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LT8304, block diagram, questions

Category: Datasheet/Specs
Product Number: LT8304

Dear people, it's me again, Mario. Last week I didn't post my questions under the 'Q&A'- heading, so now here's the same post again, now beneath the rigth section!

For my bachelorthesis, there a still a few questions left about the operation of the LT8304. Those questions are mainly based on the block diagram from the datasheet. In the following picture, I roughly framed those parts of the diagram, which are still not clear:

Now to my six questions:

1. In the datasheet, the MOSFETs M2 and M3 are described as a sense circuit, but in the block diagram, there is a constant current source of 25µA on the left side. With a ratio of 1:4, it would try to hold the IRFB-current always at 100µA. So what is the real purpose of this circuit? 

By the way, we use to call it “current mirror” in German.

2. Do the blocks A2 and gm work like simple comparators or do they have analog output values, dependent from the switch current (A2) or the voltage difference (gm)?

3. When does the block A3 reset and why it compares the output signals of A2 and gm?

4. How does the oscillator work and on which way it’s driven by the amplifier gm?

5. The datasheet gives an equation for the RFB-resistor:

 Could you please tell me or give me a tip, how you came to this formula?

I tried to derive it by a mesh circulation, but I always came to an equation, dependent from the input voltage:

6. How does the temperature compensation (TC) work and what determines the PTAT voltage?

If you could give me an answer or notice to this questions, it would really help me!

I've got to submit my thesis 'till 31. August and the operation of the LT8304 ist the only topic, that's still a bit strange to me.

Thank you in advance!

Greetings, Mario

  • Hi MarWie,

    Sorry for this late reply. As per your questions,

    1. The network is a current mirror. When M2’s source is high, there current should be close to around 100uA.

    2. It works like an analog amplifier.

    3. The block A3 controls the duty cycle.

    4. The oscillator has something to do with the control loop and switching frequency.

    5. The controller controls the Vref to 1V, then the current through that resistor will always be Vref/Rref. This same current should aso be seen through Rfb, so multiply this current through Rref resistor by the top Rfb resistor.

    6. Please look into the Output Temperature Compensation section of datasheet.