Voltage regulation circuit 12V to 5V in 5A range

[Demon]

Obviously I mistyped the part number. Yes it is a P channel.

From the datasheet the Vth seems to be 4Volts min. Have you tested yet this MosFet? ...

Nope. But I'm using a IRFZ44N n-channel to PWM the LED strip with has similar specs.

...Please check with an Osciloscope that the MosFet is fully closed at PIC low state, so that there is no voltage drop on S-D pins....

Good idea. Choosing the proper resistors to drive the MOSFET is still a black art to me.

...Was there a specific reason that you selected a P channel instead of a N channel control of the PWM signal? Usually the N channel have better characteristics in terms of Rds.

Ioannis

Because when I first started testing blue-on-white LCDs, I was PWMing both the Contrast (V0) and Backlight (LedA). I didn't even consider MOSFETs and went straight to transistors since we're not talking about a lot of current. Why I chose PNP transistors back then is a mystery; maybe I saw a schematic on google and copied that.

I've since switched to white-on-black LCDs, I don't need to have adjustable Contrast (V0). So controlling the Backlight with a PNP transistor is just "the old way of doing things". But since noise has become an issue, getting a P-channel MOSFET seemed like the logical way to replace the PNP.

But if I had thought about it for 2 seconds, I would have realized I have access to both the Anode and Cathode on the Backlight. Since I already have an IRFZ44N on the circuit, it would cost less at JLCPCB to use another IRFZ44N instead of a second type of component.

So that's the long-winded way of me saying that now I'm going to swap the 2N2907A PNP to a IRFZ44N n-channel.

Funny story, I finally realized the letter at the end of a MOSFET seems to indicate the channel. I don't know if it holds true for all, but it's working for the first two that I researched.

[Ioannis]

Switching to MosFet from bipolar, as Richard noted, it will introduce MORE noise because they are faster.

So, do add capacitors (100nF plus 10uF) as close as possible to the Mosfet power supply, taking care of ground loops at the same time.

Ioannis

[richard]

So, do add capacitors

a waste of time until he address' the correct current limiting resistors for each display

if its really 90mA per display then a 1000uF might help a little.
The peak current as it stands could be very high depending on led vf , could be amps , who knows

[Ioannis]

OK, but having a 4.000uF on the 5V rail is too much, isn't it?

Ioannis

[richard]

You seem to be moving the goal posts. One 1000uF at the pnp tranny emitter since all four back lights are parallel driven from that source

[Ioannis]

By "if its really 90mA per display then a 1000uF might help a little" seemed to me that you meant 1000uF per display.

In any case though, the 1084 or even the humble 7805, do not have any problem with large capacitance.

I bet a reverse biased diode, across output/input pin, would be needed to protect the regulator, in case the Reg. input is grounded for whatever reason while the outuput cap is still charged.

Ioannis

[Demon]

...
I bet a reverse biased diode, across output/input pin, would be needed to protect the regulator, in case the Reg. input is grounded for whatever reason while the outuput cap is still charged.

Ioannis

Yup, I saw that in the datasheet.

Haven't done any of the suggestions for the LCDs yet. I'm doing voltage regulator heat tests for now; looking for the best heatsink setup.


This is how I'm running the LM1084-ADJ now:




Black is from the datasheet, red is my adjustments.

[richard]

Haven't done any of the suggestions for the LCDs yet. I'm doing voltage regulator heat tests for now; looking for the best heatsink setup.

if you solve the backlight issue then you will most likely find the wall warts simple smps are not the problem and the regs and heatsinks are unnecessary

[Ioannis]

The LCD datasheets note that the backlight is 5v at 90mA, isn't it?

So I understand that no current liminting resistor is necessary, as it is included on the LCD board.

Ioannis