LT8613 datasheet(14/24 Pages) LINER

14 / 24 page

For more information www.linear.com/LT8613

LT8613

8613f

applicaTions inForMaTion

The LT8613 is capable of a maximum duty 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 LT8613 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, ~0.18V,

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 LT8613 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-circuitconditionstheLT8613safelytoleratesopera-

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:

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

(~0.18V) 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.

As a quick example, an application requiring 4A output

should use an inductor with an RMS rating of greater than

4A and an ISAT of greater than 5A. During long duration

overload or short-circuit conditons, the inductor RMS is

greater to avoid overheating of the inductor. To keep the

efficiency high, the series resistance (DCR) should be less

than 0.020Ω, and the core material should be intended

for high frequency applications.

The LT8613 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 7.5A at low

duty cycles and decreases linearly to 6A 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 –

∆IL

(8)

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

calculated as follows:

∆IL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)













(9)

where fSW is the switching frequency of the LT8613, and

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

output current that the LT8613 will deliver depends on

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

and output voltages. The inductor value may have to be

increased if the inductor ripple current does not allow

sufficient maximum output current (IOUT(MAX)) given the

switching frequency, and maximum input voltage used in

the desired application.

The optimum inductor for a given application may differ

from the one indicated by this design guide. A larger value

inductor provides a higher maximum load current and

reduces the output voltage ripple. For applications requir-

ing smaller load currents, the value of the inductor may

be lower and the LT8613 may operate with higher ripple

LT8613 датащи(PDF) 14 Page - Linear Technology

LT8613 датащи(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL