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LTC1703

1703fa

LTC1703 remains in continuous mode. See the Inductor

Selection section for a detailed description of ripple

current.

As the output load current decreases in continuous mode,

the average current in the inductor will reach a point where

it drops below half the ripple current. At this point, the

current in the inductor will reverse during a portion of the

switching cycle, or begin to flow from the output back to

the input. This does not adversely affect regulation, but

does cause additional losses as a portion of the inductor

current flows back and forth through the resistive power

switches, giving away a little more power each time and

lowering the efficiency. There are some benefits to allow-

ing this reverse current flow: the circuit will maintain

regulation even if the load current drops below zero (the

load supplies current to the LTC1703) and the output

ripple voltage and frequency remain constant at all loads,

easing filtering requirements. Circuits that take advantage

of this behavior can force the LTC1703 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 LTC1703 switches to a second mode of

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

ous mode, the LTC1703 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 LTC1703 detects that the inductor current has reached

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

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 LTC1703 minimizes this effect by ignoring the SW

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

Figure 5a. Continuous Mode

Figure 5b. Discontinuous Mode

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 LTC1703 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.

TIME

IRIPPLE

IAVERAGE

1703 F05a

TIME

IRIPPLE

IAVERAGE

1703 F05b

Figure 6. Ringing at SW Causes Discontinuous

Comparator to Trip Early

TIME

50ns

BLANK

TIME

DISCONTINUOUS

COMPARATOR

TURNS OFF BG

VSW

VBG

1703 F06

TIME

APPLICATIO S I FOR ATIO

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