Datasheet LT1511 (Analog Devices) - 9

LT1511
U U W U APPLICATIONS INFORMATION Input and Output Capacitors Soft-Start
In the 3A Lithium Battery Charger (Figure 1), the input The LT1511 is soft started by the 0.33µF capacitor on the capacitor (CIN) is assumed to absorb all input switching VC pin. On start-up, VC pin voltage will rise quickly to 0.5V, ripple current in the converter, so it must have adequate then ramp at a rate set by the internal 45µA pull-up current ripple current rating. Worst-case RMS ripple current will and the external capacitor. Battery charging current starts be equal to one half of output charging current. Actual ramping up when VC voltage reaches 0.7V and full current capacitance value is not critical. Solid tantalum capacitors is achieved with VC at 1.1V. With a 0.33µF capacitor, time such as the AVX TPS and Sprague 593D series have high to reach full charge current is about 10ms and it is ripple current rating in a relatively small surface mount assumed that input voltage to the charger will reach full package, but caution must be used when tantalum capaci- value in less than 10ms. The capacitor can be increased up tors are used for input bypass. High input surge currents to 1µF if longer input start-up times are needed. can be created when the adapter is hot-plugged to the In any switching regulator, conventional timer-based soft charger and solid tantalum capacitors have a known starting can be defeated if the input voltage rises much failure mechanism when subjected to very high turn-on slower than the time out period. This happens because the surge currents. Highest possible voltage rating on the switching regulators in the battery charger and the com- capacitor will minimize problems. Consult with the manu- puter power supply are typically supplying a fixed amount facturer before use. Alternatives include new high capacity of power to the load. If input voltage comes up slowly ceramic (5µF to 20µF) from Tokin or United Chemi-Con/ compared to the soft start time, the regulators will try to Marcon, et al., and the old standby, aluminum electrolytic, deliver full power to the load when the input voltage is still which will require more microfarads to achieve adequate well below its final value. If the adapter is current limited, ripple rating. Sanyo OS-CON can also be used. it cannot deliver full power at reduced output voltages and The output capacitor (COUT) is also assumed to absorb the possibility exists for a quasi “latch” state where the output switching current ripple. The general formula for adapter output stays in a current limited state at reduced capacitor current is: output voltage. For instance, if maximum charger plus computer load power is 30W, a 15V adapter might be VBAT current limited at 2.5A. If adapter voltage is less than 0.29 (VBAT () 1 – ) V (30W/2.5A = 12V) when full power is drawn, the adapter I CC RMS = (L1)(f) voltage will be sucked down by the constant 30W load until it reaches a lower stable state where the switching regu- For example, VCC = 16V, VBAT = 8.4V, L1 = 20µH, lators can no longer supply full load. This situation can be and f = 200kHz, IRMS = 0.3A. prevented by utilizing undervoltage lockout, set higher than the minimum adapter voltage where full power can be EMI considerations usually make it desirable to minimize achieved. ripple current in the battery leads, and beads or inductors may be added to increase battery impedance at the 200kHz A fixed undervoltage lockout of 7V is built into the VCC pin, switching frequency. Switching ripple current splits be- but an additional adjustable lockout is also available on the tween the battery and the output capacitor depending on UV pin. Internal lockout is performed by clamping the VC the ESR of the output capacitor and the battery imped- pin low. The VC pin is released from its clamped state when ance. If the ESR of COUT is 0.2Ω and the battery impedance the UV pin rises above 6.7V and is pulled low when the UV is rased to 4Ω with a bead or inductor, only 5% of the pin drops below 6.2V (0.5V hysteresis). At the same time current ripple will flow in the battery. 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 0.33µF 9