LTM8049 datasheet(11/20 Pages) LINER

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LTM8049

8049fa

For more information www.linear.com/LTM8049

APPLICATIONS INFORMATION

circuit 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 LTM8049. A

ceramic input capacitor combined with trace or cable

inductance forms a high Q (underdamped) tank circuit.

If the LTM8049 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

TheLTM8049hasanoperationalswitchingfrequencyrange

between 200kHz and 2.5MHz. The free running frequency

is programmed with an external resistor from the RT pin

to ground. Do not leave this pin open under any condition.

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

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

RT value is calculated by the following equation:

RT =

81.6

fOSC

– 1, where fOSC is 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 LTM8049 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, reduce the usable input

voltage range, generate excessive heat or even damage

the LTM8049 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. Note that

the Maximum Output Current vs Input Voltage curves

given in the Typical Characteristics section are for the

recommended operating conditions in Table 1. Using a

different operating frequency may result in a different

maximum output current.

Table 1. Recommended Component Values and Configuration (TA = 25°C)

VIN RANGE (V) VOUT (V)

CIN

COUT

RFB (Ω)

fOPTIMAL

RTOPTIMAL (Ω)

fMAX

RTMIN (Ω)

2.6V to 11.5V

2.5V

4.7µF 25V 0603 X5R

100µF 4V 0805 X5R

15.4k

600kHz

137k

2MHz

38.3k

2.6V to 9.5V

–2.5V

4.7µF 25V 0603 X5R

100µF 4V 0805 X5R

+ 47µF 6.3V 0805 X5R

30.1k

600kHz

137k

2MHz

38.3k

2.6V to 18V

3.3V

4.7µF 25V 0603 X5R

100µF 4V 0805 X5R

25.5k

650kHz

124k

2.3MHz

33.2k

2.6V to 12V

–3.3V

4.7µF 25V 0603 X5R

100µF 4V 0805 X5R

+ 47µF 6.3V 0805 X5R

39.2k

650kHz

124k

2.3MHz

33.2k

2.7V to 20V

4.7µF 25V 0603 X5R

47µF 6.3V 0805 X5R

45.3k

750k

107k

2.5MHz

31.6k

2.7V to 15V

–5V

4.7µF 25V 0603 X5R

100µF 6.3V 1206 X5R

+ 47µF 10V 1206 X5R

60.4k

750k

107k

2.5MHz

31.6k

2.7V to 20V

4.7µF 25V 0603 X5R

22µF 10V 1206 X5R

82.5k

1MHz

80.6k

2.5MHz

31.6k

2.7 to 18.5V

–8V

4.7µF 25V 0603 X5R

2x 47µF 10V 1206 X5R

95.3k

1MHz

80.6k

2.5MHz

31.6k

2.7V to 20V

12V

4.7µF 25V 0603 X5R

22µF 16V 1210 X5R

130k

1MHz

80.6k

2.5MHz

31.6k

2.7V to 20V

–12V

4.7µF 25V 0603 X5R

47µF 16V 1210 X5R

143k

1MHz

80.6k

2.5MHz

31.6k

2.7V to 20V

15V

4.7µF 25V 0603 X5R

22µF 16V 1210 X5R

165k

1.1MHz

73.2k

1.9MHz

41.2k

2.7V to 20V

–15V

4.7µF 25V 0603 X5R

47µF 16V 1210 X5R

178k

1.1MHz

73.2k

1.9MHz

41.2k

3V to 20V

18V

4.7µF 25V 0603 X5R

22µF 16V 1210 X7R

200k

1.5MHz

53.6k

1.8MHz

45.3k

3V to 20V

–18V

4.7µF 25V 0603 X5R

22µF 16V 1210 X7R

215k

1.5MHz

53.6k

1.8MHz

45.3k

3.7V to 18V

24V

4.7µF 25V 0603 X5R

22µF 25V 1206 X5R

274k

1.5MHz

53.6k

1.8MHz

45.3k

3.7V to 18V

–24V

4.7µF 25V 0603 X5R

22µF 25V 1206 X5R

287k

1.5MHz

53.6k

1.8MHz

45.3k

Note: An input bulk capacitor is required.

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