Cirrus Logic AN376 - Manual (Page 4)

AN376
4 AN376REV2
For the design example, the maximum ripple current I
ripple
specification is ±40% on the RMS output current of
250mA, which is equal to 100mA peak. Equation 10 derives the attenuation factor required by the RC filter to
reduce the output peak current I
PK
to the specified maximum ripple peak current of 100mA:
To determine the required output capacitance C
OUT
for the RC filter, the forward current versus forward voltage
characteristics of an LED light engine using ten Philips LUMILEDS DS61 LED assembled in series is required.
For example, assume that the full-scale output current of the converter is 250mA. The chart plotted in Figure 3
can be used to estimate the effective resistance of one LED by creating the tangent at the operating point.
Therefore, at the intersection of the operating point of 250mA and the LED current-to-voltage curve, the esti-
mated effective resistance is approximately 0.46
.
It should be noted that the forward voltage of an LED drops with temperature, which will affect the final ripple
current. Since the total output resistance for the LED string is approximately 4.6
, Equation 7 on page 3 is used
to calculate the required output capacitance C
OUT
:
2.3 CS1615 Example
The CRD1615-8W is a 120VAC, 60Hz LED driver with an output voltage of 27.9V and an output current of
250mA. In the design of the CRD1615-8W, the output capacitor was selected as a standard value of 680
F.
I
ripple
I
PK
---------------
100mA
205mA
------------------- 0.49 = = [Eq. 10]
0
100
200
300
400
2.4 2.45 2.5 2.55 2.6 2.65 2.7 2.75 2.8 2.85
For
w
ar
d Cur
rent
(mA
)
Forward Voltage (V)
250 mA
Dimmer
edge Trailing-
Leading edge -
Dimmer
Effective Resistance
0.46 ~ :
Figure 3. Effective Resistance of a DS61 LED
C
OUT
1
H 4 f
line
  2
-------------------------------- 1 –
754 R
LED

----------------------------------------------------
1
0.49
2
-------------- 1 –
754 4.6  
----------------------------- 513 F = = =
[Eq. 11]