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LT1769 Datasheet(PDF) 9 Page - Linear Technology |
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LT1769 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1769 APPLICATIONS INFORMATION Input and Output Capacitors In the 2A Lithium-Ion Battery Charger (Figure 1), the input capacitor (CIN) is assumed to absorb all input switching ripple current in the converter, so it must have adequate ripple current rating. Worst-case RMS ripple current will be equal to one half of the output charge current. Actual capacitance value is not critical. Solid tantalum capacitors such as the AVX TPS and Sprague 593D series have high ripple current rating in a relatively small surface mount package, but caution must be used when tantalum capaci- tors are used for input bypass. High input surge currents are possible when the adapter is hot-plugged to the charger and solid tantalum capacitors have a known failure mechanism when subjected to very high turn-on surge currents. Selecting a high voltage rating on the capacitor will minimize problems. Consult with the manu- facturer before use. Alternatives include new high capacity ceramic (5 µF to 20µF) from Tokin or United Chemi-Con/ Marcon, et al. Sanyo OS-CON can also be used. The output capacitor (COUT) is also assumed to absorb output switching ripple current. The general formula for capacitor ripple current is: IRMS = (L1)(f) VBAT VCC () 0.29 (VBAT) 1 – For example, VCC = 16V, VBAT = 8.4V, L1 = 20µH, and f = 200kHz, IRMS = 0.3A. EMI considerations usually make it desirable to minimize ripple current in the battery leads. Beads or inductors can be added to increase battery impedance at the 200kHz switching frequency. Switching ripple current splits be- tween the battery and the output capacitor depending on the ESR of the output capacitor and the battery imped- ance. If the ESR of COUT is 0.2Ω and the battery impedance is raised to 4 Ω with a bead or inductor, only 5% of the ripple current will flow into the battery. Soft-Start and Undervoltage Lockout The LT1769 is soft-started by the 0.33 µF capacitor on the VC pin. On start-up, the VC pin voltage will quickly rise to 0.5V, then ramp at a rate set by the internal 45 µA pull-up current and the external capacitor. Charge current starts ramping up when VC pin voltage reaches 0.7V and full current is achieved with VC at 1.1V. With a 0.33µF capaci- tor, the time to reach full charge current is about 10ms and it is assumed that input voltage to the charger will reach full value in less than 10ms. The capacitor can be increased up to 1 µF if longer input start-up times are needed. In any switching regulator, conventional time-based soft- starting can be defeated if the input voltage rises much slower than the time out period. This happens because the switching regulators in the battery charger and the com- puter power supply are typically supplying a fixed amount of power to the load. If the input voltage comes up slowly compared to the soft-start time, the regulators will try to deliver full power to the load when the input voltage is still well below its final value. If the adapter is current limited, it cannot deliver full power at reduced output voltages and the possibility exists for a quasi “latch” state where the adapter output stays in a current limited state at reduced output voltage. For instance, if maximum charger plus computer load power is 25W, a 15V adapter might be current limited at 2A. If adapter voltage is less than (25W/2A = 12.5V) when full power is drawn, the adapter voltage will be pulled down by the constant 25W load until it reaches a lower stable state where the switching regu- lators can no longer supply full load. This situation can be prevented by utilizing undervoltage lockout, set higher than the minimum adapter voltage where full power can be achieved. A fixed undervoltage lockout of 7V is built into the LT1769. This 7V threshold can be increased by adding a resistive divider to the UV pin as shown in Figure 2. Internal lockout is performed by clamping the VC pin low. The VC pin is released from its clamped state when the UV pin rises above 7V and is pulled low when the UV pin drops below 6.5V (0.5V hysteresis). At the same time UVOUT goes high with an external pull-up resistor. This signal can be used to alert the system that charging is about to start. The charger will start delivering current about 4ms after VC is released, as set by the 0.33 µF capacitor. A resistor divider is used to set the desired VCC lockout voltage as shown in Figure 2. A typical value for R6 is 5k and R5 is found from: |
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