Cirrus Logic AN375 - Manual (Page 17)

AN375
AN375REV4 17
Assume using a BJT with a beta equal to 20, then the maximum collector current I
collector
is equal to 300mA.
The CLAMP pin is driven high a period t
p
(approximately 300 s) prior to the predicted trailing-edge dimmer
phase cut. Assuming a worst case phase cut of 90° and peak voltage V
peak
of 350V the maximum power
dissipation is calculated using Equation 24:
The lowest collector resistor value is calculated using Equation 25:
An external resistor between the CLAMP pin and the base of the BJT should be used when less than the full
15mA drive is required, by limiting the base drive the supply voltage V
DD
current is reduced.
I
collector
V
peak
 t
p

0.5 t
line

------------------------------------------------------
300mA 350V  300 s 
8.3ms
--------------------------------------------------------------- 3.8W = = [Eq. 24]
V
peak
I
collector
----------------------
350V
0.3A
-------------- 1167  = = [Eq. 25]