Help With Constant Power Circuit

[akahrim]

Hello All,

I'm was looking through the data sheet for the LT2940 Power monitoring chip. The applications section had an interesting circuit for a constant power (8W) load. It is supposed to be constant from 8V-32V. I have a basic understanding of the LT2940. My question is what does the LM334 do in the circuit and how does it enable the circuit to be constant 8W from 8V-32V? I redrew the circuit and attached it to this post. Any help understanding this circuit would be greatly appreciated. Thanks.

Amir

 

[dougy83]

PMON sinks a current proportional to the power dissipated by the FET. The LM334 is providing ~200uA current, so if the output power PMON output is drawing less than this, the voltage at the FET gate will increase; if PMON current is more than the LM334 current, then the gate voltage (and hence power) will decrease. Because of this feedback, the output power is regulated.

You can set the output power by adjusting any of: 1) current sense resistor, 2) voltage divider resistors, or 3) the current from the LM334.

 

[akahrim]

Awesome. Your explanation is great. Thanks alot for the feedback!!!!!

 

[akahrim]

PMON sinks a current proportional to the power dissipated by the FET. The LM334 is providing ~200uA current, so if the output power PMON output is drawing less than this, the voltage at the FET gate will increase; if PMON current is more than the LM334 current, then the gate voltage (and hence power) will decrease. Because of this feedback, the output power is regulated.

You can set the output power by adjusting any of: 1) current sense resistor, 2) voltage divider resistors, or 3) the current from the LM334.

Hey Dougy83,

I was playing with the circuit today and applying your explanation. I was wondering if you could help me clarify my understanding of the circuit. So this circuit is supposed to be constant 8W from an input of 8V to 32V. Going from 8V-32V would mean the MOSFET on resistance would have to go up. This would reduce the current in order to maintain the constant 8W. To raise the on resistance, we must lower the voltage applied to the gate. This is where my understanding gets fuzzy. From 8V-32V the LM334 supplies a constant ~200uA. The LT2940 will always (From 8V-32V) sink current because the I- is at the high side of the sense resistor. This makes the input voltage across current sense resistor "negative" meaning that PMON will be negative. It will sink current? As we raise the input voltage from 8V-33V, we need the MOSFET gate voltage to go lower. PMON must sink more current in order for the gate voltage to go lower? How the PMON output maintains the power is still a little fuzzy. Would it be possible for you to explain this a little further? Can the PMON output sink more than 200uA? Any additional clarification would be greatly appreciated.

 

[dougy83]

Yes, you're correct. As the input voltage increases the on resistance of FET increases, which requires the gate voltage to decrease.

If the input voltage increases, the PMON sink current increases to >200uA (the LM334 is still always outputting 200uA); e.g. let's say PMON sinks 220uA. There is therefore 20uA which comes from the 100N capacitor on the FET gate, decreasing its voltage. This in turn increases the FET resistance which will decrease the FET current and also decrease the PMON sink current. The current from the capacitor will decrease to 0 (PMON sink current == LM334 current), and this means the measured power is again 8W.

If the input voltage decreases, the PMON sink current decreases to e.g. 190uA which means that the 10uA difference flows into the cap which will increase its voltage. This will reduce the FET resistance and increase the power, which will increase PMON back up towards 200uA - when it reaches 200uA the currents are again equal and the FET gate votlage remains constant (as does the output power).

 

[akahrim]

Yes, you're correct. As the input voltage increases the on resistance of FET increases, which requires the gate voltage to decrease.

If the input voltage increases, the PMON sink current increases to >200uA (the LM334 is still always outputting 200uA); e.g. let's say PMON sinks 220uA. There is therefore 20uA which comes from the 100N capacitor on the FET gate, decreasing its voltage. This in turn increases the FET resistance which will decrease the FET current and also decrease the PMON sink current. The current from the capacitor will decrease to 0 (PMON sink current == LM334 current), and this means the measured power is again 8W.

If the input voltage decreases, the PMON sink current decreases to e.g. 190uA which means that the 10uA difference flows into the cap which will increase its voltage. This will reduce the FET resistance and increase the power, which will increase PMON back up towards 200uA - when it reaches 200uA the currents are again equal and the FET gate votlage remains constant (as does the output power).

Thank you very much for taking the time to explain this to me. The circuit makes sense to me know. Thanks again.

Amir