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I'm having trouble with MCP73833. It's 1S lipo charger powered from USB. I followed this figure from application note:

enter image description here

My schematic:

enter image description here

Battery connectors are X4 and X5, load is connected, USB is disconnected,

The problem is on the line marked in my schematic I measure 2.1V. I don't know what might be wrong with this design and how it can get in there. D5 schottky diode prevents this situation. Also its not IRLML2244 fault. MCP73833 10, 9 pins are the only way. I checked block diagram of MCP73833 and the only thing that bothers me is the Direction Control. Is there a option that it might get though Vbat and appear on VDD?

The question is: How 2.1V appeared on marked line?

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  • \$\begingroup\$ You said D5 prevents the situation, so do you mean you only measure 2.1V when D5 is removed, (not as your drawing shows)? What are the conditions - battery connected, load connected, USB powered, etc.? What are X4 and X5? Why did you connect the battery to where the load is connected on the reference schematic? The net you pointed to is tied directly to the input connector, so again I'm wondering if that connector is powered when you measured. \$\endgroup\$ – AngeloQ Mar 11 '17 at 18:07
  • \$\begingroup\$ @AngeloQ I mean that D5 is there and current can't flow in this direction. X4, X5 are battery connectors, battery is connected, load connected, USB disconnected. VBAT is system load \$\endgroup\$ – KRol Mar 11 '17 at 18:13
  • \$\begingroup\$ OK, thanks for clarifying. So, yes D5 should prevent current flow from the battery reaching the input. There may be some leakage through the diode, but I would expect R30 to keep the voltage very low. So perhaps there is current flow from VBAT and VDD in the controller as you suggested. You could remove it to confirm if this is the case. Maybe the device is damaged? \$\endgroup\$ – AngeloQ Mar 11 '17 at 18:47
  • \$\begingroup\$ @AngeloQ I just assembled a new one and there is still this problem. So I think it's not damaged. Also I checed if pcb house did the job properly and there is everything OK. As you mentioned I should desolder 73833 and check it. \$\endgroup\$ – KRol Mar 11 '17 at 19:10
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Don't forget that there will be internal protection diodes. So this will be the reason for some voltage appearing at the line.

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