LT8613 datasheet(20/30 Pages) AD

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LT8645S/LT8646S

Rev. B

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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

advantageofhighfrequencyoperationisthatsmallerinduc-

tor and capacitor values may be used. The disadvantages

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 Elec-

trical 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/LT8646S will reduce switching

frequency as necessary to maintain control of inductor

current to assure safe operation.

The LT8645S/LT8646S 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/LT8646S 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)

1– 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 fre-

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

off-time.Notethathigherswitchingfrequencywillincrease

the minimum input voltage below which cycles will be

dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8645S/LT8646S is designed to minimize solution

size by allowing the inductor to be chosen based on the

output load requirements of the application. During over-

load or short-circuit conditions the LT8645S/LT8646S

safelytoleratesoperationwithasaturatedinductorthrough

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

outputvoltage,VSW(BOT)isthebottomswitchdrop(~0.2V)

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. In

addition, the saturation current (typically labeled ISAT)

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