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LM2575HV-ADJ min value for Vin
I'm about to use the LM2575HV-ADJ in my circuit design set up for a 3,3V output voltage. In my country this part is easier to find than the LM2575HV-3.3. My question is what is the minium Vin for the 3.3V output. I'd like to confirm if it really is 4.75V (value for the LM2575HV-3.3) as it should only be related to the minimum duty cycle of its switching circuit.
Yes, the -ADJ version will work for your 3.3V application. The minimum VIN to support 3.3V depends on the load current and temperature.
The following graph (taken from page 9 of the datasheet) shows the dropout voltage for 1A and 200mA loads. The minimum VIN would be equal to 3.3V + Vdropout.
In terms of the minimum operating voltage for the -ADJ part, please refer to this graph from page 10 of the datasheet:
Please let us know if you have any other questions.
Thanks for the quick answer. However it still isn't 100% clear to me. As for the first figure, min Vin at 25 degrees Celsius and Ioad of 200mA would be Vout + 0.9V. Instead, in the second figure, this value is 3.5V which is Vin(1.23V) + 1.77V. Anyway, the base circuit for these graphics has a 5V output so that increases a little bit my confusion.
Could you then confirm that this chip would work with Vin=5V, Vout=3.3V for Iload=200mA and Iload=1A?
Yes, the chip will work for Vin=5V, Vout=3.3V over the full load range.
The plots I mentioned describe two constraints:
1. Minimum operating input voltage to bias the chip properly. This is described in the second figure and the value is 3.5V (at 25C). This needs to be satisfied regardless of the output voltage. Since your input voltage is at 5V, this requirement is satisfied.
2. Minimum input voltage to ensure the output is regulated at the target voltage. This is described in the dropout voltage figure. The assumption in the dropout figure is that constraint 1 is already satisfied, and it is in your case.
From the dropout voltage graph, the worst case dropout voltage for full load (1A) and -40C is 1.375V.
To regulate the output at 3.3V, you need to provide at least 3.3V + 1.375V = 4.675V of input voltage.This is worst case.
At 25C, the dropout voltage at 1A load goes down to 1.25V. This means that at 25C, the input voltage has to be at least 4.55V in order to regulate the output at 3.3V.
I hope this helps.
Please let us know if you have other questions.
Perfect. Thans a lot!
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