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LTC4006 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4006 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LTC4006 sn4006 4006is OPERATIO 6 NTC LTC4006 S1 R9 32.4k C7 0.47 µF RTH 10k NTC – + – + – + 60k ~4.5V CLK 45k 15k TBAD 4006 F03 D C Q Figure 3 Figure 2. IMON vs ICHARGE ICHARGE (% OF MAXIMUM CURRENT) 0 0 0.2 0.4 0.6 0.8 4006 F02 1.0 1.2 20 40 60 80 100 1.19V 0.309V This voltage is stored by C7. Then the switch is opened for a short period of time to read the voltage across the thermistor. tHOLD = 10 • RRT • 17.5pF = 54µs, for RRT = 309k When the tHOLD interval ends the result of the thermistor testing is stored in the D flip-flop (DFF). If the voltage at NTC is within the limits provided by the resistor divider feeding the comparators, then the NOR gate output will be low and the DFF will set TBAD to zero and charging will continue. If the voltage at NTC is outside of the resistor divider limits, then the DFF will set TBAD to one, the charger will be shut down, and the timer will be suspended until TBAD returns to zero (see Figure 4). off for about 300ns at 40 µs intervals. This is done to prevent audible noise when using ceramic capacitors at the input and output. Charger Startup When the charger is enabled, it will not begin switching until the ITH voltage exceeds a threshold that assures initial current will be positive. This threshold is 5% to 15% of the maximum programmed current. After the charger begins switching, the various loops will control the current at a level that is higher or lower than the initial current. The duration of this transient condition depends upon the loop compensation but is typically less than 100 µs. Thermistor Detection The thermistor detection circuit is shown in Figure 3. It requires an external resistor and capacitor in order to function properly. The thermistor detector performs a sample-and-hold func- tion. An internal clock, whose frequency is determined by the timing resistor connected to RT, keeps switch S1 closed to sample the thermistor: tSAMPLE = 127.5 • 20 • RRT • 17.5pF = 13.8ms, for RRT = 309k The external RC network is driven to approximately 4.5V and settles to a final value across the thermistor of: V VR RR RTH FINAL TH TH () .• = + 45 9 CLK (NOT TO SCALE) VNTC tSAMPLE VOLTAGE ACROSS THERMISTOR tHOLD 4006 F04 COMPARATOR HIGH LIMIT COMPARATOR LOW LIMIT Figure 4 |
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