WLED and CLED Driver Electrical Characteristics

Symbol

Parameter

Conditions

Min

Typ

Max

Units

I

RANGE

Sink Current Range

V

0–25.5

mA

I

MAX

Maximum Sink Current

(Note 16)

25.5

30

mA

I

LEAKAGE

Leakage Current

V

0.03

1

µA

I

Sink Current Matching (Note

17)

I

WLED1...4 or CLED1...2

0.5

2.7

%

Note 16: A minimum voltage, Dropout Voltage, is required on the WLED and CLED outputs for maintaining the LED current. The current reduction at lower voltages

is shown in the graph WLED Output Current vs Voltage

Note 17: Match % = 100% * (Max − Min)/Min

Adjustment

WLED[7:0] or

CLED[7:0]

WLED or CLED

Current (Typical)

Units

0000 0000

0

mA

0000 0001

0.1

mA

0000 0010

0.2

mA

0000 0011

0.3

mA

••

•

••

•

1111 1101

25.3

mA

1111 1110

25.4

mA

1111 1111

25.5

mA

WLED or CLED Output Current vs Voltage

Temperatures −40˚C, 25˚C, 85˚C, 100˚C

20080523

Recommended External

Components

OUTPUT CAPACITOR, C

OUT

The output capacitor C

the output ripple voltage. In general, the higher the value of

C

ramic capacitors with low ESR are the best choice. At the

lighter loads, the low ESR ceramics offer a much lower V

OUT

ripple than 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

much lower than the tantalums under all load conditions.

Capacitor voltage rating must be sufficient, 10V is recom-

mended.

INPUT CAPACITOR, C

IN

The input capacitor C

input ripple voltage and to a lesser degree the V

higher value C

age rating must be sufficient, 10V is recommended.

OUTPUT DIODE, D

OUT

A Schottky diode should be used for the output diode. To

maintain high efficiency the average current rating of the

schottky diode should be larger than the peak inductor cur-

rent (1A). Schottky diodes with a low forward drop and fast

switching speeds are ideal for increasing efficiency in por-

table applications. Choose a reverse breakdown of the

schottky diode larger than the output voltage. Do not use

ordinary rectifier diodes, since slow switching speeds and

long recovery times cause the efficiency and the load regu-

lation to suffer.

INDUCTOR, L

The LP3933’s high switching frequency enables the use of

the small surface mount inductor. A 10 µH shielded inductor

is suggested. Values below 4.7 µH should not be used. The

inductor should have a saturation current rating higher than

the peak current it will experience during circuit operation

(

Ω ESR is suggested for high effi-

ciency. Open core inductors cause flux linkage with circuit

components and interfere with the normal operation of the

circuit. This should be avoided. For high efficiency, choose

an inductor with a high frequency core material such as

ferrite to reduce the core losses. To minimize radiated noise,

use a toroid, pot core or shielded core inductor. The inductor

should be connected to the SW pin as close to the IC as

possible. Examples of suitable inductors are TDK types

LLF4017T-100MR90C and VLF4012AT-100MR79 and Coil-

craft type DO3314T-103.

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