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LTC4006 Datasheet(PDF) 8 Page - Linear Technology |
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LTC4006 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC4006 4006fa Overview The LTC4006 is a synchronous current mode PWM step- down (buck) switcher battery charger controller. The charge current is programmed by the sense resistor (RSENSE) between the CSP and BAT pins. The final float voltage is internally programmed to 8.4V (LTC4006-6), 12.6V (LTC4006-2) or 16.8V (LTC4006-4) with better than ±0.8% accuracy. Charging begins when the potential at the DCIN pin rises above the voltage at CLN (and the UVLO voltage) and the ACP/SHDN pin is allowed to go high; the CHG pin is set low. At the beginning of the charge cycle, if the cell voltage is below 2.5V, the charger will trickle charge the battery with 10% of the maximum programmed current. If the cell voltage stays below 2.5V for 25% of the total charge time, the charge sequence will be terminated im- mediately and the CHG pin will be set to a high impedance. An external thermistor network is sampled at regular intervals. If the thermistor value exceeds design limits, charging is suspended. If the thermistor value returns to an acceptable value, charging resumes. An external resis- tor on the RT pin sets the total charge time. The timer can be defeated by forcing the CHG pin to a low voltage. As the battery approaches the final float voltage, the charge current will begin to decrease. When the current drops to 10% of the programmed charge current, an internal C/10 comparator will indicate this condition by sinking 25 µAatthe CHG pin. The charge timer is also reset to 25% of the total charge time. If this condition is caused by an input current limit condition, described below, then the C/10 comparator will be inhibited. When a time-out occurs, charging is termi- nated immediately and the CHG pin changes to a high impedance. The charger will automatically restart if the cell voltage is less than 3.9V. To restart the charge cycle manu- ally, simply remove the input voltage and reapply it, or force the ACP/SHDN pin low momentarily. When the input voltage is not present, the charger goes into a sleep mode, dropping batterycurrentdrainto15 µA.Thisgreatlyreducesthecurrent drain on the battery and increases the standby time. The charger can be inhibited at any time by forcing the ACP/SHDN pin to a low voltage. Input FET The input FET circuit performs two functions. It enables the charger if the input voltage is higher than the CLN pin and provides the logic indicator of AC present on the ACP/SHDN pin. It controls the gate of the input FET to keep a low forward voltage drop when charging and also prevents reverse current flow through the input FET. If the input voltage is less than VCLN, it must go at least 170mV higher than VCLN to activate the charger. When this occurs the ACP/SHDN pin is released and pulled up with an internal load to indicate that the adapter is present. The gate of the input FET is driven to a voltage sufficient to keep a low forward voltage drop from drain to source. If the voltage between DCIN and CLN drops to less than 25mV, the input FET is turned off slowly. If the voltage between DCIN and CLN is ever less than – 25mV, then the input FET is turned off in less than 10 µs to prevent significant reverse current from flowing in the input FET. In this condition, the ACP/SHDN pin is driven low and the charger is disabled. Battery Charger Controller The LTC4006 charger controller uses a constant off-time, current mode step-down architecture. During normal opera- tion, the top MOSFET is turned on each cycle when the oscillator sets the SR latch and turned off when the main current comparator ICMP resets the SR latch. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current trips the current comparator IREV or the beginning of the next cycle. The oscillator uses the equation: t VV Vf OFF DCIN BAT DCIN OSC = – • to set the bottom MOSFET on time. This activity is dia- grammed in Figure 1. The peak inductor current, at which ICMP resets the SR latch, is controlled by the voltage on ITH. ITH is in turn controlled by several loops, depending upon the situation at hand. The average current control loop converts the voltage between CSP and BAT to a representative current. Error amp CA2 OPERATIO TGATE OFF ON BGATE INDUCTOR CURRENT tOFF TRIP POINT SET BY ITH VOLTAGE ON OFF 4006 F01 Figure 1 |
Similar Part No. - LTC4006_15 |
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Similar Description - LTC4006_15 |
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