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LTM8053

8053fc

For more information www.linear.com/LTM8053

APPLICATIONS INFORMATION

Capacitor Selection Considerations

The CIN and COUT capacitor values in Table 1 are the

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, 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. In Burst Mode

operation, the LTM8053’s switching frequency depends

on the load current, and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since the

LTM8053 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 LTM8053. A

ceramic input capacitor combined with trace or cable

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

If the LTM8053 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.

Frequency Selection

TheLTM8053usesaconstantfrequencyPWMarchitecture

that can be programmed to switch from 200kHz to 3MHz

by using a resistor tied from the RT pin to ground. Table 2

provides a list of RT resistor values and their resultant

frequencies.

Table 2. SW Frequency vs RT Value

fSW (MHz)

RT (k)

0.2

232

0.3

150

0.4

110

0.5

88.7

0.6

71.5

0.7

60.4

0.8

52.3

1.0

40.2

1.2

33.2

1.4

28.0

1.6

23.7

1.8

20.5

2.0

18.2

2.2

15.8

3.0

10.7

Operating Frequency Trade-Offs

It is recommended that the user apply the optimal RT

value given in Table 1 for the input and output operating

condition. System level or other considerations, however,

may necessitate another operating frequency. While the

LTM8053 is flexible enough to accommodate a wide range

of operating frequencies, a haphazardly chosen one may

result in undesirable operation under certain operating or

fault conditions. A frequency that is too high can reduce

efficiency, generate excessive heat or even damage the

LTM8053 if the output is overloaded or short-circuited. A

frequencythatistoolowcanresultinafinaldesignthathas

too much output ripple or too large of an output capacitor.

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