Pad on the Bottom

[NZ0I]

I came across this part that looks like it might be suitable for a 12V buck-boost switching supply for transmitters, it could accommodate battery voltages from 5V to 15V, and could be more than 80% efficient: http://www.digikey.com/products/en?keywords=LTC3112EDHD%23PBF-ND

The part has a large ground pad on the bottom, for both electrical connection and thermal heat flow. It needs to be soldered to a pad beneath the chip.

The switching regulator being used in the receiver has a similar pad, but it isn't required to have an electrical connection, it is used for thermal only. I didn't worry about trying to solder it to the underlying pad in the receiver since we are using the device well below its rated current limit. It has never gotten warm. Still, it would be wise to solder the bottom pad.

To facilitate soldering the bottom pad in the new boards, I've removed the solder resist from the rear of the PCB at the location of the vias beneath the thermal pad. I've added as many vias as I can, and with the solder resist removed from the opposite side of the board, I'm thinking (hoping) it might make it easier to solder the pad from below. The vias should serve as little tubes that might wick the solder in... or not. There is also the question of how to keep the part in place while soldering it from below.

Does either of you have any experience with soldering this type of device? Any suggestions or pointers?

[Gerald Boyd]

I had an analog devices part on another project that had a bottom pad that needed connected.to accommodate that I put a plated through hole that was large enough to get the tip of the iron to touch both the device pad and the plated hole. Then applied solder using thin solder. Had to use flux to get good flow. The hole also needs to be big enough for both the tip and solder.

The converter would keep us running longer if the battery is going out. If the transmitter monitored the battery maybe it could send the letter "L" at the end of the cycle and someone could fix it just in time before it goes off line. 

Hope to get back on the presentation tonight and will modify based on the comments made the other day . 

Thanks.

Jerry


Sent from my iPhone

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[Charles Scharlau]

I will see how big a plated through-hole I can put under those parts. Thank you for the tip. How did you ensure the part remained in place while soldering the pad? Did you solder all the pins first? I suspect that might work, but I'd be a little concerned that the solder holding the pins in place might liquify while soldering the pad underneath... but perhaps that is unlikely since it would require a lot of heat going into the board to cause that to happen.

An efficient switching supply that can handle input voltages between 5V and 15V would allow many different types of battery to be used, and used efficiently. You could use alkaline cells from AA's to D's or a 9V battery, 6V or 12V SLA batteries, a multi-cell LiPo pack, etc. I'll look into whether this (or another part) can support voltages down to 3V while putting out the current we need, so that single-cell LiPos could be used. In this application (a transmitter) we shouldn't have to worry much about noise, as long as the hunters can't DF the noise coming from the switching supply.

I think the processor should monitor the battery, and the SWR. It might also monitor the forward output power in order to close the feedback loop for setting transmit power. It might also make sense to monitor the temperature of the final output transistor.

If the battery gets too low, or the SWR or transistor temperature gets too high, the processor can throttle back the power to prevent damage to the transmitter or extend the battery long enough to assist with finding the transmitter. I like the "L" idea, and it could even send out a number indicating how many more minutes the transmitter has before total shutdown. If some other problem is indicated (e.g., SWR) it could send some other diagnostic character.

 

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