EVAL-CN0536-ARDZ comparator confusion

Question

Hi, 
 I cannot make sense of the feedback section of the schematic. How are VHYS=1.145V and V_POWER GOOD =1.025V obtained if there's no path to ground through DGND via R12 and R13? The input leakage current to the inverting input of U5 seems to be so small that it should have a negligible effect on the voltages. 
 Therefore either 
 a) VREF=VHYS=V_POWER GOOD=1.182V and R12, R13 and C13 are pretty much useless, or 
 b) There should be another 3.7-3.8M resistor in parallel with C13 
 
 What adds to the confusion is that in Figure 1 of CN-0536 the voltage divider is drawn correctly, but in Figure 15 has the same problem as in the full schematic, but the V_POWER GOOD is shown as 1.182V.

MarkThoren · Accepted Answer

Hi JesusC, we did some more digging and indeed there is an issue. The initial prototypes of this circuit had the following connections: 
 REF pin (1.182V) 
 127k 
 HYST pin (held at 1.145V) 
 453k 
 Power Good threshold (1.025V) 
 3.01M 
 1.025V (to another comparator input, not used in the final circuit) 
 866k 
 GND 
 
 The 3.01M and 866k got omitted on the production circuit. We'll update the circuit on the next build, in the meantime, the expected effect is that the BOOST ENABLE period will be shorter than expected, with correspondingly lower ripple on the high voltage output, and slightly higher quiescent current consumption. 
 If this is an issue in your application, you can connect a ~4M resistor across C13. 
 -Mark

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EVAL-CN0536-ARDZ comparator confusion

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