UC3843 current mode pwm hack

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enjen77

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dear all,

my project was designing boost controller for automotive purposes.
The problem are when the voltage from battery dropped to 7 Volt, because of cracking, suddenly UC3843 won't work and high voltage produced by this PWM is 0 volt.

I am trying to solve the problem by this circuit. can this circuit work?
 

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It will not work. First, to keep it operating you must keep pin 5 above 7V. Second, the line between REF and ground is shorted and will break everything.

You might consider maintaining Vcc with a winding on T3 as in the app notes.
 
It is a safety feature in the switcher and should not be defeated. If anything it should be enhanced since drawing off a car battery when it is below 10V will weaken the battery. 10V is under 1.7V on a 2V cell: dead battery.

What you are trying to do will destroy your battery!
 
We do not understand if you want the PWM to work below 7 volt or to stop below 7 volts. It is a good idea to stop the PWM when the battery is too low.

I normally pull 'COMP' pin low to shut off the PWM.
Your circuit will not work. I think you are trying to pull 'FB' high to shut off the PWM. This will work but is not good for a accurate 300 volts.
 
There is another control IC in the family with part no. "UCC28C40" from TI. The "43" has an undervoltage lockout of around 7.6 to 8.4 volts. But the "40" has a UVLO from 6.6 to 7.0 volts, which sounds like it will work for your application. The maximum on this device however is 18 volts. If you go with the UCC28C40, you can operate all the way down to 7.0 volts. Have I helped?

Claude
 

i want the PWM work below 7 V, as cranking voltage is only below 7 V causing the PWM won't work
 
mneary said:
It will not work. First, to keep it operating you must keep pin 5 above 7V. Second, the line between REF and ground is shorted and will break everything.

You might consider maintaining Vcc with a winding on T3 as in the app notes.
Click to expand...

could you explain more detail this sentences "will break everithing"

if the voltage less then UVLO i believe that the reference voltage will be 0 V, but it doesn't mean that this shorted to ground may be there is no current flow there, and with internal complex circuit i believe it is not short to ground

at least i hope

rgds
 

i will try your solution sir,

thank you
 
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