While you wait for parts, have you thought of simulating the circuit? TI should have tools for doing this, and of course LTSpice if there is a model for the 3845 (someone probably has made one). Great way to evaluate what is going on without the bother of actually letting the smoke out of parts 🙂
Simulation is very good thing, but i doubt if software has a inductor saturation current value parameter. At least for microcap i've not saw that.
Its a fet. I tested using a dmm. I do it all tge time before using it for any test. Now don't have any.
looks like current sense resistor is only 10 mili ohms - way too small
Max threshold is 3V -- that a possible 300 amps which is possible in transient or unstable conditions and not sure how device can even control
Max threshold is 3V -- that a possible 300 amps which is possible in transient or unstable conditions and not sure how device can even control
I would use 30 mili ohm current sense and clamp comp pin with diode to .7V. That would reduce max drain current to 23 amps
Actually it is internally limited to 1V for current limit comparator but with 10 mili ohm sense that allows a possible 100 amps. Pretty sure your choke will saturate well before that. Even 30 mili ohm is kind of dicey depending on your buck inductor design
For instance i thought this could be the case. I will see in the morning when i get to the workshop.
The general theory of a current mode converter is that an analog of the inductor current (the voltage across the sense resistor) is compared to the output of the error amp to turn the switch off. The switch is turned on at the start of each cycle. This can be seen in the block diagram posted by Blujay. The point here is that the voltage arcross the current sense resistor must be large enough, say a hundred millivolts or so, for the current sense comparator to have a decent signal when the converter is working with a nominal load. If this condition is not met, the converter may still appear to work, even regulate voltage to a degree, but it is far from optimal. Most would say it is "unstable" as the duty cycle varies widely from cylcle to cycle.
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with 50% pedestal current I guess switch current 9 amps peak or so-- to step 12V to 24V, 150 watts
That is only 90mV signal against 1V comparator range -- certainly no transient or overload protection and would be prone to noise and instability.
It may not even be operating in current mode and the fact that comp is type 1 single pole really invites poor phase margin and even oscillation
That is only 90mV signal against 1V comparator range -- certainly no transient or overload protection and would be prone to noise and instability.
It may not even be operating in current mode and the fact that comp is type 1 single pole really invites poor phase margin and even oscillation
Thanks blujay. You pinted out my issue. My current sensing register was not of correct value on the test board. It was shorting out mosfets. I had used 0.1 in my previous boards but used 0.01 on my test board mistakenly. It works now. Thanks.