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BQ24650, will this chip work on a two layer board?

Adrian Morgan
Prodigy 10 points
Other Parts Discussed in Thread: BQ24650

I created a schematic and board layout in Eagle using the BQ24650 IC. In the reference design, some components had an analog ground and a digital ground. I decided to connect all of them to analog ground and create a 2 layer board with ground planes on the top and bottom layer. Will the circuit still operate properly if it is connected this way? I noticed the EVM version created by TI used 4 layers. Is there a specific reason?

Here are the files:

Schematic:

5074.MPPTcharger.sch

PCB Layout:

1754.MPPTcharger.brd

Also, here are some pictures of the schematic and the board.


Thanks.

  • 0
    Prodigy 50 points
    excerpt from: www.onmyphd.com

    Large voltage and current spikes from power circuits can generate disruptive interference in the control circuits, which are usually low voltage and current. For each power supply stage, you should keep power ground and control ground separate. If you tie them together in the PCB, do it in a point near the end of the supply path (most notably near the PCB ground connection).

    In case you are thinking about using a multilayer circuit board, use the top layer for power traces, the bottom layer for control traces and an inner layer for the control ground. The large ground plane in the middle layer will offer a small impedance path for any interference from the power circuits and protect the control paths from it.

    As for the case of power circuits, you should keep the digital and analog grounds separate. The reason is the same: voltage and current spikes from digital circuits can generate interference (noise) in the analog circuits, affecting their performance. If you tie them together in the PCB, do it in a point near the end of the supply path (most notably near the PCB ground connection). Although there are other points of view to this problem, this is the most accepted.

    Any interference in the analog ground has the same effect as it would be in the signal lines (all that matters is the differential voltage between any point and ground). You want a large ground plane to reduce its resistance, but that also makes it more susceptible to capacitive coupling to lines routed above or beneath it. To that end, analog ground should only have analog lines crossing it and the same for digital. That reduces the capacitive coupling between analog and digital circuits.

    Hope this helps, I am currently working a design with the Bq24650 as well using two layers. I will update once the boards come in and I can verify performance.
  • 0
    TI__Expert 5565 points
    Adrian,

    This device can be used on a 2-layer PCB. The EVM design uses 4 layers since it allows for a demonstration of a smaller solution size. Since you are using a 2-layer PCB, it's very critical to follow the layout guidelines listed in the datasheet for this device. The biggest challenge with a 2-layer PCB is to keep the AC loop (see figure 24) small between the input capacitor, high side MOSFET, low side MOSFET, inductor, and output capacitor.

    It is recommended that you follow the analog and digital ground connections as shows in the EVM. The analog ground connections carry large current while the more sensitive signals are tied to digital ground. It not properly connected, the current ripple from the analog ground will disrupt the sensitive signals tied to digital ground. From the picture of your layout, it doesn't seem like you are following rule #7 from the PCB layout guidelines for the device (see page 25) since I don't see the thermal pad tied to the ground pin.
  • 0
    Prodigy 10 points
    Did you ever get this to work on the two layer board? I'm trying to do something similar now but I'm not sure it'll work.