OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 32)

32
Note: For a precise calculation, the V
CEsat
of Q
2
should also be considered. For a
BC847 transistor, the voltage drop is in the range between 40 and 50 mV depending
on the temperature of the device. In fi gure 25, the voltage drop relates to a temperature
range between -55 and +150 °C. In the described application solution, however, the
temperature of Q
2
is generally within a tighter range when it is active. Therefore, the
voltage drop variation will be smaller than in fi gure 25.
The Rset
EQ
model should be as follows:
Figure 25: BC847 – collector-emitter saturation voltage vs. collector current
(NXP reference data).
BC847B; l
C
/l
B
= 20.
(1) T
amb
= 150 °C.
(2) T
amb
= 25 °C.
(3) T
amb
= -55 °C.
Considering the calculated Rset
1
and Rset
2
and introducing a
voltage drop of Q
1
= 45 mV :
The error introduced by the transistor affects the overall control
accuracy of the ECG by about 0.2 % (to be added to the 5 % of the
ECG itself). The severity of the error depends on the working point
and the transistor Vce. Choosing a lower output current increases
the severity of the error (i.e. Iout = 35 % of Inom
max
0.3 %).
and therefore Iout = 70.4 % of Inom
max
LEDset APPLICATIONS
Vset 7.37 V
Rset
2
Iset + V
CEsat
1 +
Rset
2
Rset
1