AN2823 datasheet(7/33 Pages) STMICROELECTRONICS

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AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

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2.1

Designing a high efficiency current source (topology 1)

The reference of the embedded error amplifier of the L597x family is 1.235 V.

The sense resistor value when the circuit proposed in Figure 4 is used to implement a

350 mA current source is:

Equation 1

and the power losses across it are:

Equation 2

So if the current generator is powering a string composed of 1 W LEDs, the total efficiency

of the conversion is affected by the sense resistor at a percentage of 43%. Obviously this

circuit could be rendered acceptable by increasing the number of LEDs composing the

string, but is, in any case, limited when the load is a single 3 W LED.

2.1.1

DC GAIN design

Figure 5 shows the guiding principle to implement an improved current generator using the

L5973AD and two additional resistors.

Figure 5.

Improved schematic of the current generator

R1 and R6 resistors are used to decrease the magnitude of the regulated voltage across the

sense resistor. Applying the superimposition of the effects, the equation of the FB voltage

can be written as:

Equation 3

SENSE

LED

-----------

1.235

0.35

---------------

3.52

LOSSES

SENSE

LED

430mW

≅

⋅

REF

--------------------

⋅

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