OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 28)

28
3.3.1.4. Application solution 4 – SI S-5841 solution
The SI S-5841 (SI – Seiko Instruments Inc.) series is a tem-
perature switch IC which detects a certain temperature and
sends a signal to an external device. Various combinations
of the parameters such as the detection temperature, output
form and output logic can be selected.
In package SOT-23-5, fi ve Tset
TH
temperatures are available
as factory settings (+55 °C, +65 °C, +75 °C, +85 °C, +95 °C).
The temperature hysteresis is set by the HYS1 pin (and/or
HYS2 pin), offering the choice between four values: 0 °C,
2 °C, 4 °C, 10 °C (the confi guration depends on ordering part
number and package type).
More than the previous ICs, the S-5841 series also has a
noise suppression time (t
delay
). If the temperature is lower than
the detection temperature (+TD), the DET pin is at a low level.
Although the output from the comparator goes active due to
noise if the period in which this status continues is shorter
than the noise suppression time, the DET pin keeps its low
level. However, if the period in which the output of the inter-
nal comparator is active is longer than the noise suppression
time, the DET pin is set to a high level. In case of +12 V
range supply voltage, the noise suppression time is about
1.2 –1.5 s.
In fi gure 20a, a reference schematic is shown. The maximum
operating voltage of the IC is +12 V and the open drain
output can withstand 12 V maximum. The control could be
directly used as in fi gure 20a. By the way, a safer and highly
recommended implementation is shown in fi gure 20b where
the output pin is connected to the Vset via a transistor and
the supply voltage is simply reduced by a Zener diode and a
resistor (+12 V can have a tolerance of +10 %).
LEDset APPLICATIONS
Figure 20a: Thermal protection – S-5841 solution (direct connection). Figure 20b: Thermal protection – S-5841 solution (via output transistor and voltage regulator).
A maximum leakage current of 1 µA on the open drain output
allows the use of the IC without affecting the control accuracy
(over the Iset).
Example:
Considering a Tset
TH
of 75 °C and a hysteresis of 4 °C,
the following devices can be used:
• S-5841B75D-M5T1U (open drain) in case of direct
connection to LEDset (case a – not recommended)
• S-5841B75A-M5T1U (CMOS out) in case of connection
to LEDset via transistor (case b)
In both cases, pin 1 must be tied to Vcc (pin 4).
For more information, please see the general notes on page 33.