Datasheet LT1505 (Analog Devices) - 10

LT1505
U OPERATION
The LT1505 is a synchronous current mode PWM step- At input current limit, CL1 will supply the programming down (buck) switcher. The battery DC charge current is pro- current IPROG, thus reducing battery charging current. grammed by a resistor RPROG (or a DAC output current) at To prevent current shoot-through between topside and the PROG pin and the ratio of sense resistors RS2 over RS1 lowside switches, comparators A3 and A4 assure that one (see Block Diagram). Amplifier CA1 converts the charge cur- switch turns off before the other is allowed to turn on. rent through RS1 to a much lower current IPROG (IPROG = Comparator A12 monitors charge current level and turns IBAT • RS1/RS2) fed into the PROG pin. Amplifier CA2 com- lowside switch off if it drops below 20% of the programmed pares the output of CA1 with the programmed current and value (20mV across R drives the PWM loop to force them to be equal. High DC S1) to allow for inductor discontinu- ous mode operation. Therefore sometimes even in con- accuracy is achieved with averaging capacitor CPROG. Note tinuous mode operation with light current level the lowside that IPROG has both AC and DC components. IPROG goes switch stays off. through R1 and generates a ramp signal that is fed to the PWM control comparator C1 through buffer B1 and level Comparator E6 monitors the charge current and signals shift resistors R2 and R3, forming the current mode inner through the FLAG pin when the charger is in voltage mode loop. The BOOST pin supplies the topside power switch gate and the charge current level is reduced to 20%. This charge drive. The LT1505 generates an 9.1V V complete signal can be used to start a timer for charge GBIAS to power drives and V termination. BOOSTC. BOOSTC pin supplies the current amplifier CA1 with a voltage higher than VCC for low dropout appli- The INFET pin drives an external P-channel FET for low cation. For batteries like lithium that require both constant- dropout application. current and constant-voltage charging, the 0.5% 2.465V reference and the amplifier VA reduce the charge current When input voltage is removed, VCC will be held up by the when battery voltage reaches the preset level. For NiMH and body diode of the topside MOSFET. The LT1505 goes into NiCd, VA can be used for overvoltage protection. a low current, 10µA typical, sleep mode as VCC drops below the battery voltage. To shut down the charger The amplifier CL1 monitors and limits the input current, simply pull the VC pin or SHDN pin low with a transistor. normally from the AC adapter, to a preset level (92mV/RS).
U U W U APPLICATIONS INFORMATION Input and Output Capacitors
capacitor will minimize problems. Consult the manufac- turer before use. Alternatives include new high capacity In the 4A Lithium Battery Charger (Figure 1), the input ceramic (at least 20µF) from Tokin or United Chemi-Con/ capacitor (CIN) is assumed to absorb all input switching Marcon, et al. ripple current in the converter, so it must have adequate ripple current rating. Worst-case RMS ripple current will The output capacitor (COUT) is also assumed to absorb be equal to one half of output charging current. Actual output switching current ripple. The general formula for capacitance value is not critical. Solid tantalum capacitors capacitor current is: such as the AVX TPS and Sprague 593D series have high ripple current rating in a relatively small surface mount VBAT 0.29 (VBAT () 1 – ) package, but caution must be used when tantalum capaci- VCC tors are used for input bypass. High input surge currents IRMS = (L1)(f) can be created when the adapter is hot-plugged to the charger and solid tantalum capacitors have a known For example, VCC = 19V, VBAT = 12.6V, L1 = 15µH, failure mechanism when subjected to very high turn-on and f = 200kHz, IRMS = 0.4A. surge currents. Highest possible voltage rating on the 1505fc 10