LM2611 needs a min of 2.7V, didn't want that to limit the min VDD at which I could talk to my part, so I run if off my 6.6V rail that the TPS61085 is putting out. Not sure where my big mistake is, but the joy of getting the 61085 working after realizing that NXP puts stripes on the CATHODE of its Schottky diodes unlike the rest of the world soon evaporated when I saw what the LM2611 was doing to the boost output. That's a big lesson - you need to think about high current issues - a charge pump is probably way better off here in terms of working right off - especially when you've got a fixed voltage rail. What was I thinking? I thought - hey, a Qpump needs a regulated input, I'm better off with a switcher. What I didn't tell National in their feedback form was that the Cuk is pulling about 1A out of the boost. The table is getting hot!
I am using the LM2611A driven by an on-board boost (TPS61085) to realize a -5V voltage. I am using components according to the datasheet Figure 1. The boost output is correct when the LM2611 is disabled - it is 6.6V solid. However, when the LM2611 is enabled, there is large ripple on the boost output and the feedback node voltage of the LM is about 0.4V and the output is about 2V. The boost output average value is still 6.6V when the boost is overcompensated - with a 0 ohm and 1 uF cap on COMP. With 51k and 1 nF, the ripple is about 10V p2p. There is no other load on either the boost of the Cuk. Any help I can get in resolving this issue will be greatly appreciated.
Looking at the LM2611 datasheet - it looks like the startup current (despite what is claimed as soft-start) is way up there in the 1.5A range. So it could be that the 61085 is cycling in and out of UVLO all the time. So nothing really works. I wonder if a charge pump would really be better of here - if you can find a low current part, go with it.
Their response :
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The issue is you have a boost regulator (TPS61085) is driving a dynamic load (the LM2611). So first you should probably put a filter between them - for example a cap to further reduce any existing ripple from the boost regulator. The other thing to check is the inductor you've selected for the boost regulator. Depending on the current load of the LM2611, it will also determine the value of inductor of the TPS61085. Also, you probably should first check the LM2611A circuit by itself, with a test bench power supply, and then you can see thecurrent levels you can expect (both average current and the transients). From then you can optimize the boost regulator circuit inductor and other parts (output cap, etc) appropriately.
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What they missed (failing to see things from the customer's point of view can sometimes be fatal. But, a big outfit like NSM is probably safe) :
1) I've done my layout already. If they know something like this probably won't work, why not specify the source better on their datasheet?
2) Who really has the luxury of trying out different caps and inductors? That's nuts - this is not like running spice on your ckt.
My mistake was in not using jumpers to isolate the different modules and allowing test-sources (clean, expensive) for verifying the modules. What I did try was to force the 6.6V output of the TI boost and see if that helped - but I was forcing through a really long cable, so it probably had almost no effect.
Any, end of the story is that most of the bench triple supplies have a negative supply that you mostly can't use for anything else, so all I had to do - and here, my great idea of putting down some non-standard pads at the last minute really helped me out - I was able to put down a 2N7002, a resistor and some wires do give the MCU more control. Stuff works pretty well actually.
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