OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 26)

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3.3.1.2. Application solution 2 – LM26 solution
LM26 (National Semiconductor) is a digital output tempera-
ture switch IC with a factory-programmed trip point ranging
from -55 °C to 110 °C (in increments of 1 °C). LM26 can have
four different confi gurations of the digital output. Version C
(active-high, push-pull on OS output) is needed for this appli-
cation.
In fi gure 16, a reference schematic is shown. Since the maxi-
mum operating voltage is limited to 5.5 V, a voltage regulator
is needed in order to supply LM26 from the LEDset interface.
The simplest solution is to clamp the supply voltage on pin 4
with a Zener diode (D
1
). In order to ensure a suffi cient reverse
current to the diode, a proper value of R
1
has to be selected.
A by-pass capacitor (C
1
) between pin 4 and ground (pin 2) is
recommended.
The hysteresis of the temperature threshold can be set at:
• 10 °C if pin 1 is grounded;
• 2 °C if pin 1 is connected to pin 4 (Vcc).
The LM26 digital output drives a transistor (Q
1
) which pulls
the Vset node down when an overtemperature condition is
detected. Q
1
can either be a bipolar or a logic-level FET de-
vice with a voltage rating above 12 V. A device with low leak-
age current (namely lower than 1 µA) is mandatory. For this
reason, bipolar transistors are preferred (i.e. BC847).
LEDset APPLICATIONS
Figure 16: Thermal protection – LM26 solution. Figure 17: LM26 solution – output characteristic.
Figure 18: LM26 solution – real circuit (1) appearance with connector
block (2).
For more information, please see the general notes on page 33.
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