LT8640 datasheet(20/32 Pages) AD

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LT8640S/LT8643S/

LT8640SA/LT8643SA

Rev. D

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APPLICATIONS INFORMATION

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 disad-

vantages are lower efficiency and a smaller input voltage

range.

The highest switching frequency (fSW(MAX)) for a given

application can be calculated as follows:

fSW(MAX)=

VOUT + VSW(BOT)

tON(MIN) VIN – VSW(TOP) + VSW(BOT)

(

)

(5)

where VIN is the typical input voltage, VOUT is the output

voltage, VSW(TOP) and VSW(BOT) are the internal switch

drops (~0.4V, ~0.15V, respectively at maximum load)

and tON(MIN) is the minimum top switch on-time (see the

Electrical Characteristics). This equation shows that a

slower switching frequency is necessary to accommodate

a high VIN/VOUT ratio.

For transient operation, VIN may go as high as the abso-

lute maximum rating of 42V regardless of the RT value,

however the LT8640S/LT8643S/LT8640SA/LT8643SA will

reduce switching frequency as necessary to maintain con-

trol of inductor current to assure safe operation.

The LT8640S/LT8643S/LT8640SA/LT8643SA is capable

of a maximum duty cycle of approximately 99%, and the

VIN-to-VOUT dropout is limited by the RDS(ON) of the top

switch. In this mode the LT8640S/LT8643S/LT8640SA/

LT8643SA skips switch cycles, resulting in a lower switch-

ing frequency than programmed by RT.

For applications that cannot allow deviation from the pro-

grammed switching frequency at low VIN/VOUT ratios use

the following formula to set switching frequency:

VIN(MIN) =

VOUT + VSW(BOT)

1– fSW • tOFF(MIN)

– VSW(BOT) + VSW(TOP) (6)

where VIN(MIN) is the minimum input voltage without

skipped cycles, VOUT is the output voltage, VSW(TOP) and

VSW(BOT) are the internal switch drops (~0.4V, ~0.15V,

respectively at maximum load), fSW is the switching

frequency (set by RT), and tOFF(MIN) is the minimum

switch off-time. Note that higher switching frequency will

increase the minimum input voltage below which cycles

will be dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8640S/LT8643S/LT8640SA/LT8643SA is designed

to minimize solution size by allowing the inductor to be

chosen based on the output load requirements of the

application. During overload or short-circuit conditions

the LT8640S/LT8643S/LT8640SA/LT8643SA safely toler-

ates operation with a saturated inductor through the use

of a high speed peak-current mode architecture.

A good first choice for the inductor value is:

VOUT + VSW(BOT)

fSW

⎛

⎝⎜

⎞

⎠⎟

• 0.7

(7)

where fSW is the switching frequency in MHz, VOUT is

the output voltage, VSW(BOT) is the bottom switch drop

(~0.15V) and L is the inductor value in µH.

To avoid overheating and poor efficiency, an inductor must

be chosen with an RMS current rating that is greater than

the maximum expected output load of the application.

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