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Registered Member #56586
Joined: Thu Aug 20 2015, 01:43AM
Location:
Posts: 2
I'm trying to design a schematic using the LT3750 to charge a 300uF cap to 100v as fast as possible. It's a work in progress and need a little help.
1) I'm not understanding the DONE pin. I want to add an LED to come on when the cap is finished charging. How would I use the DONE pin for this operation?
2) I've got an RC snubber that I've added in but also read about an active snubber for efficiency. I'm not too familiar with snubbers like this and am borrowing from other schematics. Any thoughts, comments or recommendations?
3) Last, I want to trigger with an IGBT or Mosfet using low voltage. I've added in the IR2117 gate driver. The LED in the diagram is for continuity. I'd like to hear some thoughts about this and any suggestions for improvement.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Regarding the DONE pin, it is maybe a better idea to use a simple voltage indicator mounted directly on the capacitor.
I say this because it is also used as an error indication, so it can both mean charged capacitor or not charged capacitor... useless :) "DONE (Pin 2): Open Collector Indication Pin. When target output voltage is reached, an NPN transistor turns on. Requires a pull-up resistor or current source. Any fault conditions such as thermal shutdown or undervoltage lockout will also turn on the NPN."
You might need to add a transistor to drive a LED if you want to use the done pin afterall. "The DONE bit is pulled up to VIN through a high impedance 100K resistor. This may not be compatible with all electrical systems."
Registered Member #56586
Joined: Thu Aug 20 2015, 01:43AM
Location:
Posts: 2
So the DONE pin connects to an internal NPN, which turns on through the 100k pull up resistor when the targeted output voltage is reached. When the NPN is turned on, I'm assuming voltage is flowing through it from VIN. Couldn't an LED be connected in series with the 100k so when the NPN switches on the LED should switch on as well. Or do I have this wrong? Although the 100K is a bit much for the LED.
edit: Ok so after looking at the specs it seems the DONE pin can only handle 1mA of current. Can't use it to sink current into an LED directly. Looking at a PNP transistor.
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LT3750 Capacitor Charge/Discharge Circuit Help
