LT8640 datasheet(20/32 Pages) AD

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LT8636/LT8637

Rev. C

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APPLICATIONS INFORMATION

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 disadvan-

tages 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 (~0.4V, ~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 LT8636/LT8637 will reduce switching fre-

quency as necessary to maintain control of inductor cur-

rent to assure safe operation.

The LT8636/LT8637 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

LT8636/LT8637 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 (~0.4V, ~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 LT8636/LT8637 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 LT8636/LT8637 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:

L =

VOUT + VSW(BOT)

fSW

⎛

⎝

⎜

⎞

⎠

⎟ • 0.7

(6)

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.

In addition, the saturation current (typically labeled ISAT)

rating of the inductor must be higher than the load current

plus 1/2 of in inductor ripple current:

IL(PEAK) =ILOAD(MAX) +

ΔIL

(7)

where

∆IL is the inductor ripple current as calculated in

Equation 9 and ILOAD(MAX) is the maximum output load

for a given application.

As a quick example, an application requiring 3A output

should use an inductor with an RMS rating of greater than

3A and an ISAT of greater than 4A. During long duration

overload or short-circuit conditions, the inductor RMS

rating requirement is greater to avoid overheating of the

inductor. To keep the efficiency high, the series resistance

(DCR) should be less than 0.02Ω, and the core material

should be intended for high frequency applications.

The LT8636/LT8637 limits the peak switch current in

order to protect the switches and the system from over-

load faults. The top switch current limit (ILIM) is 10A at

low duty cycles and decreases linearly to 7A at DC = 0.8.

The inductor value must then be sufficient to supply the

desired maximum output current (IOUT(MAX)), which is a

function of the switch current limit (ILIM) and the ripple

current.

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