LT8625SP datasheet(21/39 Pages) AD

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LT8625SP/LT8625SP-1

Rev. C

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Input Capacitors

The PVIN of the LT8625SP/LT8625SP-1 should be

bypassed with at least three ceramic capacitors for best

performance. Two small ceramic capacitors can be placed

close to the part (COPT1, COPT2). These capacitors should

be 0402 or 0603 in size.

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 LT8625SP/LT8625SP-1 is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT8625SP/LT8625SP-1’s

voltage rating. This situation is easily avoided (see Analog

Device’s Application Note 88). When SVIN and PVIN are

powered from the same supply, a small RC filter (e.g.,

10Ω and 1µF) from the supply to SVIN can be added for

particularly noise sensitive applications. If SVIN and PVIN

are powered from independent supplies, SVIN should also

be bypassed with a single small ceramic capacitor of at

least 1µF placed as close to the pin as possible.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT8625SP/LT8625SP-1 to produce the DC output. In

this role it determines the output ripple, thus low imped-

ance at the switching frequency is important. The second

function is to store energy in order to satisfy transient

loads and stabilize the LT8625SP/LT8625SP-1’s con-

trol loop. Ceramic capacitors 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.

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 per-

formance 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.

The LT8625SP/LT8625SP-1 will typically be operated

at a switching frequency of 2MHz. Table 2 shows some

examples of output capacitors with ideal frequency char-

acteristics for when operating at switching frequencies

around 2MHz. Figure 6 shows the frequency character-

istics of these capacitors; it can be seen that a combi-

nation of these capacitors will minimize the impedance

at the switching frequency on the output and keep the

impedance low enough to suppress any higher frequency

harmonics near the switching frequency, thus achieving

the lowest output ripple.

Table 2. Examples of Output Capacitors with Desirable

Frequency Characteristics for 2MHz Operation

PART DESCRIPTION

MANUFACTURER/PART NUMBER

22µF, X7R, 10V, 20% 1206

MURATA, GRM31CR71A226ME15

10µF, X7R, 25V, 10% 1206

MURATA, GRM31CR71E106KA12

4.7µF, X7S, 16V, 10% 0603

MURATA, GRM188C71C475KE21

GRM31CR71A226ME15K (22µF)

GRM31CR71E106KA12 (10µF)

GRM188C71C475KE21 (4.7µF)

FREQUENCY (Hz)

100

10k

100k

10M 100M

10m

100m

100

10k

8625sp1 F06

Figure 6. Frequency Characteristics of Example

Output Capacitors for 2MHz operation

APPLICATIONS INFORMATION

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