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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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You had used a P-channel Mosfet IRLML2244, so due to resistor R30 the gate pin of MOSFET is pulled down to the ground with 10k resistance as shown with a red circle. So the P channel MOSFET will be turned on and a small amount of current will flow from battery positive terminal to gate of MOSFET as shown with a blue line. so you are able to measure voltage on that line.Circuit Issue

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  • \$\begingroup\$ Can you elaborate on the current flow from source (VBAT) to gate of the MOSFET? btw, just to make things clear, X4 and X5 are battery terminals but battery voltage will also appear on VBAT because of p-mos body diode \$\endgroup\$ – KRol Jun 29 '20 at 20:42
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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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You only have 5 components connected to that line. I would check first for leakage in the diode and the FET. The other thing I would check would be that all grounds lead back to the same point - preferably all the same physical point as well as electrical to eliminate ringing and ground loops. From where are you getting your USB source? Are you certain it will supply the power demands placed upon it by this circuit? You also MUST have a limiting resistor for no-load conditions.

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  • \$\begingroup\$ This is 2 yeats old post but I remember that neither diode nor FET was leaking any current. Also I'm sure that all GND was connected to the same point. I can't event check it since I don't have this hardware anymore. Device needed max 100mA, I suplied it via lab power supply \$\endgroup\$ – KRol Feb 3 '20 at 0:27
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I know this is an old post, but wanted to corroborate that we're also observing the exact same behavior from MCP73833 as described in OP.

Our best guess is that Vbat flows out through VDD when there is no charge voltage input present, though not sure why this would be as the datasheet doesn't describe this behavior.

Confirmed via scoping & multiple test boards (and about a dozen different MCP73833), same behavior is present on all. We addressed the issue by placing a suitable diode on the input to VDD, as this is tied to Vbus on our particular circuit and was holding the line high.

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I read your circuit and the on thing you asked about marked place voltage. How does this come?
i think you should consider components parellel to 10k resistor and c2 which means both have same volts across c2 and 10k resistor .
Just measure c2 volt across input side and the controls is that this is feedback controls just follow basic to that and you’ll have rational solution.

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