LT8640 datasheet(18/26 Pages) AD

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LT8642-1

Rev. 0

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APPLICATIONS INFORMATION

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

switching frequency.

Input Capacitors

The VIN of the LT8642-1 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 LT8642-1

near the VIN and GND pins.

A third, larger ceramic capacitor of 4.7µF or larger should

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

detail. X7R or X5R capacitors are recommended for best per-

formance 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 LT8642-1 circuit is plugged into a live sup-

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

possibly exceeding the LT8642-1’s voltage rating. This

situation is easily avoided (see Analog Devices 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 LT8642-1 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 LT8642-1’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and pro-

vide the best ripple performance. For good starting val-

ues, see the Typical Applications 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 capac-

itor 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 Applications 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

bias and temperature. A physically larger capacitor or one

with a higher voltage rating may be required.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT8642-1 due to their piezoelectric

nature. When in Burst Mode operation, the LT8642-1’s

switching frequency depends on the load current, and

at very light loads the LT8642-1 can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT8642-1 operates at a lower current limit

during Burst Mode operation, the noise is typically very

quiet to a casual ear. If this is unacceptable, use a high

performance tantalum or electrolytic capacitor at the out-

put. Low noise ceramic capacitors are also available.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT8642-1. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high quality

(underdamped) tank circuit. If the LT8642-1 circuit is

plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT8642-

1’s rating. This situation is easily avoided (see Analog

Devices Application Note 88).

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