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I have a design wherein the basic idea is to control individual LED strings and dim (analog dimming) them using Pi Pico via I2C interface.

High Level Block Diagram

This board is the LED driver which will be connected to the LED array board where the strings of LED are populated.

The LED driver is a constant current one, one is Recom and another one is from Diodes Inc.

More detailed info can be found in the attached schematic.

Schematic Page 1

Schematic Page 2

Parts Datasheet Links -

Recom's datasheet says "Don't connect the V- output to Gnd"

I had assigned a separate net to it (provided jumpers for flexibility of using two different drivers.)

I have connected the high side switch according to the datasheet and app note, but to control the load switch the gate or input pin of the load switch is being controlled by the I2C expander.

Where should I connect the I2C expander GND: GNDD or GND?

If I want to control the load switch I must connect the IO expander's ground to the GNDD net but that would mean for I2C it won't use the GND of Pi Pico.

Am I missing something here or is my schematic wrong? Please advise.

I found the following post useful.

If there's any shortcoming in the post understandability please let me know.

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  • \$\begingroup\$ You said it yourself ”Don't connect the V- output to Gnd”. It probably does low side current sense and/or low side switching, which you are effectively shorting out. Get a different driver. \$\endgroup\$
    – winny
    May 23, 2022 at 12:45
  • \$\begingroup\$ If you refer to the post I have linked at the end it's evident, but that's not my question. Where do I connect the Gnd pin of the I2C expander. \$\endgroup\$
    – JAGADISH K
    May 23, 2022 at 13:04
  • \$\begingroup\$ Somewhere you must connect both GND anyway, what exact is your concern? \$\endgroup\$
    – Jens
    May 23, 2022 at 13:49
  • \$\begingroup\$ This probably won't work with a constant current driver \$\endgroup\$
    – user253751
    May 23, 2022 at 15:33
  • \$\begingroup\$ @user253751 My question was since I'm using the load switch to connect and disconnect the String anode to LED+ of the recom driver, and the LED - is connected to the common cathode of the LED strings, what would happen to the gate of the MOSFET when I use an I2C expander. Which ground will the I2C expander use, GNDD or GND because it has to use GND for I2C and GNDD to switch the Mosfet Gate? \$\endgroup\$
    – JAGADISH K
    May 24, 2022 at 4:52

2 Answers 2

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With a constant current source, you would rather connect different LED channels in series than in parallel. To turn them off, you'd short them out individually with a MOSFET (given that the driver can operate at very low load voltage - yours is protected against short-circuits). When arranged in parallel, the driver must not output more current than one single of your 8 LED channels can take (you're probably aware of that, just saying). Another problem is that you likely get into big problems once you switch between channels. Having different parallel strands where some are hot and some are cold is literally the worst case scenario for thermal runaway.

Concerning your actual question: to get around the "don't connect DIM- to LED-"-problem you can use an optocoupler to drive the MOSFETs. It doesn't matter if it's a high-side or a low-side switch in that case, but switching low-side would be preferable because you can use n-channel MOSFETs, which typically have higher current capability and are easier and cheaper to get.

In principle, something like this should work for your "parallel approach":

schematic

simulate this circuit – Schematic created using CircuitLab

The values of the resistors depend on your component selection and if you use an optocoupler with high enough voltage rating you can omit R1 completely.

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  • \$\begingroup\$ The thing is the LED Strings are populated on a separate board and it's manufactured already, there are 8 strings and each string has 4 LEDs in series, and their respective anodes are brought out in a connector, and the cathodes are connected to each other and again brought out on the same connector, Because string cathodes are commoned I had to use high side switches, \$\endgroup\$
    – JAGADISH K
    May 24, 2022 at 4:49
  • \$\begingroup\$ My question was since I'm using the load switch to connect and disconnect the String anode to LED+ of the recom driver, and the LED - is connected to the common cathode of the LED strings, what would happen to the gate of the MOSFET when I use an I2C expander. Which ground will the I2C expander use, GNDD or GND because it has to use GND for I2C and GNDD to switch the Mosfet Gate? \$\endgroup\$
    – JAGADISH K
    May 24, 2022 at 4:50
  • \$\begingroup\$ So there is nothing much I can do now about the LED structure now. Although one thing to consider is only one string would be active at a time, the CC driver will control only one string and the rest are disconnected by the high side switches. \$\endgroup\$
    – JAGADISH K
    May 24, 2022 at 4:51
  • \$\begingroup\$ GNDD would have to be GND in your application, but that's problematic as the driver might do high-side current sensing and switching. Note, that the datasheet explicitly mentions "All LED Drivers may not be used without a load. They must be switched on the primary side only." - switching the LEDs on the secondary side is not allowed. I personally would design a (or 8 of it) DCDC drivers that do both current sensing and switching on the high-side. \$\endgroup\$
    – Sim Son
    May 24, 2022 at 6:18
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    \$\begingroup\$ I don't want to rain on your parade but I would expect Recom to answer that you are using their driver way out of specified use and recommend you to get a different driver. \$\endgroup\$
    – winny
    May 24, 2022 at 12:47
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I think this will not work at all because of the very special input stage of the TBD62786. It's input signal is relative to VCC, not to GND. To control it by the I2C expander you must connect VCC of TBD62786 (= VS) with VDDP of the PCAL6408A (= +5V). Active low output signals from PCAL6408A would activate the PMOS driver in TBD62786. The driver's GND must be connected to -VOUT of the Recom module. BUT this is only possible, if the Recom module would be an isolating converter. It isn't, and so VS will rise +5V to VDC and everything will be destroyed.

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  • \$\begingroup\$ I think you are right, I didn't read that "Vcc=0" condition... @JAGADISHK, you really should use an optocoupler to circumvent all those ground reference problems. \$\endgroup\$
    – Sim Son
    May 24, 2022 at 15:24
  • \$\begingroup\$ @SimSon If I use an Optocoupler will it mitigate these issues? The VCC referenced Input Pins of Load switch driver wouldn't pose any problems right. \$\endgroup\$
    – JAGADISH K
    May 31, 2022 at 7:08
  • \$\begingroup\$ Hi, I stumbled into this part mouser.in/datasheet/2/408/… Do you think this part and I2C expander will get the job done? \$\endgroup\$
    – JAGADISH K
    May 31, 2022 at 10:06
  • \$\begingroup\$ @JAGADISHK: I think these couplers can be used as load switches \$\endgroup\$
    – Jens
    May 31, 2022 at 11:00

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