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Adjusting charge current by adjusting control voltage into LT1248 PFC controller
moffy:
The diodes are probably there for protection purposes, stopping +ve high voltage inputs damaging the internals. Or they could just be a by product of the manufacturing process. As you say, who knows why? But at least we know they are there.
ocset:
Thanks but surely they cant really be there?...the one on the VSENSE pin would be stopping the feedback signal from getting to the error amplifier?
Jay_Diddy_B:
Hi,
The diodes are there and they are real. The reason that they are not documented beyond that, is that they are matched.
Consider this model:
The Error Amplifier, EA, has three inputs. There is the reference input, the soft start input and the feedback node, Vsense.
The EA will regulate the lowest of the two voltage, the fixed reference (7.5V) and the voltage on the SS pin.
When the voltage on the SS pin is greater the 7.5V the reference in the controlling input.
D1 and D2 are needed to OR the fixed reference and the soft start inputs.
The voltage at the non-inverting input of the EA is lower of the two inputs plus one diode drop.
D4 add the diode drop to the Vsense input.
The diodes are biased by the input currents of the EA.
Modelling Result
General Comments
You would be much better using an IC that is designed to be an LED driver, like the LT3799, instead of adding work arounds to a PFC chip.
Given the other topics that you have started on the EEVBlog in the last few weeks, I am sure that you are not designing a battery charger. You would get much better help if you told the truth and asked for help for an LED driver.
Regards,
Jay_Diddy_B
ocset:
Thankyou very much Jay_Diddy_B.
Your post was great, and i see now, amongst your other comments, the bit showing the current sources I1 and I3 really clears it up for me.
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