Bi-directional constant current device

[betwixt]

Here is an interesting challenge, I'm looking or ideas.

I've been asked to design a very low cost lighting device that uses voltage reversal to select which set of LED lights are illuminated when they are wired in two parallel chains of alternately reversed parallel LEDs. The chains will be driven with a 2KHz square wave with a short dead time at transitions and each chain will have about 500 LEDs, 250 wired one way and the other 250 wired in opposite polarity across the two wires. The maximum voltage allowed is +4V or -4V +/- 5% on one wire relative to the other. The chains might be shorter or longer depending on the end users chosen configuration but the current required has to be fixed at 300mA (600mA for both chains) from source. So essentially it is a voltage limited constant current but reversing polarity generator.

My first thought is a single constant current source supplying a H-Bridge with the four switches driven by an MCU. Given the low voltage it can be done without bootstrap supply. I wonder if any readers have alternative ideas. Cost is paramount. A single 12V unregulated supply is available.

Brian.

 

[BigBoss]

An NMOS/PMOS pair charge pump might be a good starting point, though.
Non-Inverted and Inverted signals will commutate the transistors.

 

[crutschow]

My first thought is a single constant current source supplying a H-Bridge with the four switches driven by an MCU.

Can't think of a better way, as that's how I would do it.
A simple bridge such as below, such work.

 

[dick_freebird]

I think I'd go with a dual MOSFET driver and run the channels complementary, "poor boy H bridge" minus any phase control / BBM. Headroom and resistors for constant(-ish) current. Probably need back-diodes as many LEDs rate lower reverse than forward voltage and need protecting.

 

[Easy peasy]

H bridge is the standard way to approach this

 

[betwixt]

Can't do it that way Dick_freebird, there are only two wires and the LEDs self protect by being across each other so Vf is all they will ever have across them, Resistors to limit the current are not an option either as the number of LEDs can be different in each installation.

I'm thinking of a linear regulator, something like a 7805 wired for constant current then a H bridge with complimentary NPN & PNP so top and bottom of the arms can be driven B-E directly from a micro with 4 IO pins. If the bridge can run at the same voltage as the MCU it should be possible to drive the transistors by inverting the signal to the two top transistors, I'm not sure if that's feasible given the constant current requirement though as VCC would be variable. The prototype I was shown used direct drive from two MCU pins but obviously the current available was much less than needed in a production unit.

Brian.