LTC1702IGN datasheet(14/36 Pages) LINER

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LTC1702

ripple voltage and frequency remain constant at all loads,

easing filtering requirements. Circuits that take advantage

of this behavior can force the LTC1702 to operate in

continuous mode at all loads by tying the FCB (Force

Continuous Bar) pin to ground.

Discontinuous Mode

To minimize the efficiency loss due to reverse current flow

at light loads, the LTC1702 switches to a second mode of

operation: discontinuous mode (Figure 5b). In discontinu-

ous mode, the LTC1702 detects when the inductor current

approaches zero and turns off QB for the remainder of the

switch cycle. During this time, the voltage at the SW pin

will float about VOUT, the voltage across the inductor will

be zero, and the inductor current remains zero until the

next switching cycle begins and QT turns on again. This

prevents current from flowing backwards in QB, eliminat-

ing that power loss term. It also reduces the ripple current

in the inductor as the output current approaches zero.

The LTC1702 detects that the inductor current has reached

zero by monitoring the voltage at the SW pin while QB is

APPLICATIONS INFORMATION

Figure 6. Ringing at SW Causes Discontinuous

Comparator to Trip Early

Figure 5a. Continuous Mode

Figure 5b. Discontinuous Mode

TIME

50ns

BLANK

TIME

DISCONTINUOUS

COMPARATOR

TURNS OFF BG

VSW

VBG

1702 F06

TIME

on. Since QB acts like a resistor, SW should ideally be right

at 0V when the inductor current reaches zero. In reality, the

SW node will ring to some degree immediately after it is

switched to ground by QB, causing some uncertainty as to

the actual moment the average current in QB goes to zero.

The LTC1702 minimizes this effect by ignoring the SW

node for a fixed 50ns after QB turns on when the ringing

is most severe, and by including a few millivolts offset in

the comparator that monitors the SW node. Despite these

precautions, some combinations of inductor and layout

parasitics can cause the LTC1702 to enter discontinuous

mode erratically. In many cases, the time that QB turns off

will correspond to a peak in the ringing waveform at the

SW pin (Figure 6). This erratic operation isn’t pretty, but

retains much of the efficiency benefit of discontinuous

mode and maintains regulation at all times.

Burst Mode Operation

Discontinuous mode removes the resistive loss drop term

in QB, but the LTC1702 is still switching QT and QB on and

off once a cycle. Each time an external MOSFET is turned

on, the internal driver must charge its gate to VCC. Each

time it is turned off, that charge is lost to ground. At the

high switching frequencies that the LTC1702 operates at,

the charge lost to the gates can add up to tens of milliamps

from VCC. As the load current continues to drop, this

quickly become the dominant power loss term, reducing

efficiency once again.

TIME

IRIPPLE

IAVERAGE

1702 F05a

TIME

IRIPPLE

IAVERAGE

1702 F05b

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