AN2823 datasheet(8/33 Pages) STMICROELECTRONICS

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The L5973AD in LED driving applications

AN2823

8/33

Doc ID 15015 Rev 1

so the regulated VSENSE is:

Equation 4

The accuracy of the regulated current is mainly affected by the tolerance of the resistors R6,

R1 and RS.

If the selected sense resistor value is equal to RS=1.5 Ω, the required voltage drop on the

sense resistor to set 350 mA flowing through the LED string is 525 mV. The required resistor

ratio is:

Equation 5

Given R6 = 6.8 k

Ω and R1 = 20 kΩ, the nominal LED current is 355 mA. The power wasted

on the sense resistor is so decreased from 430 mW to:

Equation 6

improving the efficiency of the conversion.

2.1.2

The dimming operation

The best way to dim an LED device is by PWM dimming performed on the current flowing

through it. The device is powered with a constant current for minimal color shift, even when

the amount of emitted light is decreased.

This requirement must be converted in the specifications for the dimmable current source.

Devices specifically tailored for the LED market (see the LED7706 and LED7707 as

examples) implement additional circuitry to meet these specifications and offer exceptional

dimming performance.

The implementation of the dimming feature in a constant current generator developed with

standard switching regulators can be tricky, as no dedicated circuitry is embedded in the

device to provide this operation. As a consequence, the design of the compensation

network must be a trade-off between the response to the small signal necessary for system

stability and the large signal during the dimming operation.

The LC filter of the output stage filters the output voltage but the output capacitor value

affects the dimming performance. In fact, even if the PWM input signal disables the current

generator, the current flowing through the LED string drops to zero only when the voltage

across the capacitor does not guarantee the minimum forward voltage across each LED.

The L5973AD is an asynchronous device: the rectification of the inductor current is given by

the external diode. The device is not capable of sinking current and so it can not quickly

discharge the output capacitor to power off the emitting diodes.

As a consequence, a large output capacitor value better filters the current flowing in the LED

from the current ripple, but significantly limits the dimming performance as it introduces

additional delay in the falling edge of the LED current.

-------

REF

–

() 1.235

------- 2.065

⋅

–

⋅

–

-------

2.9

LOSSES

SENSE

LED

184mW

≅

⋅

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