LT8640 datasheet(17/28 Pages) AD

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LT8640A

Rev. 0

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APPLICATIONS INFORMATION

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 Equation 1.

= R2

VOUT

0.970V

– 1

⎛

⎝⎜

⎞

⎠⎟

(1)

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

If low input quiescent current and good light-load effi-

ciency 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 given by Equation 2.

IQ = 1.7µA +

VOUT

+R2

⎛

⎝⎜

⎞

⎠⎟

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

(2)

where 1.7µA is the quiescent current of the LT8640A and

the second term is the current in the feedback divider

reflected to the input of the buck operating 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 current 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.

The fixed output versions of the LT8640A have the feed-

back resistor network and phase lead capacitor integrated

within the part. The FB pin is replaced with a VOUT pin for

these regulators. The VOUT pin can be connected directly

to the inductor and output capacitor. The LT8640A-3.3

regulates to 3.3V and has a total of 11.33MΩ of internal

feedback divider resistance from the VOUT pin to ground.

Setting the Switching Frequency

The LT8640A 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

table showing the necessary RT value for a desired switch-

ing frequency is in Table 1.

The RT resistor required for a desired switching frequency

can be calculated using Equation 3.

RT =

46.5

fSW

– 5.2

(3)

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

quency in MHz.

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

232

0.3

150

0.4

110

0.5

88.7

0.6

71.5

0.7

60.4

0.8

52.3

1.0

41.2

1.2

33.2

1.4

28.0

1.6

23.7

1.8

20.5

2.0

17.8

2.2

15.8

3.0

10.7

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off

between efficiency, component size, and input voltage

range. The advantage of high frequency operation is that

smaller inductor and capacitor values may be used. The

disadvantages are lower efficiency and a smaller input

voltage range.

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