UCC28704: VCL and Region working in UCC28704

Raty Milly

Prodigy 40 points

Part Number: UCC28704

Hello team,

How do I calculate VCL?

How does Control Law works?

How can I determine which region to work in?

Best regards

over 6 years ago

0 Hong Huang over 6 years ago

TI__Guru 73455 points

VCL is an internal parameter. You can estimate its value based on switching frequency and CS-pin voltage, Vcs. When Vcs at 750mV, the operation is in Region 4. when Vcs is at 188mV, the operation is in Region 2 and 1. When Vcs is between 188mV and 750mV, the operation is in Region 3.

0 Raty Milly over 6 years ago in reply to Hong Huang

Prodigy 40 points

And how is RS1 and RS2 related with VCL?

0 Hong Huang over 6 years ago in reply to Raty Milly

TI__Guru 73455 points

RS1 can affect VCL through IVSLS which is IVSL(run) in equation (1), page 10.

IVSLS / KLC (page 9 diagram) will source through CS pin to RLC (equation (3), page 11) to superpose a voltage on top of voltage Rcs. The voltage on CS pin is obtained to affect the control law.

0 Raty Milly over 6 years ago in reply to Hong Huang

Prodigy 40 points

I think my problem is that I don´t understand the graphic in the page 15, the values in x axis are VCL or are VSS(Page 9 diagram)?

0 Hong Huang over 6 years ago in reply to Raty Milly

TI__Guru 73455 points

It is VCL not VSS. VCL is an internal variable. You can refer to Figure 12, page 12, of the datasheet, to see where VCL is.

0 Raty Milly over 6 years ago in reply to Hong Huang

Prodigy 40 points

So, if I want to work in region 3, VCL is between 2.2V and 3V, what voltage have to be VSS?

0 Hong Huang over 6 years ago in reply to Raty Milly

TI__Guru 73455 points

To work in region 3, you will need to adjust Rcs and Lp, as well as transformer turns ratio, and basically, using equation (17) as well as associated equations. I suggest you use the UCC28704 design tool to make your design.

VSS is used for a relative comparison and not directly proportional to VCL. So VSS cannot help your design and VSS shown in the diagram is for help how VCL is obtained but not a straight forward like that.

Please follow the application section in the datasheet for your design, and also use the design tool to help calculation.

0 Hong Huang over 6 years ago in reply to Hong Huang

TI__Guru 73455 points

The datasheet page 12 and 13 has explanation how VS (VSS) works. You can see from there VS signal is used from its relative changes.

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UCC28704: VCL and Region working in UCC28704