LT8650SPJVPBF datasheet(24/34 Pages) AD

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LT8650SP

Rev. B

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APPLICATIONS INFORMATION

Synchronization

To select low ripple Burst Mode operation, tie the SYNC pin

below 0.4V (this can be ground or a logic low output). To

select forced continuous mode (FCM), float the SYNC pin.

To select FCM with spread spectrum modulation (SSM),

tie the SYNC pin above 2.8V (SYNC can be tied to VCC).

To synchronize the LT8650SP oscillator to an external

frequency connect a square wave (with 20% to 80% duty

cycle) to the SYNC pin. The square wave amplitude should

have valleys that are below 0.4V and peaks above 1.5V

(up to 6V). When synchronized to an external clock the

LT8650SP will use FCM.

Channel 1 will synchronize its positive switch edge transi-

tions to the positive edge of the SYNC signal, and channel

2 will synchronize to the negative edge of the SYNC signal.

The LT8650SP may be synchronized over a 300kHz to

3MHz range. The RT resistor should be chosen to set the

LT8650SPswitchingfrequencyequaltoorbelowthelowest

synchronization input. For example, if the synchronization

signal will be 500kHz and higher, the RTshouldbeselected

for nominal 500kHz.

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

minimum slope compensation required to avoid sub-

harmonic 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.

A synchronizing signal that incorporates spread spectrum

may reduce EMI. The duty cycle of the SYNC signal can

be used to set the relative phasing of the two channels

for minimizing input ripple.

Forced Continuous Mode

Forcedcontinuousmode(FCM)isactivatedbyeitherfloat-

ing the SYNC pin, tying the SYNC pin to VCC, applying a

DC voltage above 2.8V to the SYNC pin, or applying an

external clock to the SYNC pin.

While in FCM discontinuous mode operation is disabled

and the inductor current is allowed to go negative so that

the regulator can switch at the programmed frequency

all the way down to zero output current. This has the

advantage of maintaining the programmed switching

frequency across the entire load range so that the switch

harmonics and EMI are consistent and predictable. The

disadvantage of FCM is that the light load efficiency will

be low compared to Burst Mode operation.

At low input voltages when the part enters dropout, the

programmed switching frequency will be maintained

and off time skipping will not be allowed. This keeps the

switching frequency controlled, but the dropout voltage

will be higher than in burst mode, due to maximum duty

cycle constraints.

The negative inductor current is limited to a maximum of

about –2.5A, so the LT8650SP can only sink a maximum

of about –1.3A. This prevents boosting an excessive

amount of current back from the output to the input. FCM

is disabled if the input voltage is greater than 37V to pre-

vent overvoltaging the LT8650SP if the input capacitor is

charged when sinking current from the output. Additional

safety features include disabling FCM when the SS pin

voltage is below 1.8V to prevent discharging the output

when starting up into a pre-biased output, and a bottom

FET current limit to prevent over charging the output if

the minimum on time is violated.

Spread Spectrum Modulation

Spread spectrum modulation (SSM) is activated by tying

the SYNC pin to VCC or applying a DC voltage from 2.8V

to 4V to the SYNC pin. SSM reduces the EMI emissions

by modulating the switching frequency between the value

programmed by RT to approximately 20% higher than that

value. The switching frequency is modulated linearly up

and then linearly down at a 5kHz rate. This is an analog

function, so each switching period will be different than

the previous one. For example, when the LT8650SP is

programmed to 2MHz and the SSM feature is enabled,

the switching frequency will vary from 2MHz to 2.4MHz

at a 5kHz rate. When in SSM, the part will also operate in

forced continuous mode.

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