I'm looking to design a four channel dimmer using the On Semi FL5150/FL5160 dimmer chip.

This will be controlled by a digital signal sent over an isolated I2C interface, with my initial design using the MCP4728 quad channel I2C DAC.

However the reference designs on the datasheet tie the Source pins of the output MOSFET pair to the GND of the dimmer control chip. As I would have to tie the GND of the DAC to each dimmer chip to correctly reference the analog voltage, all four MOSFET pairs would have their Source pins linked, so the circuit would fail to operate as expected (all four channels would operate as one).

Can somebody explain:

  • why are the Source pins connected to GND ?
  • is it possible to omit this connection ?
  • is this connection required for ZC detection, if so could this be provided via a different method ?

3 wire reference design

NB: this is a different question to FL5150 (LED dimmer) circuit explanation


FL5150 is phase cut dimmer, not a LED dimmer. Output is AC switch. Connection to ground is reference for gate controlling voltage. Then transistor are opened, current goes through diode of one transistor and drain-source the second. Another polarity - transistors switching places. Transistor are controlled by DC signal. The ground is not real earth, it is minus of IC power supply.

  • \$\begingroup\$ Thx, it is obvious in hindsight that the gate circuit needs to be completed through the source pin. \$\endgroup\$ – David Moorhouse Dec 28 '20 at 21:31

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