LTM8080

16

Rev. 0

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For most applications, the design process is straightfor-

ward, summarized as follows:

1. See Table 1 and find the row with the desired input

range and output voltage.

2. Apply the recommended CIN, CBUS, COUT, CSET, RSET,

and RT values.

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. Remember that the

maximum output current is limited by junction tempera-

ture, the relationship between the input and output volt-

age magnitude, and other factors. See the graphs in the

Typical Performance Characteristics section for  more

details.

The recommended frequency (and attendant RT value) at

which the LTM8080 is optimal for efficiency over the given

input condition is shown in the fSW column in Table 1.

Additional conditions must be satisfied if the synchroniza-

tion function is used. Please refer to the Synchronization

section for details.

Capacitor Selection Considerations

The CIN and COUT capacitor values in Table 1 are the min-

imum recommended values for the associated operating

conditions. Applying capacitor values below those shown

in Table 1 is not recommended and may result in undesir-

able operation. Using larger values is generally acceptable

and can yield improved dynamic response if necessary.

Again, it is incumbent upon the user to verify proper oper-

ation over the intended system’s line, load, and environ-

mental 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

applied voltage and give dependable service. Other types,

including Y5V and Z5U, have a very large temperature

and voltage coefficients of capacitance. In an application

circuit, they may have only a tiny fraction of their nominal

capacitance resulting in a much higher output voltage

ripple than expected.

Ceramic capacitors are also piezoelectric. In Burst Mode

operation, the LTM8080’s switching frequency depends

on the load current and can excite a ceramic capacitor

at audio frequencies, generating audible noise. Since

the LTM8080 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-performance electrolytic capacitor at VBUS. It may

also be a parallel combination of a ceramic capacitor and

a low-cost electrolytic capacitor.

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

VIN*

(V)

VOUT

(V)

VBUS

(V)

RSET

(kΩ)

CSET

CIN**

CBUS

COUT

fSW

RT

(kΩ)

3.5 to 40

0.8

2.5

8.06

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

800kHz

52.3

3.5 to 40

1

2.5

10

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

800kHz

52.3

3.5 to 40

1.2

2.5

12.1

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

800kHz

52.3

3.5 to 40

1.5

2.5

15

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

800kHz

52.3

3.5 to 40

1.8

2.8

18.2

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

900kHz

46.4

4.5 to 40

2

3

20

4.7µF 4V

1µF 50V

22µF 6.3V

10µF 4V

1MHz

40.2

5 to 40

2.5

3.5

24.9

4.7µF 6.3V

1µF 50V

10uF 10V

10µF 6.3V

1.1MHz

36.5

6 to 40

3.3

4.3

33.2

4.7µF 6.3V

1µF 50V

10uF 10V

10µF 6.3V

1.2MHz

33.2

9 to 40

5

6

49.9

4.7µF 10V

1µF 50V

10uF 16V

10µF 10V

1.3MHz

30.1

15 to 40

8

9

80.6

4.7µF 16V

1µF 50V

10uF 16V

10µF 16V

1.5MHz

24.3

*The LTM8080 may be capable of lower input voltages but may skip switching cycles or reduce its operating frequency.

**An input bulk capacitor is required.

APPLICATIONS INFORMATION