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LT1769 Datasheet(PDF) 12 Page - Linear Technology |
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LT1769 Datasheet(HTML) 12 Page - Linear Technology |
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12 / 16 page  12 LT1769 APPLICATIONS INFORMATION Figure 5. Current Comparator for Initiating Float Time Out NEGATIVE EDGE TO TIMER 1769 F04 3.3V OR 5V ADAPTER OUTPUT 3 8 7 1 4 2 D1 1N4148 C1 0.1 µF BAT SENSE R1* 1.6k RS1 0.05 Ω R4 470k R3 430k R2 560k LT1011 D2 1N4148 * TRIP CURRENT = = ≈ 270mA R1(VBAT) (R2 + R3)(RS1) (1.6k)(8.4V) (560k + 430k)(0.05 Ω) + – VBAT BAT RS3 200 Ω RS2 200 Ω LT1769 IBAT R1 2.465 2000 I R2 2.465 2000 II LOW HI LOW = ()( ) = ()( ) − All battery chargers with fast charge rates require some means to detect full charge in the battery and terminate the high charge current. NiCd batteries are typically charged at high current until a temperature rise or battery voltage decrease is detected as an indication of near full charge. The charging current is then reduced to a much lower value and maintained as a constant trickle charge. An intermediate “top off” current may also be used for a fixed time period to reduce total charge time. NiMH batteries are similar in chemistry to NiCd but have two differences related to charging. First, the inflection characteristic in battery voltage as full charge is approached is not nearly as pronounced. This makes it more difficult to use – dV/dt as an indicator of full charge, and an increase in battery temperature is more often used with a temperature sensor in the battery pack. Secondly, con- stant trickle charge may not be recommended. Instead, a moderate level of current is used on a pulse basis ( ≈ 1% to 5% duty cycle) with the time-averaged value substitut- ing for a constant low trickle. Please contact the Linear Technology Applications department about charge termi- nation circuits. If overvoltage protection is needed, R3 and R4 can be cal- culated according to the procedure described in Lithium- Ion Charging section. The OVP pin should be grounded if not used. When a microprocessor DAC output is used to control charge current, it must be capable of sinking current at a compliance up to 2.5V if connected directly to the PROG pin. Thermal Calculations If the LT1769 is used for charging currents above 1A, a thermal calculation should be done to ensure that junction temperature will not exceed 125 °C. Power dissipation in the IC is caused by bias and driver current, switch resis- tance and switch transition losses. The GN package, with a thermal resistance of 35 °C/W, can provide a full 2A charging current in many situations. A graph is shown in the Typical Performance Characteristics section. For 1A full current, the current sense resistor (RS1) should be increased to 0.1 Ω so that enough signal (10mV) will be across RS1 at 0.1A trickle charge to keep charging current accurate. For a 2-level charger, R1 and R2 are found from: Nickel-Cadmium and Nickel-Metal-Hydride Charging The 2A Lithium-Ion Battery Charger shown in Figure 1 can be modified to charge NiCd or NiMH batteries. For ex- ample, if a 2-level charge is needed; 1A when Q1 is on and 100mA when Q1 is off. Figure 6. 2-Level Charging R2 5.49k R1 49.3k 300 Ω PROG 1 µF Q1 LT1769 1769 F05 |
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