LTM8051

16

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Capacitor Selection Considerations

mum recommended values for the associated operating

conditions. Applying capacitor values below those indi-

cated in Table 1 is not recommended and may result in

undesirable operation. Using larger values is generally

acceptable, and can yield improved dynamic response,

if it is necessary. Again, it is incumbent upon the user to

verify proper operation over the intended system’s line,

load and environmental conditions.

Ceramic capacitors are small, robust and have very low

ESR. However, not all ceramic capacitors are suitable. X5R

and X7R types are stable over temperature and applied

voltage and give dependable service. Other types, includ-

ing Y5V and Z5U have very large temperature and voltage

coefficients of capacitance. In an application circuit they

may have only a small fraction of their nominal capaci-

tance resulting in much higher output voltage ripple

than expected.

(V)

(kΩ)

BIAS

(kHz)

(kHz)

(kΩ)

0.8

Open

1µF 50V X5R 0805 2 x 100uF 4V X5R 0805

3.2V to 10V

47pF

450

75

1200

24.9

1

1000

1µF 50V X5R 0805 2 x 100uF 4V X5R 0805

3.2V to 10V

33pF

550

60.4

1400

21

1.2

499

1µF 50V X5R 0805 2 x 100uF 4V X5R 0805

3.2V to 10V

22pF

650

49.9

1400

21

1.5

287

1µF 50V X5R 0805 2 x 100uF 4V X5R 0805

3.2V to 10V

22pF

800

40.2

1400

21

1.8

200

1µF 50V X5R 0805

100uF 4V X5R 0805

3.2V to 10V

-

800

40.2

1800

15

2

165

1µF 50V X5R 0805

100uF 4V X5R 0805

3.2V to 10V

-

900

34.8

1800

15

2.5

118

1µF 50V X5R 0805

100uF 4V X5R 0805

3.2V to 10V

-

1100

27.4

2000

13.3

3.3

78.7

1µF 50V X5R 0805

100uF 4V X5R 0805

3.2V to 10V

-

1200

24.9

2800

8.06

5

47.5

1µF 50V X5R 0805

47uF 6.3V X5R 0805

3.2V to 10V

-

1300

22.1

3000

7.15

8

27.4

1µF 50V X5R 0805

22uF 10V X5R 0805

3.2V to 10V

-

1700

16.5

3000

7.15

Note 1: The LTM8051 may be capable of the operating at lower input voltages but may skip switching cycles.

Note 2: A bulk input capacitor is required.

Ceramic capacitors are also piezoelectric. In Burst Mode

operation, the LTM8051’s switching frequency depends

on the load current, and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since the

LTM8051 operates at a lower current limit during Burst

Mode operation, the noise is typically very quiet to a

casual ear.

If this audible noise is unacceptable, use a high perfor-

mance electrolytic capacitor at the output. It may also be

a parallel combination of a ceramic capacitor and a low

cost electrolytic capacitor.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM8051. A

ceramic input capacitor combined with trace or cable

inductance forms a high-Q (underdamped) tank circuit.

If the LTM8051 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possi-

bly exceeding the device’s rating. This situation is easily

avoided; see the Hot-Plugging Safely section.