET)intotheloaduntilthenextclockcycle.

The peak inductor current is controlled by the internally

compensated ITH voltage, which is the output of the error

amplifier. This amplifier compares the VFB pin to the 0.6V

internal reference. When the load current increases, the

VFB voltage decreases slightly below the reference. This

decrease causes the error amplifier to increase the ITH

voltage until the average inductor current matches the

new load current.

The main control loop is shut down by pulling the RUN

pin to ground.

Low Current Operation

Two discontinuous-conduction modes (DCMs) are avail-

able to control the operation of the LTC3607 at low output

currents. Both modes, Burst Mode operation and pulse-

skipping, automatically switch from continuous operation

to the selected mode when the load current is low.

To optimize efficiency, Burst Mode operation can be se-

lected by floating the MODE/SYNC pin or setting it to 1V

or greater. When the load is relatively light, the LTC3607

automaticallyswitchesintoBurstModeoperationinwhich

the PMOS switch operates intermittently based on load

demand with a fixed peak inductor current. By running

cycles periodically, the switching losses, which are domi-

nated by the gate charge losses of the power MOSFETs,

are minimized. The main control loop is interrupted when

the output voltage reaches the desired regulated value.

A voltage comparator trips when the ITH voltage drops

below an internal clamp voltage, shutting off the switch

and reducing the power. The output capacitor and the

inductor supply the power to the load until ITH exceeds

an internal clamp voltage, turning on the switch and the

main control loop, which starts another cycle.

To optimize ripple, pulse-skipping mode can be selected

by grounding the MODE/SYNC pin. In the LTC3607, pulse-

skipping mode is implemented similarly to Burst Mode

operation with the ITH clamp set to a lower internal clamp

voltage. This results in lower ripple than in Burst Mode

operationwiththetrade-offbeingslightlylowerefficiency.

Dropout Operation

Whentheinputsupplyvoltagedecreasestowardtheoutput

voltage, the duty cycle increases to 100% which is the

dropout condition. In dropout, the PMOS switch is turned

on continuously with the output voltage being equal to the

input voltage minus the voltage drops across the internal

P-channel MOSFET and the inductor.

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