I'm developing a board to drive 5 LED strips with 1A maximum current. I need some help for design validation and component selection. The circuit is the following (repeted 5 times). The PWM signals (5 kHz) come from a PCA9685 or directly from the MCU (I have not decided yet). The MOSFET is a RFD14N05LSM and the driver IC is a MCP1416.

driver circuit

From MOSFET datasheet I calculated the following values. Are they correct? Should I consider some safety factor in the calculations?

  1. Turn-on current Ig= Qg(5)/t(ON) = 25nC/60ns = 417mA. Qg(5) is the total gate charge at 5V Vgs and t(ON) is the turn-on time from datasheet. For t(on) should I consider the rise time from MCP1416 datasheet, figure 2-1?
  2. Power dissipation Pds= rds(on)*Ids^2 = 0.1 Ohm * 1A^2 = 100mW. From this, considering a junction to case thermal resistance of 3.125 °C/W, the temperature of the junction should be just 0.3 °C above the ambient temperature.

Regarding the driver circuit:

  1. Could the following selection values be right for TVS diode D1? Reverse stand-off voltage 5.3V, breakdown voltage 7V, clamping voltage 21V. Should I consider anithing else in the selection of the diode?
  2. Should I place a resistor between driver IC and the gate? If yes, how can I determine the rigth value and the power dissipation?
  3. If I don't put the resistor RG, where are eventually dissipated the voltage spikes? I mean, if a 10V peak comes from the driver, the TVS forces the gate to 7V, where are dissipate the other 3V?
  4. Can I add anything else to this circuit to increase overall reliability?
  5. The datasheet of the MCP1416, sec. 4.3 (Decoupling Capacitors), recommends putting a low ESR film capacitor for decoupling. Since checking the datasheets of different film capacitors I did not find any reference to the ESR, what can be considered as low ESR? Can I take any film cap?
  • \$\begingroup\$ For 1A I'd get a logic level FET with lower Rdson / gate charge and drop the gate driver and TVS. Also add a flywheel diode just in case, Rg anywhere between 10R~47R and maybe a pull down? \$\endgroup\$ – Wesley Lee May 25 at 20:09
  • \$\begingroup\$ Built in reverse diode will handle 1A flyback easily. Lspice Rgate is in the ballpark of I2C driver Ron so none needed for matching for high speed transitions MPT. 3.3V for Vgs may be a little more than 0.1 ohm perhaps 0.3 max and Rja is 100’C/W with no sink. So 30’C rise is expected for Tj \$\endgroup\$ – Tony Stewart EE75 May 25 at 20:29
  • \$\begingroup\$ All film caps are low ESR \$\endgroup\$ – Tony Stewart EE75 May 25 at 21:04
  • \$\begingroup\$ @WesleyLee do you mean a diode in reverse bias across J1? Is this recommended also for non-inductive loads? \$\endgroup\$ – Marco May 25 at 22:47
  • 1
    \$\begingroup\$ @Marco - Yes that is what I meant - diodes are cheap, I always just add them, you don't need them for non inductive loads but long cables can be inductive and you never know what you might want to drive later. \$\endgroup\$ – Wesley Lee May 25 at 22:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.