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I have a problem with choosing most appropriate transistor to control backlight of my LCD display. According to datasheet its \$I_f = 60mA\$, and \$V_f = 3.2V\$. My circuitry is all 3.3V based. I want to supply PWM signal from my STM32 F411CEU6, but it can only supply max of 25mA per GPIO pin. Due to rather large \$V_f\$ there is not a lot of space left for Current limiting resistor, and \$V_{DS}\$ or \$V_{CE}\$. I have found 2SC5585TL NPN BJT (datasheet). According to it, if I were to supply 6mA at Base (\$\frac{I_C}{I_B} = \frac{60mA}{6mA} = 10\$) \$V_{CE}\$ should be around 25mV, which leaves 75mV for current limiting resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

Will this work as intended? Are my calculations correct? What is the preferred transistor type here BJT, or MOSFET?

toolic
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Andrzej Komor
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  • Do you have any higher voltage available? For example, the 3.3 V might have been derived from a 5 V supply. The backlight probably doesn't need a tightly regulated supply, so the 3.3 V regulator would have an easier life. – Andrew Morton Jun 30 '23 at 14:54
  • @AndrewMorton unfortunately I only have 3.3V power supply available at the moment. What do you mean by „tightly regulated supply”? – Andrzej Komor Jun 30 '23 at 15:33
  • A tightly-regulated supply would not have much voltage ripple. – Andrew Morton Jun 30 '23 at 18:09

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