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LT8645S-2

Rev. A

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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

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)

(

)

(4)

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

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

drops (~0.3V, ~0.2V, 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 65V regardless of the RT value,

however the LT8645S-2 will reduce switching frequency

as necessary to maintain control of inductor current to

assure safe operation.

The LT8645S-2 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 LT8645S-2 skips switch cycles, resulting in a lower

switching 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)

− fSW • tOFF(MIN)

− VSW(BOT) + VSW(TOP) (5)

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 (~0.3V, ~0.2V,

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 LT8645S-2 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 LT8645S-2 safely tolerates

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

⎛

⎝

⎜

⎞

⎠

⎟ •0.4

(6)

where fSW is the switching frequency in MHz, VOUT is

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

(~0.2V) and L is the inductor value in μH.

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