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LT8620

8620fa

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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

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, ~0.15V, 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 65V regardless of the RT value,

however the LT8620 will reduce switching frequency as

necessary to maintain control of inductor current to as-

sure safe operation.

The LT8620 is capable of a maximum duty cycle of ap-

proximately 99%, and the VIN-to-VOUT dropout is limited

by the RDS(ON) of the top switch. In this mode the LT8620

skipsswitchcycles,resultinginalowerswitchingfrequency

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, ~0.15V,

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 LT8620 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-circuitconditionstheLT8620safelytoleratesopera-

tion 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

(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.

Toavoidoverheatingandpoorefficiency,aninductormust

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) +

∆IL

(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.

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