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# Description of efficiency measurement in LTC3731 datasheet

Hi all,

I am asking about the Description of efficiency measurement in LTC3731 datasheet.

In the formula for Pmain on page 27 of the data sheet, if the value of δ is calculated as [0.005*65] and the value of Rds(on) as 9 mΩ, the answer is 0.87W. The value of Rds(on) at 90°C is multiplied by the temperature coefficient.

Is it correct?

Please tell me what should be substituted for (2) and (3) in the following equation. Thanks

Takahiro

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• Hello Takahiro-san,

1. The calculation shown in the data sheet has an error. The Rds(on) at 25°C (7mΩ) should be used in the Pmain equation, not the Rds(on) at 90°C (9mΩ) since the (1+δ) term will adjust the Rds(on) for temperature. With an Rds(on) of 7mΩ the calculated loss will be 0.68W. The formula itself is correct.
2. Do not substitute anything for (2) or (3). These are the actual numbers used in the formula.

Best regards,

Mike

• Hello mike,

>>Do not substitute anything for (2) or (3). These are the actual numbers used in the formula.

You say I have to use actual values, but what parameters should I use?
What do you use to get the answer to 6.3W?

Best regards,

Takahiro

• Hello Takahiro-san,

The calculation is re-written below: Note, VIN = 20V for the calculation. 20V was used because it is the highest input voltage. The switching losses are the highest at maximum Vin.

Best regards,

Mike

• Hello Mike,

So it's not the value of the design example.
I understand that the maximum input voltage is to be substituted. Takahiro

• Hello Takahiro-san,

The power loss calculations on page 27 are for the application described on page 23. On page 23, the typical input voltage is specified to be 12V and the maximum input voltage is specified to be 20V.

Best regards,

Mike