LT8640 datasheet(19/32 Pages) AD

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LT8636/LT8637

Rev. C

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APPLICATIONS INFORMATION

The LT8636/LT8637 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 regulation. The LT8636/LT8637 may be syn-

chronized over a 200kHz to 3MHz range. The RT resistor

should be chosen to set the LT8636/LT8637 switching

frequency equal to or below the lowest synchronization

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 sub-

harmonic oscillations is established by the inductor size,

input voltage and output voltage. Since the synchroniza-

tion 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 synchro-

nization 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

⎛

⎝⎜

⎞

⎠⎟

(1)

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

For the LT8636/LT8637, if low input quiescent current 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

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

(2)

where 1.7µA is the quiescent current of the LT8636/

LT8637 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.

Setting the Switching Frequency

The LT8636/LT8637 uses a constant frequency PWM

architecture 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 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

(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

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