OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 37)

37
Example:
Requirements:
Tset
H
= 70 °C
Tset
L
= 55 °C
ΔT
Hyst
= 5 °C
Iout = Inom
max
Iout
warning
= 60 % Inom
max
Iout
fault
= 0 mA
LEDset APPLICATIONS
Figure 33: Thermal derating – NTC + OPAMP solution: two-step output (example).
By selecting the following components,
the result shown in fi gure 33 can be achieved:
R
1
= 68.1 kΩ 0603 1 % (general purpose)
R
2
= 270 kΩ 0603 1 % (general purpose)
R
3
= 560 kΩ 0603 1 % (general purpose)
R
4
= 20.5 kΩ 0603 1 % (general purpose)
R
5
= 560 kΩ 0603 1 % (general purpose)
R
6
= 36 kΩ 0603 1 % (general purpose)
Rset
1
= 18 kΩ 0603 1 % (general purpose)
Rset
2
= 22 kΩ 0603 1 % (general purpose)
R
NTC
= NTCS0805E3683GXT (680 kΩ 2 % 0805) (glass-protected – Vishay)
U
1
= LM2904M – SO8 (general purpose)
Q
1
, Q
2
= BC847 – SOT23 (general purpose)
C
1
, C
2
= 100 nF X7R 0603 (general purpose)
(by-pass capacitor to be placed as close as possible to the power pins of U1)
Notes:
• Rset = Rset
1
+ Rset
2
sets the Inom.
• The dividing rate between Rset
1
and Rset
2
allows adjusting the Iout
warning
output level.
• The distribution of the resistance values of R
1
, R
4
and R
6
allows setting and
adjusting the Tset
H
and Tset
L
temperature trip thresholds.
• Adjusting the values of R
5
(for Tset
H
) and R
3
(for Tset
L
) allows changing the respective
ΔT
Hyst
(in principle, ΔT
Hyst
can be set with different values for Tset
H
and Tset
L
).
Increasing the resistance value decreases the ΔT
Hyst
and vice versa.
Figure 34: NTC + OPAMP solution: two-step output – real circuit (1)
appearance with connector block (2).
2
1