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# LM5160DNTFBK-CALC: Maximum Recommended value for Rr

Part Number: LM5160DNTFBK-CALC

I think "Maximum Recommended Value for Rr" is calculated so that injection level becomes 50mV.

I heard injection level should be 20mV to 50mV.

Injection level decreases as Rr value increases.

So I think  this "Maximum Recommended Value for Rr" is "Min recommended value". Because injection level becomes over 50mV if we use lower resistor value than this recommended value.

Is my understand wrong?

Best Regards,
Kohei Sasaki

• Hi Sasaki-san,

While your understanding is true.If it was stated as  "minimum" a larger resistor wouldn't provide enough ripple for stable operation. The equation takes minimum Vin in ensure enough ripple is produce because ripple decreases at lower Vin. This is why you see this equation on the datasheet for type 3 ripple configuration. For more information see this application report on ripple gereration (http://www.ti.com/lit/an/snva166a/snva166a.pdf

Regards,

Ethan

• Sasaki-san,

Here is another relevant app note: http://www.ti.com/lit/an/snva874/snva874.pdf

--

Tim

• Ethan-san, Tim-san,

Thank you for your reply.

If my understand is correct, I think Rr value range is calculated as the following,

Rr_min= ((Vin_min - Vo_pri)*Ton_max) / (50mV * Cr)

Rr_max= ((Vin_min - Vo_pri)*Ton_max) / (25mV * Cr)

What do you think about this opinion?

I think we can calculate injection level from the following equation. Is this correct?

Vinj=((Vin_min - Vo_pri)*Ton_max) / (Cr * Rr)

Best Regards,
Kohei Sasaki

• Hi Sasaki-san,

Yes, I do believe the ranges will be sufficient for your design. The last equation is good to confirm that the level desired is always achievable across input voltage, Cr, and Rr.

Regards,

Ethan