Lithium batteries are widely used in portable electronics such as cell phones, PDAs and digital cameras due to their high bulk and energy densities and single-cell voltages up to 4.2V. To ensure the safety of use, Li-ion batteries must have appropriate battery management circuitry in the application to prevent battery overcharge, overdischarge and overcurrent. The ultra-small package of Li-ion battery protection ICs and the requirement of few external devices make them widely used in the design of single-cell Li-ion battery protection circuits. However, at present, whether forward (self-developed) or reverse (copied and developed) domestic lithium battery protection IC design, the actual parameters are usually very different from the standard parameters due to technology and process. Therefore, in the forward design IC, it is particularly prominent that the actual operating parameters of the Li-ion battery protection IC must be tested. Currently, dedicated Li-ion battery protection board testers are available in the market, but the price is usually high and the IC must be soldered to the board during the test. Therefore, this paper designs a simple test circuit that can be used to complete the testing of Li-ion battery protection ICs with the help of common electronic instruments. The working principle of Li-ion battery protection IC[/ b] [b]The application circuit of single-cell Li-ion battery protection IC is very simple, only two resistors, two capacitors and two MOSFETs need to be connected. Its typical application circuit is shown in Figure 1. Figure 1 Typical application circuit diagram of lithium battery protection IC [2] The design of the test circuit of the lithium battery protection IC based on the operating principle of the lithium battery protection IC is The test circuit designed as shown in Figure 2. Figure 3 illustrates the circuit of module B in Figure 2. 2 in detail. When testing the overcurrent protection, module A provides voltage to the CS pin to simulate the voltage detected by the CS pin in Figure 1. A variable potentiometer in the regulation module provides variable power to the CS pin, and a jump switch in the control module provides variable power to the CS pin. Provide a sudden voltage to CS. Module B is the power supply and emulation provides the operating voltage for the IC. Adjusting the variable potentiometer R7 in the circuit can provide variable voltage for the whole circuit for testing the overcharge protection voltage and overdischarge protection voltage. The closure of switch S1 in the control module provides a jump power supply to the test circuit for testing the overcharge, overdischarge and overcurrent delays of the IC. Jumper ports P1 and P2 are used to test the operating current of the IC, while switch S2 can be turned on when testing other parameters. To test the operating current of the IC, an ammeter is connected to P1 and P2 and switch S2 is connected and disconnected. Module C is a microcurrent source made of 2 MOSFETs. It draws current to this pin when testing OD and OC outputs high and low. With the correct choice of MOSFETs, the test requirements can be met. Module D is a 2-chip MOSFET integrated chip, equivalent to M1 and M2 in Figure 1. two ports are used when testing the MOSFET leakage current, and both ports should be shorted when testing other parameters. Module E is an IC socket. This socket is used to hold the ICs to be tested. It can hold up to 4 ICs (only one IC can be placed during the test). After testing, the ICs can be removed without leaving any traces, which will not affect the sale of the ICs. and tested again. Figure 3 Circuit diagram of module B. In the design of the test circuit, the resistors should be carefully selected. Due to the presence of variable potentiometers in modules A, B and C, failure to properly select other resistors can easily lead to circuit burnout, especially since the variable potentiometers in modules A and B are selected to test various voltages . The accuracy impact is significant. The two variable potentiometers in this circuit are both 1K / 10 turns for higher accuracy. The MOSFETs in Module C should be selected with regard to their operating current range. There are only two levels of current required for testing, one of a few microamps and the other of tens of microamps, so it is usually necessary to provide less than that. In addition, the stability of the power supply has a significant impact on the overall IC test parameters. Therefore, a power supply with good stability should be used in the test whenever possible. Features of this design[/ b] [b]This design has the following three features.
● For testing IC overcharge, overdischarge, and overcurrent delays, this switch is used to short or break the resistor for abrupt changes in circuit power, while the oscilloscope is used to simultaneously grasp the power, OC, and OD transition waveforms for measuring delay times.
● To absorb and sink current to the pins at high and low levels of OC and OD during testing, the circuit uses MOSFETs to make two simple microcurrent sources. The MOSFET model chosen is the TN0201T, which uses the gate voltage to control the leak and source levels. The current, using the drain and source currents as current sources, can be accurate up to 0.1μA, which basically meets the test requirements.
● When testing the overcurrent protection voltage, test the voltage at the CS pin to change the OD pin from high to low. The short-circuit current protection voltage is much higher than the over-current protection voltage. The circuit jumps up when the voltage reaches the overcurrent protection voltage and the OD output stays low, so the conventional method cannot test the short-circuit current protection voltage, so the indirect approximate test method is used in this paper. The delay time of the IC used for overcurrent protection is about a few milliseconds to more than ten milliseconds, while the short-current delay time is about ten microseconds, so the short-circuit test can be approximated based on the difference between the overcurrent delay time and the short-current delay Time. Current protection voltage. This parameter cannot be measured using a dedicated lithium battery protection board tester. The test circuit also has some drawbacks. First, the accuracy of the IC test is related to the stability of the power supply and the accuracy of the ammeter. Among them, the accuracy of various voltage tests is also related to the accuracy of the variable potentiometer. The second is that the short-circuit protection voltage is an approximation.
Summary
Although there are many lithium battery protection board testers on the market, they are expensive and have fixed test parameters that cannot meet the needs of practical testing. In practical applications, the main parameters of Li-ion battery protection ICs that customers are most concerned about are: overcharge, overdischarge and overcurrent protection voltages, quiescent operating current and brown-out current, overcharge, overdischarge and overcurrent protection delays, and output high and low levels on the OD and OC pins. The test method provided in this paper can accurately measure the above parameters beyond those that can be tested by a Li-ion battery protection board tester. Therefore, the test circuit and test method provided in this paper is a good choice when the requirements of Li-ion battery protection IC parameters are very comprehensive or the conditions are relatively limited. The above test circuit and test method have been put into use and more than 1,000 Li-ion battery protection ICs have been successfully tested. From the test results, except for the short-circuit protection voltage which is an approximate test, the rest of the parameters tested are in good agreement with the results measured by the dedicated test equipment. From the customer's point of view, the parameters measured by the test circuit are accurate and can meet the customer's requirements. Since this test circuit is not encapsulated (plus housing), it is possible to add appropriate circuitry to test additional parameters (such as testing MOSFET leakage current as mentioned in this paper) according to the customer's needs.
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