UC28025: SS pin

Kuramochi Tadahiko

Guru 28715 points

Part Number: UC28025

Hi,

My customer hope to control duty ratio by SS pin.

Is this possible?

Does duty ratio change according to SS pin voltage?

Best Regards,

Kuramochi

over 8 years ago

0 Colin Gillmor over 8 years ago

TI__Mastermind 25570 points

Hi Kuramochi-san

The SS pin may be used to control the MAXIMUM duty cycle - but not to control the duty cycle during normal regulation.

Regards
Colin

0 John Griffin1 over 8 years ago

TI__Mastermind 34465 points

Hi Kuramochi,

The answer is yes. The voltage on the SS pin directly controls the maximum output voltage of the error amplifier.

And this sets the maximum duty cycle. So you can directly set the duty cycle by varying the SS pin voltage.

Regards

John

0 Kuramochi Tadahiko over 8 years ago in reply to John Griffin1

Guru 28715 points

Colin-san, John-san,

Thank you for your support!

Could you tell me the duty ratio vs the SS pin voltage?

Best Regards,
Kuramochi

0 John Griffin1 over 8 years ago in reply to Kuramochi Tadahiko

TI__Mastermind 34465 points

The ramp input is usually set at 1.8V pp and it has an internal 1.25V offset.
This means that with 1.5V to 2V on the SS pin the duty cycle is 0%
and with about 4V on SS the duty cycle is maximum.
The max value of SS should be 5.1V.

Regards
John

0 Kuramochi Tadahiko over 8 years ago in reply to John Griffin1

Guru 28715 points

John-san,

Thank you for your advice.

Reading the datasheet, I can't understand follows.
Could you tell me where of the datasheetI should read in detail?

>This means that with 1.5V to 2V on the SS pin the duty cycle is 0%
Why is 1.5V to 2V?
>and with about 4V on SS the duty cycle is maximum.
Why is 4V?
>The max value of SS should be 5.1V.
Why 5.1V?

Best Regards,
Kuramochi

0 Kuramochi Tadahiko over 8 years ago in reply to Kuramochi Tadahiko

Guru 28715 points

John-san,

How is this situation?

Best Regards,
Kuramochi

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UC28025: SS pin