LT8640 datasheet(20/28 Pages) AD

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LT8638S

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Input Capacitors

The VIN of the LT8638S 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

(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 LT8638S near the VIN and GND pins.

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

be placed close to COPT1 or COPT2. See Low EMI PCB

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 LT8638S circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT8638S device’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 LT8638S 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 sta-

bilize the LT8638S device’s control loop. Ceramic capaci-

tors have very low equivalent series resistance (ESR) and

provide the best ripple performance. For good starting

values, see the Typical Applications section.

Use X5R or X7R types. This choice will provide low output

ripple and good transient response. Transient performance

can be improved with a higher value output capacitor 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 sug-

gested 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 LT8638S due to their piezoelectric

nature. When in Burst Mode operation, the LT8638S’s

switching frequency depends on the load current, and

at very light loads the LT8638S can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT8638S 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 output. Low noise

ceramic capacitors are also available.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT8638S. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high qual-

ity (underdamped) tank circuit. If the LT8638S circuit is

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

twice its nominal value, possibly exceeding the LT8638S

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

Devices Application Note 88).

Enable Pin

The LT8638S is in shutdown when the EN pin is low and

active when the pin is high. The rising threshold of the EN

comparator is 0.98V, with 40mV of hysteresis. The EN pin

can be tied to VIN if the shutdown feature is not used, or

tied to a logic level if shutdown control is required.

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