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MC34674B Datasheet(PDF) 16 Page - Freescale Semiconductor, Inc |
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MC34674B Datasheet(HTML) 16 Page - Freescale Semiconductor, Inc |
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16 / 26 page  Analog Integrated Circuit Device Data 16 Freescale Semiconductor 34674 FUNCTIONAL DESCRIPTION FUNCTIONAL DEVICE OPERATION current needs to stay below the EOC threshold for more than 0.5 seconds. The charger is turned off and the LED indication is green when charge completes. If the total fast charge time limit is exceeded, the charger also enters the charge completion state. RECHARGE MODE If the battery voltage drops below the recharge threshold after charge completion, the charger will try to recharge the battery to 4.2V. Because the battery voltage drop can also be caused by the removal of the battery, before starting recharge, the charger tries to verify if the battery is still present. If the battery is not found, then the connection fault is issued again. If the battery is still connected, the charger restarts charging to bring the battery to a full state. The LED indication remains green in this mode. The recharge mode has the same total charge time limit as the fast charge mode. For any reason the battery voltage falls below the trickle-charge threshold in the recharge mode, the charger will enter the battery connection verification state again, as shown in Figure 23. TEMPERATURE AND OVER-VOLTAGE FAULT The NTC interface block offers an interface to an external NTC thermistor circuit to monitor the battery temperature. When the battery temperature is out of a user-programmable window, the charger is disabled and a fault condition is issued with a yellow LED indication. When the fault conditions are removed, the charger enters the battery connection verification state. More detailed description on the NTC interface is offered later in this datasheet. The charger has an 11V (typ.) input OVP threshold. When the input voltage is higher than this threshold, the charging is stopped and a fault condition is issued with a yellow LED indication. When the input voltage falls below the OVP threshold, the charger restarts charging and resets the internal digital logic control block. TIMEOUT FAULT The TIMEOUT fault can only occur when the charger stays in the trickle-charge mode for a period longer than the time limit. The charger is turned off and a yellow LED indication is issued when this fault occurs. The only path to exit this fault is by toggling the EN input or by recycling the power input. DETAILED FUNCTIONAL DEVICE OPERATION NTC INTERFACE The MC34674 offers an interface to an external NTC thermistor to monitor the battery temperature. The low and high temperature thresholds in the Table 4 allow users to set a temperature window (such as 0°C to 50°C), within which the charging is allowed. If the battery temperature is out of such a window, a temperature fault is issued and the LED indicates a yellow color. Figure 19 shows the internal equivalent circuit for the NTC interface and the external NTC thermistor circuit. An internal resistor divider that is powered by the VREF pin voltage, VVREF, creates two reference voltages, 1/3 VVREF and 2/3 VVREF. An external resistor divider also powered by VVREF generates the voltage VTEMP to represent the battery temperature. Because the resistance of the NTC thermistor, RNTC, decreases as temperature rises, as shown in Figure 20, VTEMP decreases as the battery temperature increases. Assume TCOLD and THOT are the two temperature thresholds, such as 0°C and 50°C. When the battery temperature falls below TCOLD, VTEMP rises above 2/3 VVREF and an under-temperature fault is issued. Similarly, when the battery temperature rises above THOT, VTEMP falls below 1/3 VVREF, so an over-temperature fault is issued. The relationship between the internal and the external divider voltages at the triggering points can be expressed as the following: equ. 1 where RNTC is the thermistor resistance at the given temperature, and KX is the ratio of the internal divider at the given triggering points (see Table 4). RU and RS represent a pull-up resistor and a series resistor in the external resistor divider respectively. The resistance selection of RU and RS can be figured out by the following two equations: equ. 2 equ. 3 where KHOT and KCOLD are the resistor divider ratios for the temperature thresholds THOT and TCOLD respectively; RHOT and RCOLD are the NTC thermistor resistance at THOT and TCOLD respectively. The typical values for KHOT and KCOLD are 1/3 and 2/3 respectively, as given in Table 4. Refer to the Application Information section for more details regarding the RU and RS selection. R NTC R S + R NTC R S R U ++ ----------------------------------------- K x = R HOT R S + R HOT R S R U ++ ----------------------------------------- K HOT = R COLD R S + R COLD R S R U ++ --------------------------------------------- K COLD = |
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Similar Description - MC34674B |
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