Dual Comparator Thermally Protects Li+ Battery

Abstract: A dual comparator and thermistor set safe temperature limits for a lithium-ion battery charging circuit. The single-cell lithium-ion (Li+) battery charger can be powered from a USB port or external supply.

Most manufacturers recommend that Li+ (lithium-ion) batteries not be charged below 0°C or above +50°C. You can monitor both of these limits by adding a thermistor and dual (window) comparator to a Li+ battery charger (Figure 1).

Figure 1.This lithium battery charger derives its power from from a USB port and provides thermal protection for the battery.
Figure 1.This lithium battery charger derives its power from from a USB port and provides thermal protection for the battery.

The circuit has a low-temperature trip point of +2.5°C and a high-temperature trip point of +47.5°C. A precision voltage reference is not necessary. Because the comparator resistor network is ratiometric, variations in the supply voltage (VBUS) do not affect the trip thresholds.

By driving the charger's enable input (EN), the comparators' open-drain outputs inhibits charging when the battery temperature is out of range. As an alternative, you can substitute a dual comparator with push-pull CMOS outputs (such as the MAX9032) if you also add a tiny, SOT-323 dual diode, as shown in dashed lines.

The dual comparator shown and the MAX9032 are available in tiny SOT23 packages, and both offer a built-in hysteresis of 2mV or 4mV respectively.

IC2 is a single-cell lithium-ion (Li+) battery charger that can be powered directly from a USB port or from an external supply up to 6.5V. The accuracy of its battery-regulation voltage (0.5%) allows maximum utilization of the battery capacity.

The charger's internal FET delivers up to 500mA of charging current, and its SELV input can be configured for charging a 4.1V or 4.2V battery. The SELI input sets the charge current to either 100mA or 500mA, and an open-drain output (CHG) indicates the charge status. For near-dead batteries, a preconditioning capability soft-starts the cell before charging. Other safety features include continuous monitoring of voltage and current, and initial checking for fault conditions before charging.

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APP 2769:
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