vne147
Member
Hello again. I’m having some difficulty with a project I’m working on, and I’m hoping someone can help me find a solution.
I’m attempting to use the LTC3623 as a programmable current source by replicating the circuit shown here:
http://www.linear.com/solutions/7390
My test circuit is slightly different but I believe equivalent to the schematic shown at the link above. I’ve replaced VSHUNT in the lower left hand portion of the schematic with an ADR130 0.5V precision voltage reference and a 1kΩ potentiometer. I attached another schematic showing the modification I made highlighted in red. Everything else in the schematic is the same.
My problem is that the circuit isn’t working at all. Right now I have it connected to a supply voltage of 6V, and a load of .22 Ω. No measurable current flows for any potentiometer setting and when I measure the output voltage it's around 10 mV. When I remove the .22 Ω load resistor and measure the output voltage, it’s around 100 mV and only changes about ± 10 mV over the full adjustment range of the potentiometer. Obviously this is not what I expected because I used the same op amp, MOSFET, and component values shown in Linear’s schematic.
So naturally I have a few questions:
Here are links to some datasheets:
Please let me know if I need to clarify anything. Thanks in advance for any help you can provide.
I’m attempting to use the LTC3623 as a programmable current source by replicating the circuit shown here:
http://www.linear.com/solutions/7390
My test circuit is slightly different but I believe equivalent to the schematic shown at the link above. I’ve replaced VSHUNT in the lower left hand portion of the schematic with an ADR130 0.5V precision voltage reference and a 1kΩ potentiometer. I attached another schematic showing the modification I made highlighted in red. Everything else in the schematic is the same.
My problem is that the circuit isn’t working at all. Right now I have it connected to a supply voltage of 6V, and a load of .22 Ω. No measurable current flows for any potentiometer setting and when I measure the output voltage it's around 10 mV. When I remove the .22 Ω load resistor and measure the output voltage, it’s around 100 mV and only changes about ± 10 mV over the full adjustment range of the potentiometer. Obviously this is not what I expected because I used the same op amp, MOSFET, and component values shown in Linear’s schematic.
So naturally I have a few questions:
- Correct me if I’m wrong, but the MOSFET symbol shown in the schematic is for a P-Channel MOSFET, yet the part number called out (BSC019N02KS) is for an N-Channel Power MOSFET. Can anyone infer enough about what is going on in this circuit to say for sure whether that MOSFET should be N channel or P channel?
- Also, the MOSFET called out in the schematic is for a pretty beefy SMD MOSFET rated for over 100W. I can’t figure out for the life of me why that would be necessary. Can anyone shed light on that?
- I have the LTC2054 op amp V+ connected to 3.3V and V- connected to ground. I have also tried it with V+ connected to the same 6V supply as the LTC3623. Both configurations yielded equally unsuccessful results. Does anyone know how I should connect the LTC2054?
- Can anyone see any problems with my test set up or potential issues that I didn’t address?
Here are links to some datasheets:
- BSC019N02KS
- LTC3623 (Refer to Figure 9 on Page 23)
- LTC2054
- ADR130
Please let me know if I need to clarify anything. Thanks in advance for any help you can provide.