LT8636 datasheet(19/28 Pages) AD

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LT8640A

Rev. 0

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APPLICATIONS INFORMATION

a function of the switch current limit (ILIM) and the rip-

ple current (Equation 8).

IOUT(MAX) = ILIM –

ΔIL

(8)

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

calculated Equation 9.

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(9)

where fSW is the switching frequency of the LT8640A, and

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

output current that the LT8640A 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.

In order to achieve higher light load efficiency, more energy

must be delivered to the output during the single small

pulses in Burst Mode operation such that the LT8640A

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 independent of switch-

ing frequency when choosing an inductor. For example,

while a lower inductor value would typically 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 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 LT8640A 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 discontinu-

ous mode operation, which further reduces maximum

load current.

For more information about maximum output current and

discontinuous operation, see ADI’s 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)

4 • fSW

(10)

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

switching frequency.

Input Capacitors

The VIN of the LT8640A 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 at the VIN1/GND1 pins and a second at VIN2/GND2

pins. 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

LT8640A near the VIN1/GND1 and VIN2/GND2 pins.

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

be placed close to VIN1 or VIN2. See Low EMI PCB Layout

section for more detail. X7R or X5R capacitors are rec-

ommended 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

circuit. If the LT8640A circuit is plugged into a live sup-

ply, the input voltage can ring to twice its nominal value,

possibly exceeding the LT8640A’s voltage rating. This

situation is easily avoided (see ADI Application Note 88).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

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