LP3958 Datasheet, PDF (23/36 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

LP3958
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Symbol
Parameter
2
Clock Low Time
3
Clock High Time
4
Setup Time for a Repeated START Condition
5
Data Hold Time (Output direction, delay generated by LP3958)
5
Data Hold Time (Input direction, delay generated by Master)
6
Data Setup Time
7
Rise Time of SDA and SCL
8
Fall Time of SDA and SCL
9
Set-up Time for STOP condition
10
Bus Free Time between a STOP and a START Condition
C_b
Capacitive Load for Each Bus Line
SNVS423C – JANUARY 2006 – REVISED MARCH 2013
Limit ⁽¹⁾
Min
Max
1.3
600
600
300
900
0
900
100
20+0.1C_b
300
15+0.1C_b
300
600
1.3
10
200
Unit
µs
ns
ns
ns
ns
ns
ns
ns
ns
µs
pF
Recommended External Components
OUTPUT CAPACITOR, C_OUT
The output capacitor C_OUT directly affects the magnitude of the output ripple voltage. In general, the higher the
value of C_OUT, 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 V_OUT ripple that the higher ESR tantalums
of the same value. At the higher loads, the ceramics offer a slightly lower V_OUT ripple magnitude than the
tantalums of the same value. However, the dv/dt of the V_OUT 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, C_IN
The input capacitor C_IN directly affects the magnitude of the input ripple voltage and to a lesser degree the V_OUT
ripple. A higher value C_IN will give a lower V_IN 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 C_SW, R_SW
EMI filter (R_SW and C_SW) 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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