LT8640 datasheet(18/32 Pages) AD

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LT8636/LT8637

Rev. C

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APPLICATIONS INFORMATION

FCM is disabled if the VIN pin is held above 37V 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. When FCM is disabled

in these ways, negative inductor current is not allowed

and the LT8636/LT8637 operates in pulse-skipping mode.

For robust operation over a wide VIN and VOUT range, use

an inductor value greater than LMIN:

LMIN =

VOUT

2 • fSW

• 1–

VOUT

VIN,MAX

⎛

⎝⎜

⎞

⎠⎟

Spread Spectrum Mode

The LT8636/LT8637 features spread spectrum opera-

tion to further reduce EMI emissions. To enable spread

spectrum operation, the SYNC/MODE pin should be tied

high to INTVCC (~3.4V)or an external supply of 3V to 4V.

In this mode, triangular frequency modulation is used

to vary the switching frequency between the value pro-

grammed by RT to approximately 20% higher than that

value. The modulation frequency is approximately 3kHz.

For example, when the LT8636/LT8637 is programmed to

2MHz, the frequency will vary from 2MHz to 2.4MHz 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 LT8636/LT8637 oscillator to an exter-

nal frequency, 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

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

The output load at which the LT8636/LT8637 reaches the

programmed frequency varies based on input voltage,

output voltage and inductor choice. To select low ripple

Burst Mode operation, tie the SYNC/MODE pin below 0.4V

(this can be ground or a logic low output).

Forced Continuous Mode

The LT8636/LT8637 can operate in forced continuous

mode (FCM) for fast transient response and full fre-

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

the oscillator operates continuously and positive SW

transitions are aligned to the clock. Negative inductor

current is allowed at light loads or under large tran-

sient conditions. The LT8636/LT8637 can sink current

from the output and return this charge to the input in

this mode, improving 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 signal 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.

Figure 4. LT8636 Load Step Transient Response with

and without Forced Continuous Mode

FRONT PAGE APPLICATION

100mA TO 1.1A TRANSIENT

12VIN, 5VOUT, fSW = 1MHz

COUT = 100µF

Burst Mode OPERATION

FCM

50µs/DIV

ILOAD

1A/DIV

VOUT

100mV/DIV

8636 F04

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