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Thread: Understanding part of epaper display circuit

  1. #1

    Understanding part of epaper display circuit

    I have the Adafruit e-paper display working with the teensy 3. I'm trying to make a board that has the example circuit for that epaper display. I'm trying to understand and perhaps simplify a specific part of this circuit:

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    The complete circuit diagram is here. The epaper display is spec'd to draw about 8mA, with a max inrush current of 40mA. It's a 3V device. In the portion above, I believe they are trying to switch the display power (VCC_T), which is 3.3V, using a control signal (PANEL_ON) from the microcontroller that might be 5V or even more, and moreover can source only a small amount of current.

    In other parts of the circuit, they are using a single N-MOSFET as a level shifter. Here, however, they use this trio of FETs:

    • 2N7002KW is an enhancement mode N-MOSFET. Max drain current 310 mA and gate threshold voltage 1.1 - 2.1V.
    • BSS84W is an enhancement mode P-MOSFET. Max drain current -130 mA, gate threshold voltage -0.8 - -2.0 V.
    • AP2305GN is a P-Channel Enhancement MOSFET with a drain current of > 3 A and a gate threshold voltage -0.5 - -1.16 V.

    My question is: why three FETs and what do they each do? Why a FET with such a huge current capacity?

    Moreover, if I know I'm driving this only from 3.3V logic, can I simplify this somewhat? This circuit already has a huge number of discrete components and I'd like to reduce if possible, but more I'd like to understand what's going on here.

    I know this is probably elementary, but I'm not an EE so if you can provide references that might help me understand this circuit, I'd appreciate it.


    Last edited by cmason; 06-18-2013 at 03:40 AM.

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Nov 2012
    Quote Originally Posted by cmason View Post
    Why a FET with such a huge current capacity?
    Probably for its low on resistance.

    Moreover, if I know I'm driving this only from 3.3V logic, can I simplify this somewhat?
    Yes. You can eliminate Q2 and Q3.

    Connect a resistor between the Q1 gate and a pin on Teensy3 which will control it. Use 330 or 470 ohms. Q1's gate is basically a capacitor, so this resistor will limit the surge of current flowing through the Teensy3 pin when you turn Q1 on or off. The polarity will be backwards, so driving the pin LOW will turn on the power to the display.

    The second resistor is not as important, but I'd recommend you connect either a pullup or pulldown resistor at the Teensy3 pin, so the display defaults to either on or off when your program isn't running (eg, when reprogramming or briefly during powerup). Any value in the 10K to 100K range is probably fine. You want to avoid the case where the Q1 gate is floating and might have a random voltage where Q1 is partially conducting, which could cause it to overheat.

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