LT8650SPJVPBF datasheet(18/34 Pages) AD

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LT8650SP

Rev. B

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APPLICATIONS INFORMATION

InBurstMode,theLT8650SPiscapableofamaximumduty

cycle of greater than 99%, and the VIN to VOUT dropout is

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

channel that enters dropout skips switch cycles, resulting

inalowerswitchingfrequency.Inforcedcontinuousmode,

the LT8650SP will not skip cycles to achieve a higher duty

cycle. The part will maintain the programmed switching

frequency and the dropout voltage will be larger due to

the smaller maximum duty cycle.

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)

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, ~0.12V,

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.

Note there is no minimum VIN2 voltage requirement as it

does not supply the internal common bias circuits, making

the channel 2 uniquely capable of operating from very low

input voltages as long as VIN1hasasupplyof3Vorgreater.

Inductor Selection and Maximum Output Current

The LT8650SP 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 LT8650SP 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:

L1,2=

VOUT1,2 +VSW(BOT)

2fSW

where fSW is the switching frequency in MHz, VOUT is the

outputvoltage,VSW(BOT)isthebottomswitchdrop(~0.12V)

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

where ∆IL is the inductor ripple current as calculated in

Equation 1 and ILOAD(MAX) is the maximum output load

for a given application.

As a quick example, an application requiring 1A output

should use an inductor with an RMS rating of greater than

1A and an ISAT of greater than 1.3A. During long duration

overloadorshort-circuitconditions,theinductorRMSrat-

ingrequirementmustbegreatertoavoidoverheatingofthe

inductor. To keep the efficiency high, the series resistance

(DCR) should be less than 0.04Ω, and the core material

should be intended for high frequency applications.

The LT8650SP limits the peak switch current in order to

protect the switches and the system from overload faults.

The top switch current limit (ILIM) is at least 13A at low

duty cycles and decreases linearly to 10A at DC = 0.8. The

inductorvaluemustthenbesufficienttosupplythedesired

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

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

IOUT(MAX) =ILIM –

ΔIL

The peak-to-peak ripple current in the inductor can be

calculated as follows:

IL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

(1)

where fSW is the switching frequency of the LT8650SP,

and L is the value of the inductor. Therefore, the maximum

output current that the LT8650SP will deliver depends on

the switch current limit, the inductor value, and the input

and output voltages.

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