Datasheet LT1769 (Analog Devices) - 9

LT1769
U U W U APPLICATIONS INFORMATION Input and Output Capacitors
ramping up when VC pin voltage reaches 0.7V and full current is achieved with V In the 2A Lithium-Ion Battery Charger (Figure 1), the input C at 1.1V. With a 0.33µF capaci- tor, the time to reach full charge current is about 10ms and capacitor (CIN) is assumed to absorb all input switching it is assumed that input voltage to the charger will reach full ripple current in the converter, so it must have adequate value in less than 10ms. The capacitor can be ripple current rating. Worst-case RMS ripple current will increased up to 1µF if longer input start-up times are needed. be equal to one half of the output charge current. Actual capacitance value is not critical. Solid tantalum capacitors In any switching regulator, conventional time-based soft- such as the AVX TPS and Sprague 593D series have high starting can be defeated if the input voltage rises much ripple current rating in a relatively small surface mount slower than the time out period. This happens because the package, but caution must be used when tantalum capaci- switching regulators in the battery charger and the com- tors are used for input bypass. High input surge currents puter power supply are typically supplying a fixed amount are possible when the adapter is hot-plugged to the of power to the load. If the input voltage comes up slowly charger and solid tantalum capacitors have a known compared to the soft-start time, the regulators will try to failure mechanism when subjected to very high turn-on deliver full power to the load when the input voltage is still surge currents. Selecting a high voltage rating on the well below its final value. If the adapter is current limited, capacitor will minimize problems. Consult with the manu- it cannot deliver full power at reduced output voltages and facturer before use. Alternatives include new high capacity the possibility exists for a quasi “latch” state where the ceramic (5µF to 20µF) from Tokin or United Chemi-Con/ adapter output stays in a current limited state at reduced Marcon, et al. Sanyo OS-CON can also be used. output voltage. For instance, if maximum charger plus computer load power is 25W, a 15V adapter might be The output capacitor (COUT) is also assumed to absorb current limited at 2A. If adapter voltage is less than output switching ripple current. The general formula for (25W/2A = 12.5V) when full power is drawn, the adapter capacitor ripple current is: voltage will be pulled down by the constant 25W load until V it reaches a lower stable state where the switching regu- BAT 0.29 (VBAT () 1 – ) V lators can no longer supply full load. This situation can be I CC RMS = prevented by utilizing undervoltage lockout, set higher than (L1)(f) the minimum adapter voltage where full power can be For example, V achieved. CC = 16V, VBAT = 8.4V, L1 = 20µH, and f = 200kHz, IRMS = 0.3A. A fixed undervoltage lockout of 7V is built into the LT1769. EMI considerations usually make it desirable to minimize This 7V threshold can be increased by adding a resistive ripple current in the battery leads. Beads or inductors can divider to the UV pin as shown in Figure 2. Internal lockout be added to increase battery impedance at the 200kHz is performed by clamping the VC pin low. The VC pin is switching frequency. Switching ripple current splits be- released from its clamped state when the UV pin rises tween the battery and the output capacitor depending on above 7V and is pulled low when the UV pin drops below the ESR of the output capacitor and the battery imped- 6.5V (0.5V hysteresis). At the same time UVOUT goes high ance. If the ESR of C with an external pull-up resistor. This signal can be used OUT is 0.2Ω and the battery impedance is raised to 4Ω with a bead or inductor, only 5% of the to alert the system that charging is about to start. The ripple current will flow into the battery. charger will start delivering current about 4ms after VC is released, as set by the 0.33µF capacitor. A resistor divider
Soft-Start and Undervoltage Lockout
is used to set the desired VCC lockout voltage as shown in The LT1769 is soft-started by the 0.33µF capacitor on the Figure 2. A typical value for R6 is 5k and R5 is found from: VC pin. On start-up, the VC pin voltage will quickly rise to 0.5V, then ramp at a rate set by the internal 45µA pull-up R6(V – V ) R5 = IN UV current and the external capacitor. Charge current starts VUV 1769fa 9