LTM8048 datasheet(13/22 Pages) LINER

13 / 22 page

LTM8045

8045fa

For more information www.linear.com/8045

APPLICATIONS INFORMATION

Setting Output Voltage

The output voltage is set by connecting a resistor (RFB)

from VOUT+ to the FB pin for a SEPIC and from VOUT– to

the FB pin for an inverting converter. RFB is determined

from the equation RFB = [(VOUT – 1.215)/0.0833]kΩ for

a SEPIC and from RFB = [(|VOUT| + 0.005)/0.0833]kΩ for

an inverting converter.

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 LTM8045. A

ceramic input capacitor combined with trace or cable

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

If the LTM8045 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.

Programming Switching Frequency

TheLTM8045hasanoperationalswitchingfrequencyrange

between 200kHz and 2MHz. The free running frequency is

programmed with an external resistor from the RT pin to

ground.Donotleavethispinopenunderanycircumstance.

When the SYNC pin is driven low (< 0.4V), the frequency

of operation is set by the resistor from RT to ground. The

RT value is calculated by the following equation:

RT =

91.9

fOSC

− 1

where fOSC is the typical switching frequency in MHz and

RT is in kΩ.

Switching Frequency Trade-Offs

It is recommended that the user apply the optimal RTvalue

given in Table 1 for the corresponding input and output

operating condition. System level or other considerations,

however, may necessitate another operating frequency.

While the LTM8045 is flexible enough to accommodate a

widerangeofoperatingfrequencies,ahaphazardlychosen

one may result in undesirable operation under certain op-

erating or fault conditions. A frequency that is too high can

reduceefficiency,generateexcessiveheatorevendamage

the LTM8045 in some fault conditions. 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.

Switching Frequency Synchronization

Theswitchingfrequencycanbesynchronizedtoanexternal

clocksource.Tosynchronizetotheexternalsource,simply

provide a digital clock signal at the SYNC pin. Switching

will occur at the SYNC clock frequency. Drive SYNC low

and the switching frequency will revert to the internal

free-running oscillator after a few clock periods.

Switching will stop if SYNC is driven high.

The duty cycle of SYNC must be between 35% and 65%

for proper operation. Also, the frequency of the SYNC

signal must meet the following two criteria:

1. SYNC may not toggle outside the frequency range of

200kHz to 2MHz unless it is stopped low to enable the

free-running oscillator.

2. The SYNC frequency can always be higher than the

free-running oscillator frequency, fOSC, but should not

be less than 25% below fOSC (fOSC is set by RT).

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