OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 35)

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The circuit confi guration allows setting a hysteresis bet ween
on and off state, thus avoiding spurious and unwanted light
fl ickering/toggling: focusing on the comparator circuit, the
R
1
, R
4
and R
3
resistors set the trip threshold voltages V
TRIP
+
(related to Tset
TH
- ΔT
Hyst
) and V
TRIP
- (related to Tset
TH
).
These V
TRIP
voltages are compared with the voltage
related to the divider realized by the R
2
and NTC resistors.
Based on the NTC characteristic (resistance vs. temperature)
and the pull-up resistor (R
2
in this case), it is possible to tune
the circuit to customer needs. Once the V
NTC
(voltage across
the NTC) becomes higher than V
TRIP
+, the comparator output
goes to low state. Consequently, Q
1
opens and the LED
module is supplied by the current which has been set by the
Rset resistance.
However, when V
NTC
becomes lower than V
TRIP
-, the output
of the comparator turns on Q
1
and thus the Iout becomes
0 mA.
Example:
Requirements:
Tset
TH
= 76 °C
ΔT
Hyst
= 6 °C
Iout = Inom
max
Iout
fault
= 0 mA
LEDset APPLICATIONS
Figure 30: Thermal derating – NTC + OPAMP solution (example).
By selecting the following components,
the result shown in fi gure 30 can be achieved:
R
1
= 100 kΩ 0603 1 % (general purpose)
R
2
= 270 kΩ 0603 1 % (general purpose)
R
3
= 560 kΩ 0603 1 % (general purpose)
R
4
= 36 kΩ 0603 1 % (general purpose)
Rset = 39 kΩ 0603 5 % (general purpose)
R
NTC
= NTCS0805E3683GXT (680 kΩ 2 % 0805) (glass-protected – Vishay)
U
1
= LM2904M – SO8 (general purpose)
Q
1
= BC847 – SOT23 (general purpose)
C
1
= 100 nF X7R 0603 (general purpose)
(by-pass capacitor to be placed as close as possible to the power pins of U1)
Figure 29: NTC + OPAMP solution – real circuit (1) appearance with
connector block (2).
2
1