Since it is undesirable for the power supply to start up in

current limit, a soft-start capacitor must be chosen to force

the LM2853 to start up in a more controlled fashion based on

the charging of the soft-start capacitance. In this example,

supposea3ms start time is desired. Three time constants

are required for charging the soft-start capacitor to 95% of

the final reference voltage. So in this case RC = 1 ms. The

DAC resistor, R, is 450 k

Ω so C can be calculated to be 2.2

nF. A 2.2 nF ceramic capacitor can be chosen to yield

approximatelya3ms start-up time.

SOFT-START CAPACITOR (C

CONDITIONS

Various fault conditions such as short circuit and UVLO of

the LM2853 activate internal circuitry designed to control the

voltage on the soft-start capacitor. For example, during a

short circuit current limit event, the output voltage typically

falls to a low voltage. During this time, the soft-start voltage

is forced to track the output so that once the short is re-

moved, the LM2853 can restart gracefully from whatever

voltage the output reached during the short circuit event. The

range of soft-start capacitors is therefore restricted to values

1nFto50nF.

COMPENSATION

The LM2853 provides a highly integrated solution to power

supply design. The compensation of the LM2853, which is

type-three, is included on-chip. The benefit of integrated

compensation is straight-forward, simple power supply de-

sign. Since the output filter capacitor and inductor values

impact the compensation of the control loop, the range of L

C

OUTPUT FILTER VALUES

Table 1 details the recommended inductor and capacitor

ranges for the LM2853 that are suggested for various typical

output voltages. Values slightly different than those recom-

mended may be used, however the phase margin of the

power supply may be degraded. For best performance when

output voltage ripple is a concern, ESR values near the

minimum of the recommended range should be paired with

capacitance values near the maximum. If a minimum output

voltage ripple solution from a 5V input voltage is desired, a

6.8 µH inductor can be paired with a 220 µF (50 m

Ω)

capacitor without degraded phase margin.

TABLE 1. Recommended L

V

VIN (V)

L

C

C

Ω)

Min

Max

Min

Max

Min

Max

0.8

5

4.7

6.8

120

220

70

100

0.8

3.3

4.7

4.7

150

220

50

100

1

5

4.7

6.8

120

220

70

100

1

3.3

4.7

4.7

150

220

50

100

1.2

5

4.7

6.8

120

220

70

100

1.2

3.3

4.7

4.7

120

220

60

100

1.5

5

4.7

6.8

120

220

70

100

1.5

3.3

4.7

4.7

120

220

60

100

1.8

5

4.7

6.8

120

220

70

120

1.8

3.3

4.7

4.7

100

220

70

120

2.5

5

4.7

6.8

120

220

70

150

2.5

3.3

4.7

4.7

100

220

80

150

3.0

5

4.7

6.8

120

220

70

150

3.0

3.3

4.7

4.7

100

220

80

150

3.3

5

4.7

6.8

120

220

70

150

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