LT8636 datasheet(21/32 Pages) AD

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LT8636/LT8637

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

IOUT(MAX) =ILIM –

ΔIL

(8)

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

calculated as follows:

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(9)

where fSW is the switching frequency of the LT8636/

LT8637, and L is the value of the inductor. Therefore, the

maximum output current that the LT8636/LT8637 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 cur-

rent does not allow sufficient maximum output current

(IOUT(MAX)) given the switching frequency, and maximum

input voltage used in the desired application.

In order to achieve higher light load efficiency, more

energy must be delivered to the output during the sin-

gle small pulses in Burst Mode operation such that the

LT8636/LT8637 can stay in sleep mode longer between

each pulse. This can be achieved by using a larger value

inductor (i.e., 4.7µH), and should be considered indepen-

dent of switching frequency when choosing an inductor.

For example, while a lower inductor value would typi-

cally be used for a high switching frequency application,

if high light load efficiency is desired, a higher inductor

value should be chosen. See curve in Typical Performance

Characteristics.

The optimum inductor for a given application may dif-

fer 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

requiring smaller load currents, the value of the induc-

tor may be lower and the LT8636/LT8637 may operate

with higher ripple current. This allows use of a physically

smaller inductor, or one with a lower DCR resulting in

higher efficiency. Be aware that low inductance may result

in discontinuous mode operation, which further reduces

maximum load current.

For more information about maximum output current and

discontinuous operation, see Analog Devices Application

Note 44.

For duty cycles greater than 50% (VOUT/VIN > 0.5), a

minimum inductance is required to avoid subharmonic

oscillation (See Equation 10). See Application Note 19

for more details.

LMIN =

VIN 2 •DC – 1

(

)

3.5 • fSW

(10)

where DC is the duty cycle ratio (VOUT/VIN) and fSW is the

switching frequency.

Input Capacitors

The VIN of the LT8636/LT8637 should be bypassed with at

least three ceramic capacitors for best performance. Two

small ceramic capacitors of 1µF should be placed close to

the part; one on each side of the device (CIN1, CIN2). These

capacitors should be 0402 or 0603 in size. For automotive

applications requiring 2 series input capacitors, two small

0402 or 0603 may be placed at each side of the LT8636/

LT8637 near the VIN and GND pins.

A third, larger ceramic capacitor of 2.2µF or larger should

be placed close to CIN1 or CIN2. See layout section for

more detail. X7R or X5R capacitors are recommended for

best performance across temperature and input voltage

variations.

Note that larger input capacitance is required when a lower

switching frequency is used. If the input power source has

high impedance, or there is significant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic capacitor.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank cir-

cuit. If the LT8636/LT8637 circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT8636/LT8637’s voltage

rating. This situation is easily avoided (see Analog Devices

Application Note 88).

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