Datasheet LT1510, LT1510-5 (Analog Devices) - 9

LT1510/LT1510-5
U U W U APPLICATIONS INFORMATION
capacity ceramic capacitor (5µF to 10µF) from Tokin or deliver full power to the load when the input voltage is still United Chemi-Con/MARCON, et al., and the old standby, well below its final value. If the adapter is current limited, aluminum electrolytic, which will require more microfarads it cannot deliver full power at reduced output voltages and to achieve adequate ripple rating. OS-CON can also be used. the possibility exists for a quasi “latch” state where the The output capacitor C adapter output stays in a current limited state at reduced OUT is also assumed to absorb output switching current ripple. The general formula for output voltage. For instance, if maximum charger plus capacitor current is: computer load power is 20W, a 24V adapter might be current limited at 1A. If adapter voltage is less than (20W/1A  V  = 20V) when full power is drawn, the adapter voltage will be 0 2 . 9 V BAT ( ) 1− BAT  V  sucked down by the constant 20W load until it reaches a I CC = RMS lower stable state where the switching regulators can no L1 f ( )( ) longer supply full load. This situation can be prevented by For example, with V utilizing undevoltage lockout, set higher than the minimum CC = 16V, VBAT = 8.4V, L1 = 30µH and f = 200kHz, I adapter voltage where full power can be achieved. RMS = 0.2A. EMI considerations usually make it desirable to minimize A fixed undervoltage lockout of 7V is built into the VCC pin. ripple current in the battery leads, and beads or inductors Internal lockout is performed by clamping the VC pin low. may be added to increase battery impedance at the 200kHz The VC pin is released from its clamped state when the VCC switching frequency. Switching ripple current splits be- pin rises above 7V. The charger will start delivering current tween the battery and the output capacitor depending on about 2ms after VC is released, as set by the 0.1µF at VC the ESR of the output capacitor and the battery impedance. pin. Higher lockout voltage can be implemented with a If the ESR of C Zener diode (see Figure 3 circuit). OUT is 0.2Ω and the battery impedance is raised to 4Ω with a bead of inductor, only 5% of the current V ripple will flow in the battery. IN V D1 Z VCC
Soft Start
1N4001 VC LT1510 The LT1510 is soft started by the 0.1µF capacitor on VC 2k GND pin. On start-up, VC pin voltage will rise quickly to 0.5V, then ramp at a rate set by the internal 45µA pull-up current 1510 F03 and the external capacitor. Battery charging current starts
Figure 3. Undervoltage Lockout
ramping up when VC voltage reaches 0.7V and full current is achieved with VC at 1.1V. With a 0.1µF capacitor, time to The lockout voltage will be VIN = VZ + 1V. reach full charge current is about 3ms and it is assumed For example, for a 24V adapter to start charging at 22V that input voltage to the charger will reach full value in less IN, choose V than 3ms. Capacitance can be increased up to 0.47µF if Z = 21V. When VIN is less than 22V, D1 keeps VC low and charger off. longer input start-up times are needed. In any switching regulator, conventional timer-based soft
Charging Current Programming
starting can be defeated if the input voltage rises much The basic formula for charging current is (see Block slower than the time-out period. This happens because the Diagram): switching regulators in the battery charger and the com- puter power supply are typically supplying a fixed amount  2. V 465  of power to the load. If input voltage comes up slowly I = I( )( ) 2000 = BAT PROG 2000  R  ( ) compared to the soft start time, the regulators will try to PROG 9