UCC256302: question

Hi Masazumi-san,

Thanks for your interest in UCC256302.

1. RVCC UVLO is not related to VCC voltage. 

2. You may also pull the BLK pin below 2.13V to make the controller stop switching

3. VCC will slowly decay because the VCC capacitance is no longer getting charged by the bias winding. When VCC is <10.5V, The controller will enable the internal JFET to charge VCC. If FB is held low for an extended period of time, you will see the controller periodically enable the JFET to charge VCC.

4. If FB is held low for an extended period of time, the controller treats this as a "fault" condition in its state machine. The controller will transition to the "Fault" state and when the FB pin is released, it will go through a full restart.Take a look at figure 44 and table 2 in the UCC256302 datasheet for more information

5.If FB is shorted to gnd, the controller will not switch until the short is removed. If FB is held lower than the burst mode threshold for more than 200ms, the controller will treat this as a "fault" and enter the fault state in its state machine.

6. There is an example of OVP protection in PMP21098:

7. The JFET will enable when VCC is less than 10.5V.

8.The WAKEUP state expires after an internal 150us timer. I do not have min or max values on the timer however.

9. I would advise against estimating the total VCC charge needed based on the gate charge of the primary side MOSFET's. For most applications, ~100uF is sufficient.

Best Regards,

Ben Lough

Hi Masazumi-san,

1. RVCC only becomes enabled after VCC has become greater than the VCCStartSelf threshold. Initially you should see VCC charge from the JFET and rise up to ~26V (RVCC is stil 0). After VCC becomes greater than 26V, the controller enables RVCC and switching begins.

2. Pulling FB low or pulling BLK low will cause the controller to go through soft start. I would expect the same startup behavior for both methods.

3. Yes, the JFET is periodically enabled to charge VCC when VCC becomes less than 10.5V. If the controller has been disabled by pulling down the FB pin, there will be no switching action and it will not be possible to charge VCC from the bias winding.

4. No the controller should not enable the JFET when in OV condition. Yes, the controller will enable the JFET when VCC is less than 10.3V.

5. You will want to make sure there is enough capacitance to keep VCC above 10.5V while in burst mode when there is a long burst-off period. Please keep in mind that when the controller is disabled by pulling FB low, the JFET will become enabled when VCC falls below 10.5V. If you reduce the capacitance, the controller VCC will fall at a faster rate and the JFET will enable more frequently as a result. I don't recommend sizing the VCC capacitance based on the MOSFET gate charge.

Best Regards,
Ben Lough