T
treez
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Hello
We are doing Boost converter PFC’d LED driver in Constant off time using LT8312 controller.
(0-300W, 240VAC)
LTspice sim and pdf schem attached.
We are adjusting the power throughput by adjusting a control voltage into the Vc pin. (This is possible because it’s a transconductance error amplifier) Do you know what is the control voltage range of the Vc voltage? I am guessing it is 2V max, (as that is the case with other linear.com chips) but what is the min?, and also, what is the tolerance on the max and min control voltages?
Also, the FET switch-ON is said to be caused by 80uA flowing through the DCM pin. Can you confirm that it means 80uA flowing “out” of the DCM pin? (ie DCM pin sourcing). This certainly is what the LTspice simulation shows. Also, there is a transistor in the DCM pin which appears to make it impossible for the DCM pin to actually source current, so how can the DCM pin actually source current?
Also, the DCM pin feeds a comparator with a 600mV reference. What is the significance of the 600mV reference? The datasheet makes no mention. On the LTspice simulator it actually appears that FET turn-ON is actually caused by the DCM pin going lower than 600mV….this would be the most likely way to bring about switch-ON of the FET following the complete discharge of the secondary. So why does the datasheet talk about the detection of 80uA of current in the DCM pin to instigate FET switch ON?
Also, the max current sense threshold of the LT8312 is just 100mV. This seems ridiculously low for an offline PFC chip. Surely it would suffer from noise issues?, especially in light load?
Would you thereby recommend actually using a current sense transformer rather than a current sense resistor with LT8312 in order to impart less noise to the Current sense pin of the LT8312?
LT8312 datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/8312fa.pdf
We are doing Boost converter PFC’d LED driver in Constant off time using LT8312 controller.
(0-300W, 240VAC)
LTspice sim and pdf schem attached.
We are adjusting the power throughput by adjusting a control voltage into the Vc pin. (This is possible because it’s a transconductance error amplifier) Do you know what is the control voltage range of the Vc voltage? I am guessing it is 2V max, (as that is the case with other linear.com chips) but what is the min?, and also, what is the tolerance on the max and min control voltages?
Also, the FET switch-ON is said to be caused by 80uA flowing through the DCM pin. Can you confirm that it means 80uA flowing “out” of the DCM pin? (ie DCM pin sourcing). This certainly is what the LTspice simulation shows. Also, there is a transistor in the DCM pin which appears to make it impossible for the DCM pin to actually source current, so how can the DCM pin actually source current?
Also, the DCM pin feeds a comparator with a 600mV reference. What is the significance of the 600mV reference? The datasheet makes no mention. On the LTspice simulator it actually appears that FET turn-ON is actually caused by the DCM pin going lower than 600mV….this would be the most likely way to bring about switch-ON of the FET following the complete discharge of the secondary. So why does the datasheet talk about the detection of 80uA of current in the DCM pin to instigate FET switch ON?
Also, the max current sense threshold of the LT8312 is just 100mV. This seems ridiculously low for an offline PFC chip. Surely it would suffer from noise issues?, especially in light load?
Would you thereby recommend actually using a current sense transformer rather than a current sense resistor with LT8312 in order to impart less noise to the Current sense pin of the LT8312?
LT8312 datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/8312fa.pdf
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