I am doing a 30Vin , 100Vout, 20W boost converter with UCC28C43 controller.
I am suppying it with a R/Zener regulator as follows……
As you can see, there is a 4u7 capacitor on the UCC28C43’s VREF pin (pin 8)
..(as well as on the Vcc pin)
The UCC28C43 datasheet on page 4 says that the maximum operating current of the UCC28C43 is 3mA……….
My gate drive accounts for another one mA , so therefore, the absolute maximum current that I can be drawing through the resistor of the R/Zener regulator is 4mA.
That would drop just 13.2V across R1, allowing the zener diode to regulate the Vcc voltage of UCC28C43 to 12V.
However, I have found that the UCC28C43 does not start up when the above circuit is used………and in fact , it just “ hangs” with its Vcc voltage at around 8V.
Why is this?
I found that I can make the UCC28C43 start up properly by having 728 Ohms of resistance in place of R1.
Do you agree that this can mean only one thing…….i.e. that the UCC28C43 draws a large surge of current at the instant of startup, and if the linear regulator that supplies its Vcc cannot meet the demand of this current surge, then the UCC28C43 just “hangs”?
So, why does the UCC28C43 datasheet not tell of this large current surge which occurs at start-up?
….or is the surge only happening because I have 4u7 of capacitance on the UCC28C43’s VREF pin (pin 8)?
My original circuit, with R1= 3K3 , should have worked, do you agree?
Why did it not work?
Your circuit has a mistake, I guess Vcc for the UCC should go from the BZX.
Anyway, you have neglected the current consumption of the BZX12 itself, I think that that is your problem
YES SORRY ABOUT THE MISTAKE......YOU ARE RIGHT IT SHOULD BE CONNECTED TO THE ZENER...BUT MY QUESTION STILL APPLIES...(AGAIN SORRY FOR MY POOR DRAWNG CARE)
sorry about capitals.......
The mistake in the circuit drawing is secondary, please read carefully all what I wrote, I have already answered you: You have to take into account the current that goes into the BZ12 itself. The datasheet gives an operation point of 5mA typical.
What is most likely happening is that whent eh VCC gets to about 8 volts the IC tries to start by charging the Vref cap from the VCC cap. This pulls the VCC cap below the turnoff voltage and shuts the unit down discharging the Vref cap.
Change the Vref cap to 0.1 uF and retry to see if it starts.
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