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LTM8065

8065fa

For more information www.linear.com/LTM8065

APPLICATIONS INFORMATION

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 LTM8065’s switching frequency depends

on the load current, and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since the

LTM8065 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 LTM8065. A

ceramic input capacitor combined with trace or cable

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

If the LTM8065 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 LTM8065 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

resultant frequencies.

Table 2. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

232

0.3

150

0.4

110

0.5

88.7

0.6

73.2

0.7

60.4

0.8

52.3

1.0

41.2

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

LTM8065 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 LTM8065 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.

BIAS Pin Considerations

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

power switching stage and operate other internal circuitry.

For proper operation, it must be powered by at least 3.2V.

If the output voltage is programmed to 3.2V or higher, BIAS

may be simply tied to VOUT. If VOUT is less than 3.2V, BIAS

can be tied to VIN or some other voltage source. If the BIAS

pin voltage is too high, the efficiency of the LTM8065 may

suffer. The optimum BIAS voltage is dependent upon many

factors, such as load current, input voltage, output voltage

and switching frequency. In all cases, ensure that the maxi-

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