LTM4703

14

Rev. 0

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For most applications, the design process is straightfor-

ward and summarized below.

1. See Table 1 for the row with the desired input range

and output voltage.

2. Apply the recommended CIN, COUT, RSET, and RT values.

3. Apply the CSET (from SET to AGND), or CSHARE (from

COMPa to AGND) capacitors as required.

While these component combinations have been tested

for proper operation. Ensure that the system operates

correctly within the defined line, load, and environmental

conditions. The maximum output current is limited by

the junction temperature, the relationship between the

input and the output voltage magnitude, the polarity, and

other factors. See the graphs in the Typical Performance

Characteristics section for guidance.

The maximum frequency (and attendant RT value) at

which the LTM4703 should be allowed to switch is shown

in Table 1 in the Maximum fSW column, while the recom-

mended frequency (and RT value) for optimal efficiency

over the given input condition is given in the fSW column.

Additional conditions must be satisfied if the synchroni-

zation function is used. See the Synchronization section

for details.

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 necessary. Ensure that the system operates correctly

within the defined line, load, and environmental conditions.

Ceramic capacitors are small, robust, and have very low

ESR. However, not all ceramic capacitors are suitable.

The X5R and X7R types are stable over temperature

and applied voltage and give dependable service. Other

types, including Y5V and Z5U have 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. Since the

LTM4703 operates at a lower current limit during

pulse-skipping mode (PSM) operation, the noise is typ-

ically very quiet to a casual ear. If this audible noise is

unacceptable, use a high performance electrolytic capac-

itor at the output. It may also be a parallel combination of

a ceramic capacitor and a low cost electrolytic capacitor.

A final precaution regarding ceramic capacitors con-

cerns the maximum input voltage rating of the LTM4703.

A ceramic input capacitor combined with a trace, or a

cable inductance, forms a high-Q (underdamped) tank

circuit. If the LTM4703 circuit is plugged into a live supply,

the input voltage can ring to twice its nominal value, pos-

sibly exceeding the device’s rating. This situation is easily

avoided (see the Hot-Plugging Safely section) for details.

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

VIN1

(V)

VOUT

(V)

RSET

(kΩ)

CIN2

COUT

fSW

(MHz)

RT

(kΩ)

MAX

fSW

(MHz)

MIN RT

(kΩ)

COMPENSATION3

3 to 16

1

10

10µF X7R 25V 1210

100µF ×4 X7R 6.3V 1210

0.4

287

2.5

35.7

Internal Compensation

3.1 to 16

1.8

18.2

10µF X7R 25V 1210

100µF ×4 X7R 6.3V 1210

0.8

137

2.5

35.7

Internal Compensation

4.2 to 16

2.75

27.4

10µF X7R 25V 1210

100µF ×4 X7R 10V 1210

1.0

105

2.5

35.7

Internal Compensation

5.7 to 16

3.6

36.5

10µF X7R 25V 1210

100µF ×4 X7R 10V 1210

1.2

86.6

2.5

35.7

Internal Compensation

8.3 to 16

5

49.9

10µF X7R 25V 1210

100µF ×4 X7R 10V 1210

1.5

66.5

3

28.7

Internal Compensation

8.6 to 16

6

60.4

10µF X7R 25V 1210

100µF ×4 X7R 10V 1210

2.0

47

3

28.7

Internal Compensation

1 The LTM4703 may be capable of the operating at lower input voltage but may skip switching cycles.

2 A bulk capacitor is required.

3 Short COMPa and COMPb pins for internal compensation.

APPLICATIONS INFORMATION