AN4130 datasheet(21/41 Pages) STMICROELECTRONICS

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AN4130

Design guidance

Doc ID 023315 Rev 1

21/41

Figure 31.

LED dynamic resistance vs. current

For a small voltage change the current change is about 10 times as large as for a resistive

load.

A 1 W white LED (~3.2 V at ~350 mA) has a slope resistance of about 1

Ω at full power. As

the current is reduced, the LED impedance rises accordingly, inversely proportional to the

current. But the capacitor's impedance determines the ripple voltage - the LED current

ripple percentage does not increase as the unit is dimmed.

4.3.2

Allowable ripple current in LEDs

At the converter switching frequency the output filter capacitor takes almost all the ripple

current. The ESR of the capacitor is dominant at that frequency, and is orders of magnitude

lower than the LED impedance. High frequency ripple in the LEDs is not a concern.

At twice the line frequency (120 Hz) LED ripple can have an effect on people even if it can't

be directly observed. It's common practice in the lighting industry to limit the optical ripple to

about 10% RMS of the total light, and NEMA SSL6-2010 requires a statement of ripple

percentage on the packaging if this is exceeded. So LED current ripple must be kept below

10%.

At 120 Hz capacitive reactance dominates the shunting impedance - ESR can be ignored.

Because of the low LED impedance the filter capacitors must reduce RMS ripple voltage to

about 1% of the LED string voltage.

Equation 1

For this design, with (18) 3.2 V LEDs at 175 mA, the capacitance should be about 284 µF.

The value used was 330 µF.

0.707

100

LED

⋅⋅

π 120Hz V

string

⋅⋅

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