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LMZ20501 as Dynamically Adjustable Output

Other Parts Discussed in Thread: LMZ20501, TPS63000, LMZ21700, TPS61061

Wanna use LMZ20501 as Dynamically Adjustable Output as
written in Application Report (SLVA251-August 2006) by Will Hadden.
In the report it used TPS63000 as an example, but my question is
is it also possible with LMZ20501?

The agent distributor of the IC in Japan said that it is not a recommended
circuit for LMZ20501. The main risk is damage of inrush current due to DAC's slew rate is too fast.
The system I plan to use as following.
1. DAC's Slew rate 1.2V/us.
   DAC Full Scale =2.5V
2. Time to settled-Vout = 2ms(Max)
3. Load resistance = 30-40 Ohms

4. Voltage range = 2.68 to 3.69 V

In PSpice simulation it worked as I want, but does it really work in real?.

The main reason I want to use this IC is because its small size (Inductor on package).

  • Hi Sainer,

    I don't foresee any road blocker in using LMZ20501 with DAC adjusted Vout. The only concern is that this part is voltage mode controlled, and the Cff value depends on the Vout.

    In your case, the Vout range, 2.68V to 3.69V, is not too wide. I'd think the 3.3V typical design on the datasheet should work. The other way to check is to do two designs on the Webench with LMZ20501 with your min Vout and max Vout. Use the same BOM values in both designs and run simulations to check the stability. Also you can use the EVM to perform bench test to get transient performance.  




  • Hi Yang,

    I really appreciated your reply.

    By the way after rechecking, LMZ20501 only covers Voltage Range of 0.8V to 3.6V, which means it doesn't cover 3.69V as I wanted.

    * I think the right one is LMZ21700. In regard to LMZ20501 you told me to take concern on CFF. LMZ21700 doesn't look as if it needs CFF, are there anything else that I have to concern with it? 

    The Distributor in Japan didn't want take the risk and told us that it is on our risk when using it, even if it works on PSpice.  On the first page of Datasheet it is written that "Adjustable Output Voltage" but there is no circuit example on the datasheet can give clarity what kind of "Adjustable Output Voltage" it is. I think that  the reason the Distributor in Japan doesn't have confidence to admit our "DAC's controlled Buck Converter Circuit".

    ** Does the Datasheet mean that "Adjustable Output Voltage" is "DAC's controlled Buck Converter Circuit"?.

    For your information, there was also a  case in the past that the PFET inside the DC-DC Converter broken of high inrush current due to fast slew rate of DAC, so the Distributor might not want to take the same risk. At that time we used TPS61061. 

    Our purpose to use dynamic Vout range of 2.68V to 3.69V, is to sweep Voltage around that range to find the smallest Load Loss and proper Voltage to our load Heater. After we find the proper one we set that value to microcontroller so it would stay on that voltage after. In conclusion, we only change the voltage dynamically/ freely just to find the proper Voltage/ Current to our Load.

    Thanks again for your reply.


  • The LMZ21700 does not need a Cff. The only requirement on the DS is Rfbb < 400kOhm. The same BOM should work fine for your Vout adjustable range. I think it should work well for your application.



  • Thanks Yang, your reply is very helpful.

  • Hi Yang, I have 2 more questions.

    1. What is the minimum value for the CSS?. On page 13 of datasheet it is written that 1000 pF, but it is too slow for me. Can I set the Tss around 235 us (CSS=470pF) ?

    2. Connecting VOS pin.

        On page 25 it is written that "Use a thin and short trace from the VOUT terminal of the output capacitor to the VOS pin". Is it a reason why it has to connect to the output capacitor?.  I want to sense the voltage  of my Load (not the output Capacitor), so I want to connect the VOS pin  to one pin of my Heater. Is it a problem to do that?.