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I need to build a Li-poly battery charger.

The intended schematic of the circuit is as follows: enter image description here

  • J1 is connected to the battery.
  • PMOS connects and disconnects the load to and from the battery
  • IC charges battery based on Li-poly (CC/CV) requirements
  • D1 is there to prevent any feedback from battery to USB

The intended operation is as follows:

  • When USB present and device needs to work, VSupply is directly provided by the USB and PMOS separates the battery from the supply. (R3 ensures PMOS is on when there is no USB) Primary reason for this is not to disturb the proper charging of the battery. Especially battery is in 2.5V level, it needs to be trickle charged to 3V and than maximum current can be applied. However without separating the battery from the supply, I couldn't come up with a way to make this work, hence the PMOS.

  • I need to find a PMOS that cuts off when Vgs is .5V or so. I haven't checked yet but hopefully it is possible.

  • My circuit takes less than 500mA at any given time however I cannot gurantee how much current can be allocated to the battery. This is a point I don't know how to deal with. How do I guarantee the bulk of the current goes to device and remaining used for charging? Should I put a resistor between battery and BAT pin of the IC to increase its impedance so that most of the current goes to the load.

Based on datasheet of this device (STC4054),

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  • \$\begingroup\$ Possibly related: electronics.stackexchange.com/questions/25393/… \$\endgroup\$ – Brian Carlton May 9 '12 at 20:30
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    \$\begingroup\$ What is connected to Vbus? is that Vusb ? = 5V@500mA max .So your load is on Vsupply? will run on either Lipo @3V or USB @ 4.5V? Right now if the battery is low, it will blow the USB polyfuse. \$\endgroup\$ – Sunnyskyguy EE75 May 10 '12 at 9:09
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    \$\begingroup\$ A Smart load monitor is needed in order to manage charging the battery when the load is sensed to be disconnected. THat is not possible with this circuit. \$\endgroup\$ – Sunnyskyguy EE75 May 10 '12 at 9:17
  • \$\begingroup\$ @TonyStewart VBus is the USB 5V @ 500mA max. Load is on VSupply. The load will drain less than 500mA in any given situation. If the battery is low, the PMOS will open and USB will serve power to both circuits. However I am not clamping the max to 500mA that is for sure so total current may pass 500mA. How can I limit that? \$\endgroup\$ – Ktc May 10 '12 at 9:18
  • \$\begingroup\$ @TonyStewart do you have any pointers (IC?) for smart load monitoring. \$\endgroup\$ – Ktc May 10 '12 at 9:24
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Your design will trip the PTC polyfuse in your USB port when the load demands 500mA and the battery demands anything significant. Since USB2 only supports 500mA max. USB3 supports 900mA.

It is not clear to me how your device load is switched on/off so you need a smart load sense switch to U1 CHRG using a comparator.

But it is clear to me this approach will not work. YOu want a trickle charge when your device is off, so the USB can power the unit and the external Li-Po can get a trickle charge, If I understand you are saying and then when the USB is off but your "load is still on" it can run off the Li-po as long as it is charged.

What you could do is .. add a small shunt resistor between USB (Vbus) and LOAD (VSWITCH) with a comparator to disable U1 CHRG when there is current being used. .

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  • \$\begingroup\$ Thanks Tony. I now realize there is no way to limit the input current. The circuit above won't work. My use case is such that when USB present the device must work independent of the battery level and it should also continue to charge if there is any juice available. Need a better chip, won't work with the ST IC. \$\endgroup\$ – Ktc May 10 '12 at 9:31
  • \$\begingroup\$ ST IC will work, but you need to disable CHRG when by pass to LOAD is detected with comparator across diode. or R \$\endgroup\$ – Sunnyskyguy EE75 May 10 '12 at 9:42
  • \$\begingroup\$ I will move to a more compherensive solution where I don't need to deal with these cases. I am looking at TI's BQ24072 as a potential solution. I will report it here. ti.com/lit/ds/symlink/bq24072.pdf \$\endgroup\$ – Ktc May 10 '12 at 10:14

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