I'm referring the DALI 2 schematics by Mikroe.
Within the circuit what is the function of D1 in given schematics?
And why they are using a dual diode?
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1\$\begingroup\$ Well, if you need two diodes, why not use a dual diode? It's convenient. \$\endgroup\$– HearthJun 6 at 13:28
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3 Answers
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In a DALI network, maximum system current is limited to 250 mA and a node is allowed to draw a maximum of 2 mA. This is generally the current drawn by node during receiving (receive opto's LED).
To limit the node current to somewhere around 2 mA a current sink/limiter is needed. And its formed by Q2 and its surrounding components. D1 is there to basically provide a reference voltage for the current source formed by Q2 and its surrounding components. The current limit, ILIM, will be determined by the following equation:
$$ \mathrm{2 \ V_{F(D1)} = V_{BE(Q2)}+I_{LIM}\cdot R7 \\ \Rightarrow I_{LIM}=\frac{2 \ V_{F(D1)}-V_{BE(Q2)}}{R7}} $$
The diodes are biased with a 100k resistor, so the biasing current will be way less than 1 mA and this will make the forward drop less than 0.7V, probably somewhere around 0.5V. So the reference voltage, 2 VF(D1), can be assumed as 1V. The VBE(Q2) can be assumed as 0.5~0.6V as well due to very low base current. So this will make the current sink's limit something around 2 mA.
And, as Justme stated in their answer, two diodes in a single package saves space and cost. Because there's already a diode drop due to Q2's base, the reference should be stable and higher than a diode drop. Of course a Zener or an LED could have been used instead of D1 but they require higher current for proper biasing (in mA range), and that'd cause the limit of 2 mA to be exceeded.
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Purpose of D1 is to provide a voltage drop of two diodes. And that's why it is a single component with two diodes rather than two separate diodes.
It's a reference voltage for the base of the transistor circuit used as constant current sink.
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That transistor with double diodes and two resistors is a very common circuit for a current source/current sink. There are commercial LED drivers which operate using such a configuration. As others have already said, the diodes are forming a voltage reference (with the 100k bias resistor).
One benefit of the configuration which others haven't discussed is that it provides temperature compensation: as the temperature increases, the Vf of the diodes decreases, which causes a reduction in the current being sunk. This negative temperature coefficient is beneficial with LEDs, because it prevents thermal runaway (the LED Vf also decreases with increased temperature, which can cause progressively higher power draw, which causes further self-heating, etc.) Using a Zener in reverse bias alone does not provide this critical feature.
Commercial examples of this all in a single chip package are the BCR series by Infineon (e.g. BCR420, BCR421), although they are designed for 10 mA and up I believe.
https://en.wikipedia.org/wiki/Current_source#/media/File:Const_cur_src_112.svg https://en.wikipedia.org/wiki/Current_source
