Data Sheet

LT8641A

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Rev 0

23 of 35

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

disadvantages are lower efficiency and a smaller input voltage range.

Calculate the highest switching frequency (fSW(MAX)) for a given application 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, ~0.2V, respectively at maximum load), and tON(MIN) is the minimum top switch on-time (see the Specifications).

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 absolute maximum rating of 65V regardless of the RT value, however

the LT8641A reduces switching frequency as necessary to maintain control of inductor current to assure safe

operation.

The LT8641A 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 LT8641A skips switch cycles, resulting in a lower switching frequency than

programmed by RT.

For applications that cannot allow deviation from the programmed 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, ~0.2V, 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 increases the minimum input voltage,

below which cycles are dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8641A 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 LT8641A safely tolerates operation

with a saturated inductor due to the use of a high-speed peak-current mode architecture.

A good first choice for the inductor value is:

L =

VOUT + VSW(BOT)

fSW

(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, choose an inductor with an RMS current rating 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 the inductor ripple current:

IL(PEAK) = ILOAD(MAX) +

1

2

ΔIL

(7)