LT8640 datasheet(16/28 Pages) AD

16 / 28 page

LT8640A

Rev. 0

For more information www.analog.com

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 LT8640A operates in pulse-skipping mode.

Spread Spectrum Mode

The LT8640A 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 (~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 LT8640A 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 LT8640A 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 LT8640A 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 LT8640A may be synchronized over a 200kHz

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

the LT8640A 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

Figure 3. Burst Mode Operation

Figure 4. Load Step Transient Response with

and without Forced Continuous Mode

Forced Continuous Mode

The LT8640A 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 LT8640A 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

Burst MODE

FRONT PAGE APPLICATION

100mA to 1.1A TRANSIENT

12VIN, 5VOUT

COUT = 47µF

50µs/DIV

VOUT

200mV/DIV

ILOAD

1A/DIV

8640a F04

FRONT PAGE APPLICATION

12VIN TO 5VOUT AT 10mA

VSYNC = 0V

5µs/DIV

VSW

5V/DIV

500mA/DIV

8640a F03

LT8640 Datasheet(PDF) 16 Page - Analog Devices

LT8640 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL