LTM4607 datasheet(12/28 Pages) LINER

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LTM8056

8056fa

For more information www.linear.com/LTM8056

APPLICATIONS INFORMATION

Capacitor Selection Considerations

The CIN and COUT capacitor values in Table 1 are the

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.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM8056. A

ceramic input capacitor combined with trace or cable

inductance forms a high Q (underdamped) tank circuit.

If the LTM8056 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 LTM8056 uses a constant frequency PWM architec-

ture that can be programmed to switch from 100kHz to

800kHz by tying a resistor from the RT pin to ground.

Table 3 provides a list of RT resistor values and their re-

sultant frequencies.

Table 3. Switching Frequency vs RT Value

FREQUENCY

RT VALUE (kΩ)

100

453

200

147

300

84.5

400

500

45.3

600

36.5

700

29.4

800

24.9

An external resistor within the range stated in Table 3

from RT to GND is required. Even when synchronizing to

an external clock. When synchronizing the switching of

the LTM8056 to an external signal source, the frequency

range is 200kHz to 700kHz.

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

LTM8056 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

Table 2. Electrolytic Caps Used in LTM8056 Testing

DESCRIPTION

MANUFACTURER

PART NUMBER

100µF, 6V, 75mΩ, Tantalum C Case

AVX

TPSC107M006R0075

100µF, 16V, 100mΩ, Tantalum Y Case

AVX

TPSY107M016R0100

68µF, 16V, 200mΩ, Tantalum C Case

AVX

TPSC686M016R0200

47µF, 25V, 900mΩ, Tantalum D Case

AVX

TAJD476M025R

33µF, 35V, 300mΩ, Tantalum D Case

AVX

TPSD336M035R0300

10µF, 50V, 120mΩ, Aluminum 6.3 × 6mm case

Suncon

50HVP10M

10µF, 63V, 120mΩ, Aluminum 6.3 × 5.8mm case

Panasonic

EEHZA1J100P

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