LP3958

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SNVS423C – JANUARY 2006 – REVISED MARCH 2013

Limit(1)

Symbol

Parameter

Unit

Min

Max

2

Clock Low Time

1.3

µs

3

Clock High Time

600

ns

4

Setup Time for a Repeated START Condition

600

ns

5

Data Hold Time (Output direction, delay generated by LP3958)

300

900

ns

5

Data Hold Time (Input direction, delay generated by Master)

0

900

ns

6

Data Setup Time

100

ns

7

Rise Time of SDA and SCL

20+0.1Cb

300

ns

8

Fall Time of SDA and SCL

15+0.1Cb

300

ns

9

Set-up Time for STOP condition

600

ns

10

Bus Free Time between a STOP and a START Condition

1.3

µs

Cb

Capacitive Load for Each Bus Line

10

200

pF

Recommended External Components

OUTPUT CAPACITOR, COUT

The output capacitor COUT directly affects the magnitude of the output ripple voltage. In general, the higher the

value of COUT, the lower the output ripple magnitude. Multilayer ceramic capacitors with low ESR are the best

choice. At the lighter loads, the low ESR ceramics offer a much lower VOUT ripple that the higher ESR tantalums

of the same value. At the higher loads, the ceramics offer a slightly lower VOUT ripple magnitude than the

tantalums of the same value. However, the dv/dt of the VOUT ripple with the ceramics is much lower that the

tantalums under all load conditions. Capacitor voltage rating must be sufficient, 25V or greater is recommended.

Examples of suitable capacitors are: TDK C3216X5R1E475K, Panasonic ECJ3YB1E475K, ECJMFB1E475K and

ECJ4YB1E475K.

Some ceramic capacitors, especially those in small packages, exhibit a strong capacitance reduction

with the increased applied voltage (DC bias effect). The capacitance value can fall below half of the

nominal capacitance. Too low output capacitance can make the boost converter unstable. Output

capacitors DC bias effect should be better than –50% at 18V.

INPUT CAPACITOR, CIN

The input capacitor CIN directly affects the magnitude of the input ripple voltage and to a lesser degree the VOUT

ripple. A higher value CIN will give a lower VIN ripple. Capacitor voltage rating must be sufficient, 10V or greater is

recommended.

OUTPUT DIODE, D1

A schottky diode should be used for the output diode. Peak repetitive current should be greater than inductor

peak current (800mA) to ensure reliable operation. Schottky diodes with a low forward drop and fast switching

speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown voltage of the

schottky diode significantly larger (~30V) than the output voltage. Do not use ordinary rectifier diodes, since slow

switching speeds and long recovery times cause the efficiency and the load regulation to suffer. Example of

suitable diode is: Central Semiconductor CMMSH1-40.

EMI FILTER COMPONENTS CSW, RSW

EMI filter (RSW and CSW) on the SW pin can be used to suppress EMI caused by fast switching. These

components should be as near as possible to the SW pin to ensure reliable operation. 50V or greater voltage

rating is recommended for capacitor.

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