Cirrus Logic AN376 - Manual (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.
H 4 f
line
 
1
1 754 R
LED
 C
OUT
  
2
+
-------------------------------------------------------------------------
1
1 754 4.5   680 F   
2
+
---------------------------------------------------------------------------
1
2.5
-------- = = =
[Eq. 12]
I
ripple
I
PK
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