Cirrus-logic AN376 User Manual download pdf (Page 5)

AN376

AN376REV2 5

Equation 6 on page 3 is used to calculate the attenuation factor:

Since the RC network attenuates the sinusoidal component by 40%, the resulting AC current in the LEDs is cal-

culated using Equation 10 on page 4:

Therefore the LED string current I

LED

is a 164mA peak-to-peak sinusoidal on a DC offset. For this design, the

typical current ripple is approximately ±33% on the 250mA RMS output current.

2.3.1 Measured Results

The measured output voltage of the CRD1615-8W is 28V at an RMS output current of 250mA. The switching

mode power supply is driving 10 Philips LUMILEDS DS61 LEDs in series. Figure 4 illustrates the attenuation of

the RC filter implemented in the CRD1615-8W. The final ripple current on the LED current, I_LED, is measured

at approximately 170mA peak-to-peak.

H4f

line



1 754 R

LED

 C

OUT



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

17544.5 680F

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

2.5

--------

== =

[Eq. 12]

ripple

0.4 205mA 0.4 82mA== =

[Eq. 13]

Figure 4. CRD1615-8W Measured Results

Notes: The current to voltage relationship is 100mA:20mV

Current I_LED is representative of the current flowing through the LED string

1 2 3 4 5 6 7 8 9

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Cirrus-logic AN376 User Manual Page 5