LTM8031

10

8031fa

APPLICATIONS INFORMATION

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 LTM8031’s switching frequency

depends on the load current, and can excite a ceramic

capacitor at audio frequencies, generating audible noise.

Since the LTM8031 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

performance electrolytic capacitor at the output. The input

capacitor can be a parallel combination of a 1μF ceramic

capacitor and a low cost electrolytic capacitor.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM8031. A

ceramic input capacitor combined with trace or cable

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

If the LTM8031 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.

Electromagnetic Compliance

The LTM8031 is compliant with the radiated emissions

requirements of EN55022 class B. Graphs of the LTM8031’s

EMC performance are given in the Typical Performance

Characteristics section. Further data, operating conditions

and test setup are detailed in an EMI Test report available

from Linear Technology.

Frequency Selection

The LTM8031 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2.4MHz

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

frequencies.

Operating Frequency Trade-Offs

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

LTM8031 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

LTM8031 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 unnecessarily large output

at which the LTM8031 should be allowed to switch is given

There are additional conditions that must be satisfied if

the synchronization function is used. Please refer to the

Synchronization section for details.

BIAS Pin Considerations

The BIAS pin is used to provide drive power for the internal

power switching stage and operate internal circuitry. For

proper operation, it must be powered by at least 2.8V. If

the output voltage is programmed to be 2.8V or higher,

cases, ensure that the maximum voltage at the BIAS pin

SWITCHING FREQUECNY (MHz)

0.2

187

0.3

124

0.4

88.7

0.5

69.8

0.6

56.2

0.7

47.5

0.8

39.2

0.9

34

1.0

28.0

1.2

23.7

1.4

19.1

1.5

16.2

1.8

13.3

2

11.5

2.2

9.76

2.4

8.66