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MAX713CSE Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX713CSE Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 18 page NiCd/NiMH Battery Fast-Charge Controllers 6 _______________________________________________________________________________________ ____________________Getting Started The MAX712/MAX713 are simple to use. A complete linear-mode or switch-mode fast-charge circuit can be designed in a few easy steps. A linear-mode design uses the fewest components and supplies a load while charging, while a switch-mode design may be neces- sary if lower heat dissipation is desired. 1) Follow the battery manufacturer’s recommendations on maximum charge currents and charge-termination methods for the specific batteries in your application. Table 1 provides general guidelines. 2) Decide on a charge rate (Tables 3 and 5). The slow- est fast-charge rate for the MAX712/MAX713 is C/4, because the maximum fast-charge timeout period is 264 minutes. A C/3 rate charges the battery in about three hours. The current in mA required to charge at this rate is calculated as follows: IFAST = (capacity of battery in mAh) ––––––––––––––––––––––––– (charge time in hours) Depending on the battery, charging efficiency can be as low as 80%, so a C/3 fast charge could take 3 hours and 45 minutes. This reflects the efficiency with which electrical energy is converted to chemical energy within the battery, and is not the same as the power- conversion efficiency of the MAX712/MAX713. 3) Decide on the number of cells to be charged (Table 2). If your battery stack exceeds 11 cells, see the Linear- Mode High Series Cell Count section. Whenever changing the number of cells to be charged, PGM0 and PGM1 must be adjusted accordingly. Attempting to charge more or fewer cells than the number pro- grammed can disable the voltage-slope fast-charge termination circuitry. The internal ADC’s input volt- age range is limited to between 1.4V and 1.9V (see the Electrical Characteristics), and is equal to the voltage across the battery divided by the number of cells programmed (using PGM0 and PGM1, as in Table 2). When the ADC’s input voltage falls out of its specified range, the voltage-slope termination cir- cuitry can be disabled. 4) Choose an external DC power source (e.g., wall cube). Its minimum output voltage (including ripple) must be greater than 6V and at least 1.5V higher (2V for switch mode) than the maximum battery voltage while charging. This specification is critical because normal fast-charge termination is ensured only if this requirement is maintained (see Powering the MAX712/MAX713 section for more details). 5) For linear-mode designs, calculate the worst-case power dissipation of the power PNP and diode (Q1 and D1 in the Typical Operating Circuit) in watts, using the following formula: PDPNP = (maximum wall-cube voltage under load - minimum battery voltage) x (charge current in amps) If the maximum power dissipation is not tolerable for your application, refer to the Detailed Description or use a switch-mode design (see Switch-Mode Operation in the Applications Information section, and see the MAX713 EV kit manual). 6) For both linear and switch-mode designs, limit cur- rent into V+ to between 5mA and 20mA. For a fixed or narrow-range input voltage, choose R1 in the Typical Operation Circuit using the following formula: R1 = (minimum wall-cube voltage - 5V) / 5mA For designs requiring a large input voltage variation, choose the current-limiting diode D4 in Figure 19. 7) Choose RSENSE using the following formula: RSENSE = 0.25V / (IFAST) 8) Consult Tables 2 and 3 to set pin-straps before applying power. For example, to fast charge at a rate of C/2, set the timeout to between 1.5x or 2x the charge period, three or four hours, respectively. < C/2 ∆V/∆t and/or temperature, MAX712 ∆V/∆t and/or temperature, MAX713 NiMH Batteries NiCd Batteries > 2C ∆V/∆t and temperature, MAX712 or MAX713 ∆V/∆t and/or temperature, MAX713 2C to C/2 ∆V/∆t and/or temperature, MAX712 or MAX713 ∆V/∆t and/or temperature, MAX713 Charge Rate Table 1. Fast-Charge Termination Methods |
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