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My main goal is to create constant current source as laser diode driver between 0-200mA, controllable by 0-3.3V analog input from MCU, And can be modulated (ON/OFF only) by square 0/3.3V PWM signal (changeable frequency up to 10khz and DC 0-100% from MCU).

V3 In my schematic should provide the analog 0-3.3V (MCU PWM with MCP4725 PWM to analog) and V2 should modulate the signal by PWM from MCU.

Attached are schematics of two switching concepts variations(BJT, MOSFET).

I already built the entire circuit with BJT option but since the BJT acted in saturation (the analog input 0-3.3v is smaller than the switching voltage), i received negative current in the analog input power supply and increasing the analog voltage didn't had any effect (does it make sense?)

besides that, rest of the circuit is working.

The MOSFET version use opamp since my MCU is 3.3v and unless it will be fully opened I'll ran into saturation at some point . (is it the right way to increase VGS from 3.3V?)

I'm not sure about high side switching / low side switching , BJT/MOSFET... Hopefully you will be able to help me solve the switching issue.

and last remark the opamps VCC is 7V since I'm using MCP602 and from the datasheet I understood that's the maximum voltage range.

Thank you!

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  • \$\begingroup\$ A laser diode is effectively an LED, so you might want to consider an LED driver. The most common way to accomplish 0-100% dimming is to choose a voltage that will be 100% and PWM it. This gives a comparably linear dimming. To improve on efficiency, you can use a control circuit that regulates voltage to control current down to a point above the LED shutoff voltage and switch to discontinuous mode only to go below that voltage. If you require a linear dimming response, this method requires calibration as the LED's luminous efficacy improves at lower current. \$\endgroup\$ – K H Mar 15 at 2:42
  • \$\begingroup\$ I'm not good enough to figure out what your circuit does do though, so maybe one of the engineers can help you out. \$\endgroup\$ – K H Mar 15 at 2:43
  • \$\begingroup\$ In regards to your first circuit, I would not recommend operating at the absolute maximum threshold. It should work at 5V. In regards to second circuit, I don't see why you need to amplify input M1 when the Vgs threshold is less than 2.5V. What is the laser part number? \$\endgroup\$ – Ernesto Mar 15 at 5:03
  • \$\begingroup\$ @KH Thank you for your answer, this circuit supposed to handle the aspects you raised. \$\endgroup\$ – s1ngames Mar 15 at 8:01
  • \$\begingroup\$ @Ernesto, your right, i might need to replace the MCP602 in higher voltage opamp, regarding the M1 amplifier, it wat intended to make the MCU 3.3V into 10V in order to fully open the MOSFET, in case the mosfet is not fully open i'm running into saturation in certain point adjusting V3. I'm not sure that opamp is the correct way to amplify the logic level to the requested VGS input. the laser diode part number is custom diode right now, and this driver should support variety of lasers. Thank you \$\endgroup\$ – s1ngames Mar 15 at 8:11

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