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MAX8725ETI Scheda tecnica(PDF) 17 Page - Maxim Integrated Products |
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MAX8725ETI Scheda tecnica(HTML) 17 Page - Maxim Integrated Products |
17 / 30 page Multichemistry Battery Chargers with Automatic System Power Selector ______________________________________________________________________________________ 17 Detailed Description The MAX1909/MAX8725 include all of the functions necessary to charge Li+, NiMH, and NiCd batteries. A high-efficiency, synchronous-rectified step-down DC- DC converter is used to implement a precision con- stant-current, constant-voltage charger with input current limiting. The DC-DC converter uses external p-channel/n-channel MOSFETs as the buck switch and synchronous rectifier to convert the input voltage to the required charge current and voltage. The charge cur- rent and input current-limit sense amplifiers have low- input-referred offset errors and can use small-value sense resistors. The MAX1909/MAX8725 feature a volt- age-regulation loop (CCV) and two current-regulation loops (CCI and CCS). The CCV voltage-regulation loop monitors BATT to ensure that its voltage never exceeds the voltage set by VCTL. The CCI battery current-regu- lation loop monitors current delivered to BATT to ensure that it never exceeds the current limit set by ICTL. A third loop (CCS) takes control and reduces the charge current when the sum of the system load and the input- referred charge current exceeds the power source cur- rent limit set by CLS. Tying CLS to the reference voltage provides a 7.5A input current limit with a 10mΩ sense resistor. The ICTL, VCTL, and CLS analog inputs set the charge current, charge voltage, and input current limit, respec- tively. For standard applications, internal set points for ICTL and VCTL provide a 3A charge current using a 15mΩ sense resistor and a 4.2V per-cell charge volt- age. The variable for controlling the number of cells is set with the MODE input. The MAX8725 includes a PKPRES input used for battery-pack detection. Based on the presence or absence of the AC adapter, the MAX1909/MAX8725 automatically provide an open- drain logic output signal ACOK and select the appropri- ate source for supplying power to the system. A p-channel load switch controlled from the PDL output and a similar p-channel source switch controlled from the PDS output are used to implement this function. Using the MODE control input, the MAX1909/MAX8725 can be pro- grammed to perform a relearning, or conditioning, cycle in which the battery is isolated from the charger and com- pletely discharged through the system load. When the battery reaches 100% depth of discharge, it is recharged to full capacity. The circuit shown in Figure 1 demonstrates a simple hardwired application, while Figure 2 shows a typical application for smart-battery systems with variable charge current and source switch configuration that sup- ports battery conditioning. Smart-battery systems typical- ly use a host µC to achieve this added functionality. Setting the Charge Voltage The MAX1909/MAX8725 use a high-accuracy voltage regulator for charge voltage. The VCTL input adjusts the battery output voltage. In default mode (VCTL = LDO), the overall accuracy of the charge voltage is ±0.5%. VCTL is allowed to vary from 0 to 3.6V, which provides a 10% adjustment range of the battery volt- age. Limiting the adjustment range reduces the sensi- tivity of the charge voltage to external resistor tolerances from ±1% to ±0.05%. The overall accuracy of the charge voltage is better than ±1% when using ±1% resistors to divide down the reference to establish VCTL. The per-cell battery termination voltage is a func- tion of the battery chemistry and construction. Consult the battery manufacturer to determine this voltage. The battery voltage is calculated by the equation: where VREF = 4.2235V, and CELL is the number of cells selected with the MAX1909/MAX8725s’ trilevel MODE control input. When MODE is tied to the LDO output, CELL = 4. When MODE is left floating, CELL = 3. When MODE is tied to ground, the charger enters condition- ing mode, which is used to isolate the battery from the charger and discharge it through the system load. See the Conditioning Mode section. The internal error ampli- fier (GMV) maintains voltage regulation (see Figure 3 for the Functional Diagram). The voltage-error amplifier is compensated at CCV. The component values shown in Figures 1 and 2 provide suitable performance for most applications. Individual compensation of the volt- age regulation and current-regulation loops allow for optimal compensation. See the Compensation section. Setting the Charge Current The voltage on the ICTL input sets the maximum voltage across current-sense resistor RS2, which in turn determines the charge current. The full-scale differen- tial voltage between CSIP and CSIN is 75mV; thus, for a 0.015Ω sense resistor, the maximum charge current is 5A. In default mode (ICTL = LDO), the sense voltage is 45mV with an overall accuracy of ±5%. The charge cur- rent is programmed with ICTL using the equation: I RS V V CHG ICTL =× 0 075 23 6 . . V CELL V VV BATT REF VCTL =+ − 18 952 . . |
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