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Registered Member #2843
Joined: Sun May 02 2010, 03:40PM
Location: King George, Virginia, U.S.A.
Posts: 4
For the NMOS you have on the left hand side of your schematic, you want R1 to be a pull-down to ground (and on the order of 1M-ohm). In addition, you'll also want to put a pull-down resistor between the source and ground to get a true logic signal at the 5-pin terminal block.
I'm not sure about using the standard 0.1" headers for 9A currents. At least I personally wouldn't do that.
I think that >3000mA ripple current capacitors are needed to bypass the primary side of the transformer, because in the worst case scenario, when your power supply cannot provide the charging current, you will draw it from those capacitors. If your design is for 9A of charging current, you should put 3 capacitors in parallel. As for the cost, it can always be more! For example, the 56uF Sayno capacitors suggested in the LT3750 datasheet (Farnell order code 9189220) are $14.68 each!
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
The done output NMOS is supposed to turn on/off an LED or some other type of visual indicator, so would that be okay?
With the Capacitors, I guess I have no choice then . I'm powering it from a SLA (81Amp short circuit current) so would I really need something that good?
The headers I'm using are 13.5A rms and 9A is only the peak current, the average would be somewhere around 4.5A I'm guessing. But thanks for pointing that out, I would never have seen that stupid me
Registered Member #2843
Joined: Sun May 02 2010, 03:40PM
Location: King George, Virginia, U.S.A.
Posts: 4
Sure, no problem with using the NMOS to toggle an indicator, as long as the source-drain current rating is not exceeded and a free-wheeling diode is included if applicable.
I retract my previous post. Your left NMOS should work just fine with the pull-up resistor on the gate. The ~DONE pin is an open collector, so you essentially have an NMOS buffer.
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
Hey
Don't think I've abandoned this project just yet! I've just had a HELL of a lot of problems but first, changes to design.
The main change changing the place of the IC to a DIL socket and making an adaptor board for the IC. Some more minor changes were making the power connections wire-spade rather than screw terminal and adding a socket for a replaceable Rvout, for a more flexible output voltage
My main problem was my PCB. I had 3 fail PCBs. In the first two I stuffed up the etching and in the third I slipped when I was tinning the pads and ripped off the IC tracks. For this reason and some other stuff ups due to the sub millimeter IC tracks I changed the design to use a DIL socket so I wouldn't have to worry about soldering the whole board and stuffing it up at the IC and having to make a whole new board!!! So after making the design changes I successful constructed the main board but I soldered on the pin on the IC adaptor board the wrong way around and I ripped off the track when desoldering the pins. So I got another adaptor board etched up and successfully soldered it together. So I put it into the socket, hooked it up and powered it on. No flames or explosions. Nothing, absolutely nothing. It wasn't charging my capacitor. I poked around and I found the cause. The done pin pull-up resistor was the wrong! Farnell gave me the wrong resistor and I plopped it in and it killed the IC. Good thing I used a socket!
Lesson: always check that they give you the right components!!!!
So I had to order in another sample IC which took awhile to come in. I've got it soldered into another adaptor board and swapped it with the dead one. Now all I need is the proper pull-up resistor then I'll try AGAIN. I've just ordered one off Farnell so hopefully this weekend I'll have it working!
If you would like to see the PCB file and schematic and all that just ask and I'll post em
Pictures: As you can see I have excellent SMD soldering skills
Track Side (note: I've taken out the Done pull-up)
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LT3750 Cap Charger
. I'm powering it from a SLA (81Amp short circuit current) so would I really need something that good?
stupid me
