LP87702 datasheet(80/98 Pages) TI

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8.2.2.1.3 Buck Output Capacitor Selection

Section 8.2 shows the output capacitor COUT0 and COUT1. A ceramic local output capacitor of 22 μF is

required for both outputs. Use ceramic capacitors, X7R or X7T types; do not use Y5V or F. DC bias voltage

characteristics of ceramic capacitors must be considered. The output filter capacitor smooths out the current flow

from the inductor to the load, whic helps maintain a steady output voltage during transient load changes and

reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low

ESR and ESL to perform these functions. Minimum effective output capacitance for good performance is 15 μF

for each buck, including the DC voltage roll-off, tolerances, aging, and temperature effects.

The output voltage ripple is caused by charging and discharging the output capacitor, and is also due to its RESR.

Table 8-5 shows how the RESR is frequency dependent (as well as temperature dependent); make sure the value

used for selection process is at the part's switching frequency.

POL capacitors can be used to improve load transient performance and to decrease the ripple voltage. A higher

output capacitance improves the load step behavior, reduces the output voltage ripple, and decreases the PFM

switching frequency. Note: the output capacitor may be the limiting factor in the output voltage ramp, especially

for very large (100-μF range) output capacitors. The output voltage might be slower than the programmed

ramp rate at voltage transitions for large output capacitors, because of the higher energy stored on the output

capacitance. Also, the time required to charge the output capacitor to target value might be longer at start-up.

The output voltage is discharged to 0.6 V level using forced-PWM operation at shutdown. This can increase the

input voltage if the load current is small and the output capacitor is large compared to the input capacitor. The

output capacitor is discharged by the internal discharge resistor when below the 0.6 V level, and more time is

required to settle VOUT down with a large capacitor because of the increased time constant.

Table 8-5. Recommended Buck Output Capacitors (X7R or X7T Dielectric)

MANUFACTURER

PART NUMBER

VALUE

CASE SIZE

DIMENSIONS LxWxH

(mm)

VOLTAGE RATING

Murata

GRM21BD71A226ME44

22 µF (10%)

0805

2 × 1.25 × 1.25

10 V

TDK

C2012X7S1A226M125AC

22 µF (20%)

0805

2 × 1.25 × 1.25

10 V

8.2.2.1.4 Boost Input Capacitor Selection

A ceramic input capacitor of 10 μF is sufficient for most applications. Place the input capacitor close to the

SW_BST pin of the device. Use X7R types, do not use Y5V or F. See Table 8-6.

Table 8-6. Recommended Boost Input Capacitors (X7R Dielectric)

MANUFACTURER

PART NUMBER

VALUE

CASE SIZE

DIMENSIONS L×W×H

(mm)

VOLTAGE RATING

Murata

GRM21BR71A06KA73

10 µF (10%)

0805

2.0 × 1.25 × 1.25

10 V

8.2.2.1.5 Boost Output Capacitor Selection

Use ceramic capacitors, X7R or X7T types; do not use Y5V or F. Place the output capacitor as close as possible

to the device's VOUT_BST pin and PGND_BST pin. DC bias voltage characteristics of ceramic capacitors must

be considered. DC bias characteristics vary from manufacturer to manufacturer, and DC bias curves should

be requested from them as part of the capacitor selection process. These capacitors must be selected with

sufficient capacitance and sufficiently low ESR and ESL to support load transients. See Table 8-7.

Table 8-7. Recommended Boost Output Capacitors (X7R or X7T Dielectric)

MANUFACTURER

PART NUMBER

VALUE

CASE SIZE

DIMENSIONS L×W×H (mm)

VOLTAGE RATING

Murata

GRM21BD71A226ME44

22 µF (10%)

0805

2 × 1.25 × 1.25

10 V

LP87702

SNVSBU3 – MARCH 2021

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