App note: Li-Ion/Polymer Shunt battery charger system with low battery disconnect


Li-Ion/Polymer Shunt battery charger system with low battery disconnect (PDF!) app note from Linear Technology:

The LTC®4071 allows simple charging of Li-Ion/Polymer batteries from very low current, intermittent or continuous charging sources. A near zero current low battery latching disconnect function protects even the lowest capacity batteries from deep discharge and potentially irreparable damage. The 550nA to 50mA operating current makes charging possible from previously unusable sources. With its low operating current the LTC4071 is well suited to charge low capacity Li Ion or thin film batteries in energy harvesting applications. The unique architecture of the LTC4071 allows for an extremely simple battery charger solution, requiring just one external resistor.

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  1. I’ve used this chip before in a project that I have yet to make a writeup for. Although it works quite well to both charge a SMALL li-poly cell and to protect it from low voltage discharge, it has a few issues that annoy me. First, which isn’t mentioned clearly in the datasheet, is that the charge status pin has a tendency to remain on when the charging current has been removed. It stays like this until the cell voltage drops below the “programmed” high/shunt threshold voltage minus the hysteresis voltage. If you have the pin connected to a status LED, then the LED will remain on even when removed from the charging supply voltage. The quick solution I found for this was to add a diode between the supply ground and the charging circuit ground. Recalculate the current limit resistor, and connect the charge signal (LED) ground to the charge supply ground. So when the supply is disconnected, the LED will also shut off. This would have issues if the ground is used for communications with say a host, but then this is just one option. The second issue is when turning on loads. Although the datasheet does mention this, I came across problems when trying to turn on the circuit which pulled a small but higher than normal current when first initiated. If the cell voltage is already low (but not yet to the cutoff voltage) the initial current has a tendency to trip the cutoff protection. This was a problem because the protection circuit has a hysteresis voltage to it as well, so once triggered, it would remain off until charged.

  2. By ‘charge status pin’ I reckon you mena the HBO pin – ‘High Battery Monitor Output’. When reading the data sheet it says exactly what you are describing – it goes low only when the supply voltage (which equals to the battery voltage when no power supply is attached) falls more than the hysteris voltage below the threshold voltage. So its not intended as being a charge status output (and the data sheet never even mentions it as such)

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