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LTM8073

8073fa

For more information www.linear.com/LTM8073

applicaTions inForMaTion

Capacitor Selection Considerations

The CIN and COUT capacitor values in Table 1 are the mini-

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.

Ceramic capacitors are also piezoelectric. In Burst Mode

operation, the LTM8073’s switching frequency depends

on the load current, and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since the

LTM8073 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 LTM8073. A

ceramic input capacitor combined with trace or cable

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

If the LTM8073 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

The LTM8073 uses a constant frequency PWM architec-

ture 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 resul-

tant frequencies.

Table 2. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

232

0.3

150

0.4

110

0.5

84.5

0.6

64.8

0.7

60.4

0.8

51.1

1.0

40.2

1.2

32.4

1.4

28.0

1.6

23.7

1.8

20.5

2.0

16.9

2.2

15.8

3.0

Operating Frequency Trade-Offs

It is recommended that the user apply the optimal RT

value given in Table 1 for the input and output oper-

ating condition. System level or other considerations,

however, may necessitate another operating frequency.

While the LTM8073 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 LTM8073 if the output is overloaded

or short-circuited. A frequency that is too low can result

in a final design that has too much output ripple or too

large of an output capacitor.

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