LTM8025

10

8025f

APPLICATIONS INFORMATION

For most applications, the design process is straight

forward, summarized as follows:

1. Look at Table 1 and find the row that has the desired

input range and output voltage.

3. Connect BIAS as indicated.

While these component combinations have been tested

for proper operation, it is incumbent upon the user to

verify proper operation over the intended system’s line,

load and environmental conditions. Bear in mind that the

maximum output current is limited by junction tempera-

ture, the relationship between the input and output voltage

magnitude and polarity and other factors. Please refer to

the graphs in the Typical Performance Characteristics

section for guidance.

which the LTM8025 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.

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, 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 LTM8025’s switching frequency depends

on the load current, and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since the

LTM8025 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 LTM8025. A

ceramic input capacitor combined with trace or cable

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

If the LTM8025 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 LTM8025 uses a constant frequency PWM architec-

ture that can be programmed to switch from 200kHz to

2.4MHz by using a resistor tied from the RT pin to ground.

sultant frequencies.