Logic Mosfet for display backlight

Other Hardware General Electronics
1

Hi,
Like to thank ahead of time for the assistance and would greatly appreciate if my work could be checked.

Setup: ESP32 controlling the backlight dimming for a 2.4"TFT display
Display Backlight: 19.2v @ 20ma

Questions:

  1. Is the schematic correct? (wiring and resistor value)
  2. Mosfet selection appropriate? SSM3K15ACT datasheet

ESP32_Backlight_PWM
ESP32_Backlight_PWM1245×496 7.92 KB

2
  1. The 1K pulldown is maybe a little stronger than needed. 10K would be just as good.
  2. How will you solder this MOSFET? Looks like it can only be soldered with a reflow oven, cannot be hand soldered with a normal handheld iron.
3

You need a current limiting resistor in series with the backlight LEDs.

4

thats already handled by the LED power source that provides 19.2v at 20ma.
Thanks!

5

Are you using a TFT breakout board where the display is already wired to a control chip?

6

Thank you.

ok. I'll try a 10k.

After I posted then realized how darn small that device is... now looking at an SOT23 device:
thoughts?
mmftn20 datasheet

ESP32_Backlight_PWM v2
ESP32_Backlight_PWM v21268×596 12.8 KB

I solder this with my solder station onhand.

7

yes

8

Then you shouldn't need to do any of this. Your breakout will handle all of that for you. Usually you simply need to supply a high signal to turn the backlight on, low to turn it off and a PWM signal to dim it. You can wire it directly to your ESP32. This is from the guide on the Adafruit TFT display:
image
No mosfet required.

9

MMFTN20 has an Rds(on) of 30 ohms at Vgs=2.5V
That is kind of high

10

Not necessarily. There may be a resistor built in, but we don't know that for certain, or a constant-current driver could be used.

That is impossible. "Ye canna break the laws 'o physics, Jim".

The power source could try to achieve 20mA by applying up to 19.2V or could supply 19.2V and hope that no more or less than 20mA is drawn. But no power source can do both at the same time!

Maybe you should post a link to the specs of this display.

11

Yes, the rest of the circuit provides a constant voltage constant current source rated for the series of backlight leds. This is NOT shown in the diagrams/photos here, but it is elsewhere in the circuit and provided by the connection point "19.2v@20ma".

12

Like I said. Impossible.

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13

I bet the display datasheet shows something like this...

image

...which merely means that at 20mA, there's a 19.2 v drop across the six LEDs (3.2v each).

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14

What display are you trying to use? That would clear up a lot of confusion.

15

My question was about using a MOSFET for logic PWM control of 6 LEDs in series, which in total require a 20ma/19.2v power source for full brightness. The power source circuitry for these LEDs has already been designed, tested, and working for a few years. I'm just looking to expand the circuitry to incorporate a dimming control via the MOSFET using logic PWM control coding on the ESP32. I appreciate everyone's concern for the power supply topic, but that's not an issue.

Cheers

16

Ok. thank you! I'll continue searching for a better fit. What would you think to be a more appropriate Rds level? more toward the 1.8v range?

17

Please clarify. Are you saying that the ESP32 can be directly connected to the LED string as a SINK?

19.2/20ma ---> LED string (6) ---> ESP32 IO25 (PWM coding applied for dimming)

I didn't think the ESP32 could do this.

18

Rds (on) is the resistance of the MOSFET when it is turned ON.
You should look for a MOSFET that has an Rds (on) of less than 3 ohms at a Vgs of 2.5V

19

Ok. That makes sense.
Thank you for the info and feedback!

This looks good I think. SSM3K376R Datasheet

20

PWM'ing a constant current source... Does that work?

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