LT8636 datasheet(18/28 Pages) AD

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LT8640A

Rev. 0

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APPLICATIONS INFORMATION

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

application can be calculated with Equation 4.

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 LT8640A will reduce switching frequency

as necessary to maintain control of inductor current to

assure safe operation. Note that in FCM operation, VOUT

may increase by up to 8% while VIN is greater than what’s

allowed in Equation 4.

The LT8640A is capable of a maximum duty cycle of

approximately 99%, and the VIN-to-VOUT dropout is lim-

ited by the RDS(ON) of the top switch. In this mode the

LT8640A skips switch cycles, resulting in a lower switch-

ing frequency than programmed by RT. The maximum

on-time of the top switch is approximately 14µs.

For applications that cannot allow deviation from the pro-

grammed switching frequency at low VIN/VOUT ratios use

Equation 5 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 LT8640A 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 LT8640A 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 given by

Equation 6.

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 (Equation 7).

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 LT8640A limits the peak switch current in order

to protect the switches and the system from overload

faults. The top switch current limit (ILIM) is 12A at low

duty cycles and decreases linearly to 9.5A at DC = 0.8.

The inductor value must then be sufficient to supply the

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

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