LT8640 datasheet(17/28 Pages) AD

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LT8638S

Rev. 0

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APPLICATIONS INFORMATION

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.

FRONT PAGE APPLICATION

100mA to 5.1A TRANSIENT

12VIN, 5VOUT, fSW = 1MHz

Cc = 220pF, RC = 12.1k

COUT = 47µF, CLEAD = 15pF

FCM

Burst Mode OPERATION

50µs/DIV

ILOAD

5A/DIV

VOUT

100mV/DIV

8638S F04

Figure 4. LT8638S Load Step Transient Response with

and without Forced Continuous Mode

FCM is disabled if the VIN pin is held above 37V or if the FB

pin is held greater than 7.75% 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 LT8638S operates in pulse-skipping mode.

Spread Spectrum Mode

The LT8638S features spread spectrum operation to

further reduce EMI emissions. To enable spread spec-

trum operation, the SYNC/MODE pin should be tied high

to INTVCC (or >3V). In this mode, triangular frequency

modulation is used to vary the switching frequency

between the value programmed by RT to approximately

24% higher than that value. The modulation frequency is

approximately 3kHz. For example, when the LT8638S is

programmed to 2MHz, the frequency will vary from 2MHz

to approximately 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 LT8638S 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 0.7V and peaks above 1.5V (up to 6V) with a min-

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

The LT8638S 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 reg-

ulation. The LT8638S may be synchronized over a 200kHz

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

the LT8638S 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.

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistor

values according to E1.

R1=R2

VOUT

0.6V

–1

⎛

⎝⎜

⎞

⎠⎟

(1)

Reference designators refer to the Block Diagram.

1% resistors are recommended to maintain output

voltage accuracy.

When using large FB resistors, a 4.7pF to 47pF phase-lead

capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8638S uses a constant frequency PWM architec-

ture that can be programmed to switch from 200kHz to

3MHz by using a resistor tied from the RT pin to ground. A

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