LT8607 datasheet(17/24 Pages) AD

17 / 24 page

LT8607/LT8607B

Rev. D

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APPLICATIONS INFORMATION

RT resistor should be chosen to set the LT8607 switch-

ing frequency equal to or below the lowest synchroni-

zation 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

subharmonic oscillations is established by the inductor

size, input voltage, and output voltage. Since the syn-

chronization 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

synchronization frequencies.

For some applications it is desirable for the LT8607 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 the clock. Second is that full switching frequency is

reached at lower output load than in Burst Mode operation

as shown in Figure 2 in an earlier section. These two differ-

ences come at the expense of increased quiescent current.

To enable pulse-skipping mode the SYNC pin is floated.

For some applications, reduced EMI operation may be

desirable, which can be achieved through spread spec-

trum modulation. This mode operates similar to pulse

skipping mode operation, with the key difference that the

switching frequency is modulated up and down by a 3kHz

triangle wave. The modulation has the frequency set by RT

as the low frequency, and modulates up to approximately

20% higher than the frequency set by RT. To enable spread

spectrum mode, tie SYNC to INTVCC or drive to a voltage

between 3.2V and 5V.

The LT8607 does not operate in forced continuous mode

regardless of SYNC signal. The LT8607 DFN is always

programmed for Burst Mode operation and cannot

enter pulse-skipping mode. The LT8607B DFN is pro-

grammed for pulse-skipping mode and cannot enter Burst

Mode operation.

Shorted and Reversed Input Protection

The LT8607 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. This allows for tailoring the LT8607

to individual applications and limiting thermal dissipation

during short circuit conditions.

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

source, tied high or floated, the LT8607 will stay at the

programmed 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 LT8607

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 LT8607’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 the LT8607’s internal circuitry will pull its quiescent

current through its SW pin. This is acceptable if the sys-

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

grounded the SW pin current will drop to near 0.7µA.

However, if the VIN pin is grounded while the output is

held high, regardless of EN, parasitic body diodes inside

the LT8607 can pull current from the output through the

SW pin and the VIN pin. Figure 4 shows a connection of

the VIN and EN/UV pins that will allow the LT8607 to run

only when the input voltage is present and that protects

against a shorted or reversed input.

VIN

LT8607

GND

8607 F04

EN/UV

Figure 4. Reverse VIN Protection

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