UCC2813 - start or stop the PWM controller in response to input voltage.

Hello Hirotaka,

The UCC2813 error amplifier output is current limited to roughly 500 mA.  It is designed  so you can pull it low to demand 0% duty cycle with a FET.  I suggested you use this pin to stop PWM switching.

Regards,

Mike

Mike san

Thank you for your reply!

Does it mean that REF pin connects to low to stop PWM?

And, you mentioned "The UCC2813 error amplifier output is current limited to roughly 500 mA".
Recommended Operating Conditions of REF pin output current is 5mA
500mA seems that it is too big. Is 500mA value correct?

Kind regards

Hirotaka Matsumoto

Mike san

Thank you for your support always!
Ok, we use comp pin to stop PWM output.

Kind regards,

Hirotaka Matsumoto

Mike san

We have some additional questions.

When COMP is low, an internal capacitance for softstart t=4ms is not discharged, right?
So, when COMP releases low condtion, the output operates without softstart.
That's why, is it possible to add capacitance to COMP pin for softstart?

Kind regards,

Hirotaka Matsumoto

Hello Hirotaka,

I was discussing this one of my colleagues and he pointed out that you could reactive soft start by activating the 1.5-V threshold on the current sense pin. This feature is covered in section 8.3.12 of the data sheet. 

If you are using the level shift circuitry presented in figure 26 or have a resistor between the CS pin and CS resistor you could pull up on the CS pin to trip the 1.5V over current activating the soft start. You should be able to do this with a NPN transistor collector tied to Vref and the emitter tied to the CS pin.  The based would have a pull up resistor equivalent to the resistor between the CS pin and the current sense resistor and you would control the base of the transistor with a FET.  Please note these resistors should be large enough in size as not to dissipate too much power.

The other option you have is to feed the UCC2813 with a series pass regulator using a NPN transistor setup a voltage divider. You could first the device into UVLO by pulling base of the NPN transistor low with a FET.  This will also initiate a soft start on the restart of the device.

Regards,

Mike

Mike san

Thank you for your cooperation always!

OK, we persuade that our customer consider to use the CS pin to trip the 1.5V over current activating the soft start.
On the other hand, could let us know followings?

<Question1>
How much is the threthold of COMP pin for low?

<Question2>
Please refer to the follow file.
20161216_UCC2813.pdf

Kind regards,

Hirotaka Matsumoto

Hello  Matsumoto,

The resistor values are of equal value and the total impeadence is greater than 10K.

The interntal reference to the none inverting amplifier to the comp pin is Ref/2.  However,  I believe you might be looking for how low the comp has to be pulled down to demand 0% duty cycle.  I would recommend pulling the comp pin below 600 mV to ensure the output demands 0% duty cycle.

Regards,

Mike

Mike san

Thank you for your reply!

The resistor values are of equal value and the total impeadence is greater than 10K.
->Ok, we got it, however could you let us know how much the value of each resistances are?

Kind regards,

Hirotaka Matsumoto

Mike san

We are sorry to rsuh you, however could you let us know about last update?

Kind regards,

Hirotaka Matsumoto

Mike san

If it is possible, could you let us the approxy resistor ratio?

Kind regards,

Hirotaka Matsumoto