LTM8027

11

8027f

APPLICATIONS INFORMATION

For most applications, the design process is straight

forward, summarized as follows:

1. Look at Table 2 and find the row that has the desired

input range and output voltage.

3. Connect the BIAS pins as indicated.

While these component and connection 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.

Capacitor Selection Considerations

minimum recommended values for the associated oper-

ating conditions. Applying capacitor values below those

indicated in Table 2 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. The LTM8027’s

switching frequency depends on the load current, and

at light loads it can excite a ceramic capacitor at audio

frequencies, generating audible noise.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM8027. A

ceramic input capacitor combined with trace or cable

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

If the LTM8027 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.

Input Power Requirements

The LTM8027 is biased using an internal linear regula-

Virtually all of the circuitry in the LTM8027 is biased via

internally decoupled with a low ESR capacitor to GND.

with voltages below the dropout threshold.

7.5V. This assures that the onboard regulator has ample

the BIAS pin above 8.5V allows use of externally derived

power for the IC. This effectively shuts down the internal

linear regulator and reduces power dissipation within the

LTM8027. Using the onboard regulator for start-up, then

mizes conversion efficiencies and is common practice. If

BIAS Power

The internal circuitry of the LTM8027 is powered by the

tioned internal linear regulator or the BIAS1 and BIAS2

pins, if it is greater than 8.5V. Since the internal linear

an external source through the BIAS pins reduces the

power lost within the LTM8027 and can increase overall

system efficiency.