LT8640 datasheet(13/22 Pages) AD

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LT8644S

Rev. 0

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Figure 4. Load Step Transient Response

with and without Forced Continuous Mode

APPLICATIONS INFORMATION

Forced Continuous Mode

The LT8644S can operate in forced continuous mode

(FCM) for fast transient response and full frequency

operation over a wide load range. When in FCM, the

oscillator operates continuously and positive SW transi-

tions are aligned to the clock. Negative inductor current

is allowed at light loads or under large transient condi-

tions. The LT8644S can sink current from the output

and return this charge to the input in this mode, improv-

ing load step transient response (see Figure 4). At light

loads, FCM operation is less efficient than Burst Mode

operation, but may be desirable in applications where it

is necessary to keep switching harmonics out of the sig-

nal band. FCM must be used if the output is required to

sink current. To enable FCM, float the SYNC/MODE pin.

Leakage current on this pin should be <1µA. See Block

Diagram for internal pull-up and pull-down resistance.

FCM is disabled if the PVIN pin is held above 8.5V or if

the FB pin is held greater than 8% above the feedback

reference voltage. FCM is also disabled during soft-

start until the soft-start capacitor is fully charged (~2V).

When FCM is disabled in these ways, negative induc-

tor current is not allowed and the LT8644S operates in

pulse-skipping mode.

Spread Spectrum Mode

The LT8644S features spread spectrum operation to fur-

ther reduce EMI emissions. To enable spread spectrum

operation, the SYNC/MODE pin should be tied to INTVCC.

In this mode, triangular frequency modulation is used

to vary the switching frequency between the value pro-

grammed by RT to approximately 25% higher than that

value. The modulation frequency is approximately 3kHz.

For example, when the LT8644S is programmed to 2MHz,

the frequency will vary from 2MHz to 2.5MHz at a 3kHz

rate. When spread spectrum operation is selected, Burst

Mode operation is disabled, and the part will run in forced

continuous mode.

Synchronization

To synchronize the LT8644S oscillator to an external fre-

quency, connect a square wave to the SYNC/MODE pin.

The square wave amplitude should have valleys that are

below 0.4V and peaks above 1.5V (up to 6V), with a mini-

mum on-time and off-time of 50ns.

The LT8644S will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will run forced continuous mode to maintain regu-

lation. The LT8644S may be synchronized over a 300kHz

to 3.5MHz range. The RT resistor should be chosen to set

the LT8644S switching frequency equal to or below the

lowest 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.

FRONT PAGE APPLICATION

100mA TO 8.1A TRANSIENT

VIN = 5V, VOUT = 1.2V, fSW = 2MHz

CC = 390pF, RC = 13.3k

COUT = 2 x 220µF, CLEAD = 33pF

Burst Mode OPERATION

FCM

50µs/DIV

ILOAD

50mA/DIV

VOUT

50mV/DIV

8644S F04

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