OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 39)

39
Example:
Requirements:
Tset
H
= 75 °C
Tset
L
= 60 °C
ΔT
Hyst
= 5 °C
Iout = Inom
max
Iout
warning
= from 100 % to 60 % Inom
max
in a linear way
Iout
fault
= 0 mA
LEDset APPLICATIONS
Figure 37: Thermal derating – NTC + OPAMP solution: continuous derating and
switch-off characteristic (example).
By selecting the following components,
the result shown in fi gure 37 can be achieved:
R
1
= 100 kΩ 0603 1 % (general purpose)
R
2
= 270 kΩ 0603 1 % (general purpose)
R
3
= 0R 0603 1 % (general purpose)
R
4
= 681 kΩ 0603 1 % (general purpose)
R
5
= 36.8 kΩ 0603 1 % (general purpose)
R
6
= Not mounted (general purpose)
Rset
1
= 18 kΩ 0603 1 % (general purpose)
Rset
2
= 22 kΩ 0603 1 % (general purpose)
Rset
3
= Not mounted
R
NTC
= NTCS0805E3683GXT (680 kΩ 2 % 0805) (glass-protected – Vishay)
D
1
= 1N4148 (general purpose)
U
1
= LM2904M – SO8 (general purpose)
Q
1
= 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)
C3 = 1 nF COG 0603 (general purpose)
(by-pass capacitor to be placed across D1)
Notes:
• Rset = Rset
1
+ Rset
2
sets the Inom.
• The dividing rate between Rset
1
and Rset
2
allows adjusting the Iout
warning
output slope
level.
• The distribution of the resistance values of R
1
, R
2
and R
5
allows setting and
adjusting the Tset
H
and Tset
L
temperature trip thresholds.
• Moving the values of R
4
allows adjusting the respective ΔT
Hyst
. Increasing the
resistance value decreases the ΔT
Hyst
and vice versa.
2
1
Figure 38: NTC + OPAMP solution: continuous derating and switch-off –
real circuit (1) appearance with connector block (2).