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LT8613

8613f

applicaTions inForMaTion

Figure 5. Reverse VIN Protection

The LT8613 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will pulse skip to maintain regulation. The LT8613

may be synchronized over a 200kHz to 2.2MHz range. The

RT resistor should be chosen to set the LT8613 switching

frequency equal to or below the lowest synchronization

input. For example, if the synchronization signal will be

500kHz and higher, the RT should be selected for 500kHz.

The slope compensation is set by the RT value, while the

minimum slope compensation required to avoid subhar-

monic oscillations is established by the inductor size,

input voltage, and output voltage. Since the synchroniza-

tion frequency will not change the slopes of the inductor

current waveform, if the inductor is large enough to avoid

subharmonic oscillations at the frequency set by RT, then

the slope compensation will be sufficient for all synchro-

nization frequencies.

For some applications it is desirable for the LT8613 to

operate in pulse-skipping mode, offering two major differ-

ences from Burst Mode operation. First is the clock stays

awake at all times and all switching cycles are aligned to

theclock.Secondisthatfullswitchingfrequencyisreached

at lower output load than in Burst Mode operation. These

twodifferencescomeattheexpenseofincreasedquiescent

current. To enable pulse-skipping mode, the SYNC pin is

tied high either to a logic output or to the INTVCC pin.

The LT8613 does not operate in forced continuous mode

regardless of SYNC signal. Never leave the SYNC pin

floating.

Shorted and Reversed Input Protection

The LT8613 will tolerate a shorted output. Several features

are used for protection during output short-circuit and

brownout conditions. The first is the switching frequency

will be folded back while the output is lower than the set

point to maintain inductor current control. Second, the

bottom switch current is monitored such that if inductor

current is beyond safe levels switching of the top switch

will be delayed until such time as the inductor current

falls to safe levels.

Frequency foldback behavior depends on the state of the

SYNC pin: If the SYNC pin is low the switching frequency

will slow while the output voltage is lower than the pro-

grammed level. If the SYNC pin is connected to a clock

sourceortiedhigh,theLT8613willstayattheprogrammed

frequency without foldback and only slow switching if the

inductor current exceeds safe levels.

There is another situation to consider in systems where

the output will be held high when the input to the LT8613

is absent. This may occur in battery charging applications

or in battery-backup systems where a battery or some

other supply is diode ORed with the LT8613’s output. If

the VIN pin is allowed to float and the EN pin is held high

(either by a logic signal or because it is tied to VIN), then

theLT8613’sinternalcircuitrywillpullitsquiescentcurrent

through its SW pin. This is acceptable if the system can

tolerate several μA in this state. If the EN pin is grounded

the SW pin current will drop to near 1µA. However, if the

VIN pin is grounded while the output is held high, regard-

less of EN, parasitic body diodes inside the LT8613 can

pull current from the output through the SW pin and

the VIN pin. Figure 5 shows a connection of the VIN and

EN/UV pins that will allow the LT8613 to run only when

the input voltage is present and that protects against a

shorted or reversed input.

VIN

LT8613

EN/UV

8613 F05

GND

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