LT8607 datasheet(17/24 Pages) AD

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LT8609S

Rev. D

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APPLICATIONS INFORMATION

The LT8609S 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

LT8609S may be synchronized over a 200kHz to 2.2MHz

range. The RT resistor should be chosen to set the

LT8609S switching frequency equal to or below the low-

est synchronization input. For example, if the synchro-

nization 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 synchronization 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 LT8609S 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 1b in an earlier section. These two differences

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 3 kHz

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 LT8609S does not operate in forced continuous mode

regardless of SYNC signal.

Shorted and Reversed Input Protection

The LT8609S will tolerate a shorted output. Several fea-

tures are used for protection during output short-circuit

and brownout conditions. The first is the switching fre-

quency will be folded back while the output is lower than

the set point to maintain inductor current control (only if

SYNC = 0V).. Second, the bottom switch current is mon-

itored 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 LT8609S to individual applications and lim-

iting thermal dissipation during short circuit conditions.

Frequency foldback behavior depends on the state of the

SYNC pin: If the SYNC pin is low or high, or floated the

switching frequency will slow while the output voltage

is lower than the programmed level. If the SYNC pin is

connected to a clock source, the LT8609S 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 LT8609S

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 LT8609S’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 LT8609S’s internal circuitry will pull its quies-

cent current 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 0.7µA.

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

held high, regardless of EN, parasitic body diodes inside

the LT8609S can pull current from the output through the

SW pin and the VIN pin. Figure 3 shows a connection of

the VIN and EN/UV pins that will allow the LT8609S to run

only when the input voltage is present and that protects

against a shorted or reversed input.

Figure 3.

VIN

LT8609S

GND

8609S F03

EN/UV

Reverse VIN Protection

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