LTM8021

9

8021fb

Capacitor Selection Considerations

minimum recommended values for the associated oper-

ating conditions. Applying capacitor values below those

indicated in Table 1 is not recommended, and may result

in undesirable operation. Using larger values is generally

acceptable, and can yield improved dynamic response or

fault recovery, 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 ap-

plied voltage and give dependable service. Other types,

including Y5V and Z5U have very large temperature and

voltage coefficients of capacitance. In an application cir-

cuit they may have only a small fraction of their nominal

capacitance resulting in much higher output voltage ripple

than expected.

Ceramic capacitors are also piezoelectric. At light loads,

the LTM8021 skips switching cycles in order to maintain

regulation. The resulting bursts of current can excite

a ceramic capacitor at audio frequencies, generating

audible noise.

If this audible noise is unacceptable, use a high performance

electrolytic capacitor at the output. This output capacitor

can be a parallel combination of a 1μF ceramic capacitor

and a low cost electrolytic capacitor.

A final precaution regarding ceramic capacitors con-

cerns the maximum input voltage rating of the LTM8021.

A ceramic input capacitor combined with trace or cable

inductance forms a high Q (under damped) tank circuit.

If the LTM8021 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.

Minimum Input Voltage

The LTM8021 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. For most applications at full load, the input

must be about 1.5V above the desired output. In addition,

it takes more input voltage to turn on than is required for

continuous operation. This is shown in Figure 1.

APPLICATIONS INFORMATION

Figure 1. The LTM8021 Requires More Voltage to Start Than to Run

LOAD CURRENT (A)

0.001

2.0

3.0

4.0

5.0

0.01

0.1

6.0

2.5

3.5

4.5

5.5

1

8021 F01

TO START

TO RUN

LOAD CURRENT (A)

0.001

2

3

4

6

5

0.01

0.1

8

7

1

RUN/SS

ENABLED

RUN/SS

ENABLED

TO START

TO RUN