LT8640 datasheet(19/32 Pages) AD

19 / 32 page

LT8640S/LT8643S/

LT8640SA/LT8643SA

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

LT8643S/LT8640SA/LT8643SA may be synchronized over

a 200kHz to 3MHz range. The RT resistor should be cho-

sen to set the LT8640S/LT8643S/LT8640SA/LT8643SA

switching frequency equal to or below the lowest synchro-

nization input. For example, if the synchronization 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 synchro-

nization frequency will not change the slopes of the induc-

tor 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:

=R2

VOUT

0.970V

–1

⎛

⎝⎜

⎞

⎠⎟

(2)

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

For the LT8640S/LT8640SA, if low input quiescent cur-

rent and good light-load efficiency are desired, use large

resistor values for the FB resistor divider. The current

flowing in the divider acts as a load current, and will

increase the no-load input current to the converter, which

is approximately:

IQ =1.7µA+

VOUT

+R2

⎛

⎝⎜

⎞

⎠⎟

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

(3)

where 1.7µA is the quiescent current of the LT8640S/

LT8640SA and the second term is the current in the

feedback divider reflected to the input of the buck oper-

ating at its light load efficiency n. For a 3.3V application

with R1 = 1M and R2 = 412k, the feedback divider draws

2.3µA. With VIN = 12V and n = 80%, this adds 0.8µA to the

1.7µA quiescent current resulting in 2.5µA no-load cur-

rent from the 12V supply. Note that this equation implies

that the no-load current is a function of VIN; this is plotted

in the Typical Performance Characteristics section.

When using large FB resistors, a 4.7pF to 22pF phase-lead

capacitor should be connected from VOUT to FB.

High VOUT Considerations

Use the LT8640SA/LT8643SA for applications where

the output voltage is higher than 10V. For previous

designs using the LT8640S/LT8643S where the output

voltage is higher than 10V, refer to the Reverse Current

Considerations section in Application Note AN-2582.

Setting the Switching Frequency

The LT8640S/LT8643S/LT8640SA/LT8643SA uses a

constant frequency PWM architecture that can be pro-

grammed to switch from 200kHz to 3MHz by using a

resistor tied from the RT pin to ground. A table showing

the necessary RT value for a desired switching frequency

is in Table 1.

The RT resistor required for a desired switching frequency

can be calculated using:

RT =

46.5

fSW

– 5.2

(4)

where RT is in kΩ and fSW is the desired switching fre-

quency in MHz.

LT8640 Datasheet(PDF) 19 Page - Analog Devices

LT8640 Datasheet(HTML) 19 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 29 30 31 32

Datasheet Download

Link URL