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ISL9211A Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL9211A Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 12 page ISL9211A 9 FN6702.1 August 25, 2010 in the sensing FET is forced to match the value programmed by ILIM pin. The OCP threshold can be set with the resistor RLIM as shown in Table 2. The size of the power FET Q1 is 31,250 times the size of the sensing FET. Therefore, when the current in the power FET is 31,250 times the current in the sensing FET, the drain voltage of the power FET falls below that of the sensing FET. The comparator CP2 then outputs a signal to turn off the power FET. where the 0.8V is the regulated reference voltage at the ILIM pin. The OCP comparator CP2 has a built-in 180µs delay to prevent false triggering by transient signals. The OCP function also has a 4-bit binary counter that accumulates during an OCP event. When the total count reaches 16, the power NFET is turned off permanently until the input power is recycled or the enable pin is toggled. Figures 7 and 8 illustrate the waveforms during the power-up when the output is shorted to ground. Internal Over-Temperature Protection The ISL9211A monitors its own internal temperature to prevent thermal failures. When the internal temperature reaches +150°C, the IC turns off the N-channel power MOSFET. The IC does not resume operation until the internal temperature drops below +110°C. Fault Indication Output The FAULT pin is an open-drain output that indicates a LOW signal when any of the three fault events happens. This provides a signal to the microprocessor to take further action to enhance the safety of the charging system. Applications Information The ISL9211A is designed to meet the “Lithium-Safe” criteria when operating together with a qualified Li-ion battery charger. The “Lithium-Safe” criteria requires the charger output to fall within the green region shown in Figure 14 under normal operating conditions and NOT to fall in the red region when there is a single fault in the charging system. Taking into account the safety circuit in a Li-ion battery pack, the charging system is allowed to have two faults without creating hazardous conditions for the battery cell. The output of the Li-ion charger, such as the ISL6292C, has a typical I-V curve shown with the blue lines under normal operation, which is within the green region. The function of the ISL9211A is to add a redundant protection layer such that, under any single fault condition, the charging system output does not exceed the I-V limits shown with the red lines. As a result, the charging system adopting the ISL9211A and the ISL6292C chip set can easily pass the “Lithium-Safe” criteria test procedures. The ISL9211A is a simple device that requires only three external components, in addition to the ISL6292 charger circuit, to meet the “Lithium-Safe” criteria, as shown in the “Typical Application Circuit” on page 1. The selection of the current limit resistor RILIM is given in “Overcurrent Protection (OCP)” on page 8. RVB Selection The RVB prevents a large current from the VB pin to the battery terminal, in case the ISL9211A fails. The recommended value should be between 200kΩ to 1MΩ. With 200kΩ resistance, the worst case current flowing from the VB pin to the charger output is shown in Equation 1, assuming the VB pin voltage is 24V under a failure mode and the battery voltage is 4.2V. Such a small current can be easily absorbed by the bias current of other components in the handheld system. Increasing the RVB value reduces the worst case current, but at the same time increases the error for the 4.34V battery OVP threshold. The error of the battery OVP threshold is the original accuracy at the VB pin given in the “Electrical Specifications” table on page 4 plus the voltage built across the RVB by the VB pin leakage current. The VB pin leakage current is less than 20nA, as given in the “Electrical Specifications” table on page 4. With the 200kΩ resistor, the worst-case additional error is 4mV and with a 1MΩ resistor, the worst-case additional error is 20mV. TABLE 2. RLIM (kΩ) OCP (mA) RLIM (kΩ) OCP (mA) 82.5 300 21 1200 61.9 400 19.1 1300 49.9 500 16.5 1400 41.2 600 15.4 1500 35.7 700 14 1600 31.6 800 12.4 1700 28 900 11.3 1800 24.9 1000 10.5 1900 22.6 1100 9.53 2000 24V 4.2V – () 200kΩ () ⁄ 99 μA = (EQ. 1) FIGURE 14. LITHIUM-SAFE OPERATING REGIONS 5 0 1000 BATTERY VOLTAGE (V) 13 4 ISL9211A LIMITS ISL6292C LIMITS 6 2 |
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