Simulate this circuit – Schematic created using CircuitLab I've come across this answer on stack exchange and I now can better understand sphero's and AnalogKid's answers. It's still a bit unclear to me if Q2 is supposed to be saturated (and act as a switch) or in the active region (and boost the current gain of the Darlington pair). The gain of the "high-current triple" formed by Q2, Q3 and Q4 is gigantic and as such any leakage current from Q1 (EM interference, stored charge or otherwise) might turn on the LEDs unwantedly. R3, R4 and R5 are high-valued resistors that discharge the base-emitter capacitance and insure a quick turn-off. ![]() ![]() The Darlington pair formed by Q3 and Q4 must be driven high-side, hence the use of Q2. The constant current source is used to "absorb" the variations in supply voltage. The purpose of the circuit is to ensure as big as possible of an output current, not a constant current. How would I go about analyzing the functioning of the PNP transistor and can I determine the values of the currents passing through it?Īs it stands, my current understanding of the circuit is: But the presence of the high-value resistor R3 confuses me as I don't see any clear purpose for its presence, perhaps to help with biasing the PNP? I was thinking that it exists to push current from its base to the collector of the transistor Q1 and that the whole PNP and Darlington section of the circuit acts as an active load for the constant current source centered on Q1. I understand most of the circuit's workings but I fail to understand why the PNP transistor exists. Image source: 8-Channel Darlington Current Driver TD62783A The schematic below describes the structure of a single channel from the 8-Channel Darlington Current Driver TD62783A.
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