Cirrus Logic AN372 - Manual (Page 31)

AN372
AN372REV1 31
action. At 125°C the thermistor has 2.5k  plus resistor R18 = 14k present a resistance of 16.5k at pin
eOTP, reaching the point at which a thermal shutdown fault intervenes.
Step 25) Clamp Circuit
Clamp load resistors R6 and R16 must each be 2k
2W resistors for 230V systems. This value has been
validated for optimal dimming performance.
The design process assumed T3 = 0. A circuit built as designed will deliver an output power just under the
nominal power and operate at a slightly lower frequency because time T3 lengthens the period TT slightly.
After the circuit is built, the exact value of time T3 can be measured. In this example, 0.8µs is the value of time
T3.
Then the power delivered will be close to
FB
Gain
must be recalculated:
R
FBGAIN
also needs to be set according the data sheet formula (see the CS1610/11/12/13 TRIAC Dimmable
LED Driver IC data sheet).
The peak current also needs to be increased by 10%, reducing R
Sense
by 10% to compensate for reaching
only 91% of power due to T3 being greater than zero. Leaving the inductance L unchanged would result in a
lower switching frequency of nearly 21%. A 10% increase in the switching period is due to T3. Another 10% is
due to the increased I
PK(FB)
, resulting in taking a correspondingly longer time to reach the peak and a
proportionally longer time to fall to zero. To keep the switching frequency close to the original design, the
inductor value should be reduced by 21%.
Therefore, resistor R
FBGAIN
is set to 31.6k , which is the closest standard value and can possibly be adjusted
later. Resistor R
Sense
must be set to 6.04 , the closest standard value to (6.76/1.1). The inductor must be
redefined as a 3mH inductance capable of not saturating for a current of 0.24A.
P
OUT
T1 T2 +  
T1 T2 T3 + +  
-----------------------------------------
9.6W 8 s 
8.8 s
------------------------------- 8.72W = 91% P 
OUT
 = [Eq. 71]
FB
Gain
TT
T2
-------
8.8 s
5.84 s
------------------ 1.5 = = = [Eq. 72]
R
FBGAIN
62.5k 
1.5 2    1 –  
------------------------------------- 31.25k  = = [Eq. 73]