I am currently designing my own PCB which combines a microcontroller board and a Waveshare e-Paper driver board (Schematics). I wonder what the functionality of this part here is. Can anyone help me understand its purpose?
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1\$\begingroup\$ The actual function is that it gives me headaches! That's not your fault, but this would be much easier to understand had it been drawn properly: \$\endgroup\$– Marcus MüllerJun 13, 2022 at 11:32
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1\$\begingroup\$ 1. instead of one GND, use two separate GND symbols (that means they're still connected! Just not drawn that way.) And also, make them both point down, one attached to Q32's emitter, and one to the lower end of R34/R37. \$\endgroup\$– Marcus MüllerJun 13, 2022 at 11:33
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1 Answer
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The circuit turns on VCC and 3V3 (on a part of the schematic that you have not posted, regulator U2 connected to the VCC_EN signal) when EPD_RST is high.
Simple as that.
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\$\begingroup\$ Thanks for your fast reply! Can you help me understand, how exactly it does that? I understand that when VCC_EN becomes high, the LDO gets enabled. Does VCC_INT get "connected" to VCC when EPD_RST is high? \$\endgroup\$ Jun 13, 2022 at 11:55
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\$\begingroup\$ @Schwandes When EPD_RST is high, Q32 starts conducting which grounds the gate of Q31. When the gate of Q31 is grounded, Q31 starts conducting, supplying power from VCC_INT to VCC. \$\endgroup\$ Jun 14, 2022 at 6:18
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\$\begingroup\$ Thanks, this makes sense! So the circuit only gets supplied with power, if EPD_RST is high. If during operation, the EPD_RST becomes low and then high again (to reset the EPD), the power supply seems to be interrupted as well. Does this cause any unexpected problems? Or is this an expected and desired behavior? Also: Why is VCC_EN connected via the 4 resistor bridge? What effect does this have on the function of the circuit? \$\endgroup\$ Jun 14, 2022 at 11:25
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\$\begingroup\$ @Schwandes Indeed the power supply will be interrupted if EDP_RST goes low and high. I don't know any details about the display that you'll drive with this so I don't know if it might cause any issues. But one may assume that the designer had some good intention with the design. VCC_EN is not connected via "4 resistor bridge", if the schematic was better drawn, it would be clearer that it is only connected to a 47k pull-down resistor, and driven from EPD_RST via 1k resistor. The pull-down (R36) isn't really necessary though, since the same effect would be caused by R34 anyway. \$\endgroup\$ Jun 15, 2022 at 5:53