Datasheet LT1571 (Analog Devices) - 8

LT1571 Series
U OPERATIO
The LT1571 is a current mode PWM step-down (buck) require both constant-current and constant-voltage charg- charger. The battery charge current is programmed by a ing, the 0.5%, 2.465V reference and the amplifier VA resistor RPROG (or a DAC output current) at the PROG pin reduce the charge current when battery voltage reaches (see Block Diagram). Amplifier CA1 converts the charge the preset level. For NiMH and NiCd, VA can be used for current through RS1 to a much lower current IPROG (500µA/ overvoltage protection. When input voltage is removed, A) fed into the PROG pin. Amplifier CA2 compares the the VCC pin drops to 0.7V below the battery voltage forcing output of CA1 with the programmed current and drives the the charger into a low-battery drain (5µA typical) sleep PWM loop to force them to be equal. High DC accuracy is mode. To shut down the charger, simply pull the VC pin low achieved with averaging capacitor CPROG. Note that IPROG with a transistor. has both AC and DC components. IPROG goes through R1 Comparator E6 monitors the charge level and signals and generates a ramp signal that is fed to the PWM control through the FLAG pin when charging is in voltage mode comparator C1 through buffer B1 and level shift resistors and the charge current has reduced to 20% or less. This R2 and R3, forming the current mode inner loop. The charge complete signal can be used to start a timer for BOOST pin drives the NPN switch (QSW) into saturation charging termination. and reduces power loss. For batteries like lithium-ion that
U U W U APPLICATIO S I FOR ATIO Input and Output Capacitors
 V 0 2 . (  9 V BAT ) 1− In the charger circuits in Figures 1 and 2, the input BAT  VCC  = capacitor C IRMS IN is assumed to absorb all input switching (L )1(f) ripple current in the converter, so it must have adequate ripple current rating. Worst-case RMS ripple current will For example, with VCC = 16V, VBAT = 8.4V, L1 = 33µH and be equal to one half of the output charge current. Actual f = 200kHz, IRMS = 0.18A. capacitance value is not critical. Solid tantalum capacitors EMI considerations usually make it desirable to minimize such as the AVX TPS and Sprague 593D series have high ripple current in the battery leads. Beads or inductors can ripple current rating in a relatively small surface mount be added to increase battery impedance at the 200kHz package, but caution must be used when tantalum capaci- switching frequency. Switching ripple current splits tors are used for input bypass. High input surge currents between the battery and the output capacitor depending are possible when the adapter is hot-plugged to the on the ESR of the output capacitor and the battery imped- charger and solid tantalum capacitors have a known ance. If the ESR of C failure mechanism when subjected to very high turn-on OUT is 0.2Ω and the battery impedance is raised to 4Ω with a bead of inductor, only 5% of the surge currents. Selecting a high voltage rating on the capacitor will minimize problems. Consult with the manufac- ripple current will flow into the battery. turer before use. Alternatives include new high capacity
Soft-Start
ceramic capacitors from Tokin or United Chemi-Con/ MARCON, et al. OS-CON can also be used. The LT1571 is soft-started by the 0.33µF capacitor on VC pin. On start-up, the V The output capacitor C C pin voltage will rise quickly to 0.5V, OUT is also assumed to absorb then ramp at a rate set by the internal 45µA pull-up current output switching ripple current. The general formula for and the external capacitor. Charge current starts ramping capacitor ripple current is: up when the VC pin voltage reaches 0.9V and full current 8