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I need to control TFT backlight via single GPIO output from atxmega128a4u.

The LCD backlight requires +3.0V, while the rest of my schematic runs on +3.3V. I do not have access to backlight ground pin, as it is connected to ground in the TFT module. The module consumes 50mA in total, I have no idea how much power is consumed by backlight, but it should be more than allowed max 20mA from a single MCU pin. I do not want to use more than one GPIO due to pin shortage.

So, in other words, I need to have a simple schematic to supply +3.0V with at least 50mA, that is controlled by +3.3 microcontroller output.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ "I do not have access to backlight ground pin, as it is connected to ground in the TFT module." You have shown it as a resistor and connected to +3.3 V. The question needs a tidy-up. Are you asking how to switch the positive supply to an LED? \$\endgroup\$ – Transistor Jul 11 '16 at 13:25
  • \$\begingroup\$ Why not get out the ol' multimeter and test the current that the backlight draws? \$\endgroup\$ – Bort Jul 11 '16 at 13:29
  • \$\begingroup\$ It shows around 35mA, in any case it is too much for a single MCU pin. \$\endgroup\$ – Andrejs Cainikovs Jul 11 '16 at 13:30
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) A low-side switch. Switch GPIO high to turn on LED. (b) A high-side switch. Switch GPIO low to turn on the LED.

Figure 1 shows how to switch a large load using a transistor. Any small-signal transistor of the right type should do the job.

How it works:

In Figure 1b a small current (3 mA) is pulled through the Q3 base. This causes Q3 to turn on and saturate - effectively making it an on-off switch. The current that passes through Q3 will be determined by the load of R3 and D2.

Normally R1 or R3 value would have to be calculated to limit the current to the correct value. It sounds as though your backlight may already have that internally.

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