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# LMZ12001: LMZ120x and LMZ140x dropout voltage

Part Number: LMZ12001

Looking at both the LMZ120x and LMZ140x power modules datasheets, it is not clear to me what the minimum input voltage needs to be for a given output voltage.

Does Vin need to be greater than or equal to Vout?

For example the LMZ12001 datasheet says Vin can range from 4.5V to 20V and it also says Vout can range from 0.8V to 6V.  This implies you can have Vin = 4.5V and Vout = 6V.

• Hi John,

No, It doesn't imply 4.5Vin to produce 6Vout. This is a buck converter and only can do step down. In a ideal(lossless) buck converter, the input voltage and duty cycle of the switch determine the output voltage. Vout=D*Vin=(Ton/Ts)*Vin. Toff min is 260ns in the datasheet 7.3.1 section. if you set 1MHz switching frequency, Ts=1000ns, Tonmax=Ts-Toffmin=1000-260=740ns. Dmax=740/1000=0.74, for 6Vout, Vinmin=Vout/Dmax=6/0.74=8.11V without consideration of nay losses.

Hope this helps,

Thanks,

Nancy

• Nancy,

Thanks for your response.  What page of the datasheet explains all of this?  I must be missing something here....

Basically, I am trying to figure out what the minimum Vin needs to be for a given Vout.  Your datasheet says this is a "simple switcher" and it's "easy to use", but I'm not so sure at this point.

• Hi John,

The datasheet doesn't have detail explanation. But it is basic Buck converter theory. Please review attached application note for better understanding.slup206.pdf

Thanks,

Nancy

• Nancy,

I think I see where the confusion came from.  Section 8.2.2 of the datasheet provides a detailed design procedure for implementation of this part into a design, and Step 1 indicates you need to select two resistors to set the minimum operating Vin.  The problem is you need to know the Vin (min) before you can even accomplish Step 1.  Perhaps the datasheet needs a revised Step 1?

Using the equations you provided (and not included in the datasheet), I calculated a minimum Vin = 6.82V using an Ron = 100k (125k is the max) for a Vout = 6V.  Based on these calculations, you are saying the part is guaranteed to provide a 6V output for any Vin equal to or greater than 6.82V?  Is this correct?

Given the minimum Vin = 6.82V, I can now proceed with section 8.2.2.1.1 (step 1 in datasheet) and select Rent and Renb to determine the input voltage below which the LMZ1201 will be disabled.  The datasheet is silent on how low I can set the minimum Vin, so I am assuming I could set this as low as 6.82V?

• Hi John,

Datasheet could not set min Vin for your application. The min Vin is determined by your application and it is your choice. Not our converter.

Min Vin=6.82V is ideal(lossless) condition, you need to keep some margin for MOSFET, inductor DCR and PCB IR drop. 8Vin should be safe.

Step1 is only set min EN on/off threshold, not for min Vin. Please check your application for Vin range.

Thanks,

Nancy