UCC28251: Minimum on time and EN control discussion

Hi Charles， I will reply to you tomorrow. —— Teng

Hi Teng,

Do you have any updates about this question?

Thanks.

BR,

CL

Hi Charles,

Sorry I don't have such circuit that can limit the minimum on-time less than 500 ns.

It is very strange why PWM still has output when SS pin is pulled to ground.  Can you test that will the controller shout down when pulling SS pin to ground externally during normal operation.

Regards,

Teng

Hi Feng,

This waveform is tested during normal operation. And I saw there is another thread below may have the same issue. Could you please help explain more? Thanks.

BR,

CL

Hi Feng,

Could you please help share some of your comments here? This is urgent case for my customer. Thanks.

BR,

CL

Hi Feng,

I do not see any response to this problem. Would appreciate it if you can share your comments by April 20th. Thanks.

BR,

CL

Hi Charles,

Sorry for the delay, since I just received the EVM.

See the attached oscilloscope picture I tested by shorting SS to ground directly on the EVM, it shows pulling SS to ground can disable device and terminate PWM immediately.  Could you advice customer try to shorting SS ground directly and share me the schematic at teng-feng@ti.com

Besides, the device can also be disabled by controlling EN pin. 

Hi Teng,

Thank you for your response here. Customer cannot share schematic to us, for this is confidential information for them. They still have two questions about this device,

1. Do you pull  EN pin to GND when you pull down the SS signal in the waveform you shared here?

2.  Why output is not in the max duty in the SS stage? For COMP voltage is higher than RAMP here.

CH1 COMP; CH2 RAMP; CH3 OUTPUT;

BR,

CL

Hi Charles,

1./ I only pulled the SS pin to ground in the testing. Could customer try to short SS pin to ground directly? and they can only share a part of schematic. 

2./ we can find there is an internal voltage offset at RAMP/CS pin from the block diagram.

Regards,

Teng

Hi Teng,

Thank you for your response here.

1.See SS control circuit from customer. Customer redo the test again, and they found that when pull down SS pin ,the output will be zero after 58ms. Do you have some explanation about this waveform?

2. What is the internal voltage offset value? I cannot find it in our datasheet.

Thanks.

BR,

CL

Hi Teng,

Thank you for your response here. Customer used secondary-side control and voltage control. Please see the latest waveform,

When SS is short to GND,the PWM signal terminated after comp voltage equal to zero. But there is still have 2ms time delay. Is it related to the secondary-side control here? What kind of control mode you used when you short SS to GND? Thanks.

Yellow=SS, Blue= COMP, Purple= PWM

BR,

CL