SLVA450B February   2011  – April 2021 TL7770-12 , TL7770-5 , TPS3513 , TPS3514 , TPS3606-33 , TPS3613-01 , TPS3617-50 , TPS3618-50 , TPS3620-33-EP , TPS3805H33-EP , TPS3808-EP , TPS3818G25 , TPS386000 , TPS386000-Q1 , TPS386040 , TPS386596 , TPS60120 , TPS60121 , TPS60122 , TPS60123 , TPS60124 , TPS60125 , TPS60130 , TPS60131 , TPS60132 , TPS60133 , TPS60140 , TPS60141 , TPS60204 , TPS60205 , TPS60210 , TPS60211 , TPS60212 , TPS60213 , TPS61130 , TPS61131 , TPS62050 , TPS62051 , TPS62052 , TPS62054 , TPS62056 , TPS62110 , TPS62110-EP , TPS62111 , TPS62111-EP , TPS62112 , TPS62112-EP , TPS65000 , TPS65001 , TPS65010 , TPS65011 , TPS65012 , TPS65013 , TPS65014 , TPS65020 , TPS65021 , TPS65022 , TPS65023 , TPS650231 , TPS65023B , TPS650240 , TPS650241 , TPS650241-Q1 , TPS650242 , TPS650243 , TPS650243-Q1 , TPS650244 , TPS650245 , TPS650250 , TPS65050 , TPS65053 , TPS65055 , TPS650830 , TPS65086 , TPS65720 , TPS65721 , TPS65811 , UC1543 , UC1903 , UC2543 , UC3903 , UCC2946 , UCC3946

 

  1.   Trademarks
  2. 1Introduction
  3. 2Designing with the TPS3808G01
  4. 3Example Calculations
  5. 4Other Sources of Inaccuracy
  6. 5Conclusion
  7. 6References

4 Other Sources of Inaccuracy

In the equations, calculations, and graph presented here, neither the resistor tolerance nor the tolerance on the SENSE voltage is accounted for. These factors have an effect on the accuracy of VIT (listed below as TOLVREF) as well, and its tolerance can be estimated by using Equation 13.

Equation 13. GUID-2D0CB2C6-5913-4A96-A72C-EF820C5CE51A-low.gif

The accuracy of the TPS3808G01 can be evaluated with 1% tolerance resistors (TOLR) and the 2% tolerance on VIT (TOLVREF) given in the product data sheet. The accuracy of the divider as a result of the leakage current is then added to these other sources of variation, as shown in Equation 14. Therefore, the worst-case accuracy of the 1% divider designed in the example is 4.73%.

Equation 14. GUID-6D8EB0F7-0B19-4038-B0C7-D08D6BDBA382-low.gif

Another concern is that higher-value resistances can have noise coupled into them more easily than lower ones. This additional noise can further decrease the accuracy. An IC with an internal voltage divider, however, addresses both of these concerns by having higher accuracy with a lower current draw than an IC that is set externally. Because the resistors are integrated in the IC, the coupling nodes and capacitances are much smaller, resulting in superior noise immunity. For example, the TPS3808G33 has a fixed VIT of 3.07 V, and boasts a total accuracy (including SENSE voltage variation, leakage current variation, and internal resistor tolerance) of ±1.25% along with a current consumption of 0.86 µA at a VI of 3.3 V. This amount is one-third of the current consumed with 25% lower accuracy than the example calculation discussed, which assumed an ideal SENSE voltage and ideal resistor values. Taking into account the additional variation in these values, then, the total accuracy of the example with the adjustable version is one-third that of the fixed version before the leakage error is even added.