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Multiple Current Source Arrangement for LED Driver Dimming

IP.com Disclosure Number: IPCOM000168859D
Publication Date: 2008-Mar-31
Document File: 2 page(s) / 31K

Publishing Venue

The IP.com Prior Art Database

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This is the abbreviated version, containing approximately 90% of the total text.

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Multiple Current Source Arrangement for LED Driver Dimming

Summary

New circuitry is presented which combines multiple current sources and multiple switches in an arrangement to obtain an LED dimming function and a monotonic variation of the current.

Description of the invention

N current sources are set in parallel, with N switches (C1 to CN) driven by AND gates. The inputs of these AND gates are connected to the digital word (W1, W2 … WN) and to the PWM signal (See figure 1).

W1

PWM

W2

WN

C1

C2

CN

C1 C2 CN

Figure 1. New combination of switches and logic gates to dimming and monotonic current variation

The ratios between the current sources could be any arbitrary functions. More specifically for LED driving a logarithmic function is preferred in order to accommodate for human eye sensitivity. In this case, to get a logarithm variation, the ratios are given by the following formula (for a log law with 2dB step):

Elt

= N

N

26 .
1
26
.
1
.
10

( )

1

N

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With: 2dB variation = 1.26, =current value of the Nth current source in µA.
N

Elt

To achieve a monotonic current variation, the variation of the digital word (W1---WN) is chosen as follows:

N P
for

[ ]

, 1

0

Wp P
P
if

>

,

=

0

0

,

=

1

0

In such an arrangement, the next step is always based on all the current sources needed in the previous step plus the adequate current addition to reach the new step. This insures a perfect monotonic variation o...