LT8640 datasheet(16/22 Pages) AD

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LT8644S

Rev. 0

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APPLICATIONS INFORMATION

where fSW is the switching frequency of the LT8644S, and

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

output current that the LT8644S 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 LT8644S 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 ADI Application Note 44.

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

minimum inductance is required to avoid sub-harmonic

oscillation (see Equation 9). See Application Note 19 for

more details.

LMIN =

PVIN

6 • fSW

2 • DC – 1

(

)

(9)

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

switching frequency.

Input Capacitors

The VIN of the LT8644S should be bypassed with at least

three ceramic capacitors for best performance. Two small

ceramic capacitors of <1µF can be placed close to the

part; one on each side of the device (COPT1, COPT2). 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 LT8644S

near the PVIN and PGND pins.

A third, larger ceramic capacitor of 22µF or larger should

be placed close to COPT1 or COPT2. 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

circuit. If the LT8644S circuit is plugged into a live sup-

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

possibly exceeding the LT8644S’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

the LT8644S to produce the DC output. In this role it

determines the output ripple, thus low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT8644S’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and

provide the best ripple performance. For good starting

values, see the Typical Application section.

Use X5R or X7R types. This choice will provide low out-

put ripple and good transient response. Transient perfor-

mance can be improved with a higher value output capaci-

tor and the addition of a feedforward capacitor placed

between VOUT and FB. Increasing the output capacitance

will also decrease the output voltage ripple. A lower value

of output capacitor can be used to save space and cost

but transient performance will suffer and may cause loop

instability. See the Typical Application in this data sheet

for suggested capacitor values.

When choosing a capacitor, special attention should be

given to the data sheet to calculate the effective capaci-

tance under the relevant operating conditions of voltage

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