LT8646SEVPBF datasheet(19/30 Pages) AD

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LT8645S/LT8646S

Rev. B

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APPLICATIONS INFORMATION

a 3kHz rate. When spread spectrum operation is selected,

Burst Mode operation is disabled, and the part will run in

pulse-skipping mode.

Synchronization

To synchronize the LT8645S/LT8646S oscillator to an

external frequency, connect a square wave to the SYNC/

MODEpin.Thesquarewaveamplitudeshouldhavevalleys

that are below 0.4V and peaks above 1.5V (up to 6V), with

a minimum on-time and off-time of 50ns.

The LT8645S/LT8646S will not enter Burst Mode opera-

tion at low output loads while synchronized to an external

clock, but instead will pulse-skip to maintain regulation.

TheLT8645S/LT8646Smaybesynchronizedovera200kHz

to 2.2MHz range. The RT resistor should be chosen to set

the LT8645S/LT8646S switching frequency equal to or

belowthelowestsynchronizationinput.Forexample,ifthe

synchronization signal will be 500kHz and higher, the RT

should be selected for 500kHz. The slope compensation is

setbytheRTvalue,whiletheminimumslopecompensation

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.

TheLT8645S/LT8646Sdoesnotoperateinforcedcontinu-

ous mode regardless of SYNC/MODE signal.

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

0.97V

− 1

⎛

⎝⎜

⎞

⎠⎟

(1)

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

For the LT8645S, if low input quiescent current and good

light-load efficiency are desired, use large resistor values

fortheFBresistordivider.Thecurrentflowinginthedivider

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 LT8645S 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 1pF to 10pF phase-lead

capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8645S/LT8646S uses a constant frequency PWM

architecture that can be programmed to switch from

200kHz to 2.2MHz 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.

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