Hello,
I'm designing LTC2980 as power monitor for few LTC3636 POL and have some questions:
A) It is not clear how to select the Rsns (located after the inductor of LTC3636 and connected to the LTC2980 Vsense odd channels.
Also, If I'm working with very light load ~ few mA up to 0.3A is there any specific guideline or restriction ? (I didn't see such also at LTC2977 DS)
B) It is not clear regarding overall accuracy of the system after using the supervisor to fix the LTC3636 voltages
According to the 2980 DS it is mentioned -- Margin or Trim Supplies to within 0.15% of Target
It is not clear if this is the DAC accuracy that should be added to LTC3636 VFB accuracy of 1%
Also, when using the calculations of Rdac (R30) according to LTC2977 DS I'm getting very high Rdac for getting Vdc min and Vdc max within 1% error range
Here is an Excel to calculate the above for 2 domains for example (2.5V, 0.92V) with 1% desired accuracy
I didn't use the Ifb which is neglected and don't have typical value at LTC3636 DS (it is -30nA to +30nA so 0nA is probably the typical value) and anyway 30nA over 47.5K ohm doesn't change the results more than 0.06%
| Vfb | 0.6 | |
| Vout desired | 2.500 V | 0.920 V |
| Rtop (R20) Real E192 | 47500 | 47500 |
| Rbot (R10) Real E192 | 15000 | 88700 |
| Vout Real (nom) | 2.500 V | 0.921 V |
| Deviation from target | 0.00% | 0.14% |
| Domain DC accuracy | 1.0% | 1.0% |
| Rdac (R30) < | 1140000 | 3611150.79 |
| Rdac selected | 1140000 | 3611150.79 |
| VFs_Vdac > | 0.000 | 0.000 |
| VFs(1.38/2.65) | 1.380 | 1.380 |
| Vdc min | 2.468 | 0.911 |
| Vdc min accuracy | -1.30% | -0.97% |
| Vdc max | 2.525 | 0.929 |
| Vdc max accuracy | 1.00% | 1.00% |
I can see that Vdc max is 1% (it happens when the DAC outputs GND so R30 is parallel to R10)
Vdc min is higher than 1% - Is there a way that Full scale can be limited?
The Rdac looks very high - is it reasonable?
Also - Vdcmax and min are related to Vfb (LTC3636 Vfb is 1%) so does the total accuracy is poor than that?