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MAX712CPE Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX712CPE Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 18 page NiCd/NiMH Battery Fast-Charge Controllers 10 ______________________________________________________________________________________ The 1.5V of overhead is needed to allow for worst-case voltage drops across the pass transistor (Q1 of Typical Operating Circuit), the diode (D1), and the sense resistor (RSENSE). This minimum input voltage require- ment is critical, because violating it can inhibit proper termination of the fast-charge cycle. A safe rule of thumb is to choose a source that has a minimum input voltage = 1.5V + (1.9V x the maximum number of cells to be charged). When the input voltage at DC IN drops below the 1.5V + (1.9V x number of cells), the part oscillates between fast charge and trickle charge and might never completely terminate fast-charge. The MAX712/MAX713 are inactive without the wall cube attached, drawing 5 µA (max) from the battery. Diode D1 prevents current conduction into the DRV pin. When the wall cube is connected, it charges C1 through R1 (see Typical Operating Circuit) or the current-limiting diode (Figure 19). Once C1 charges to 5V, the internal shunt regulator sinks current to regulate V+ to 5V, and fast charge commences. The MAX712/MAX713 fast charge until one of the three fast-charge terminating conditions is triggered. If DC IN exceeds 20V, add a cascode connection in series with the DRV pin as shown in Figure 5 to prevent exceeding DRV’s absolute maximum ratings. Furthermore, if Figure 19’s DC IN exceeds 15V, a tran- sistor level-shifter is needed to provide the proper volt- age swing to the MOSFET gate. See the MAX713 EV kit manual for details. Select the current-limiting component (R1 or D4) to pass at least 5mA at the minimum DC IN voltage (see step 6 in the Getting Started section). The maximum current into V+ determines power dissipation in the MAX712/MAX713. maximum current into V+ = (maximum DC IN voltage - 5V) / R1 power dissipation due to shunt regulator = 5V x (maximum current into V+) Sink current into the DRV pin also causes power dissipa- tion. Do not allow the total power dissipation to exceed the specifications shown in the Absolute Maximum Ratings. Fast Charge The MAX712/MAX713 enter the fast-charge state under one of the following conditions: 1) Upon application of power (batteries already installed), with battery current detection (i.e., GND voltage is less than BATT- voltage), and TEMP higher than TLO and less than THI and cell voltage higher than the UVLO voltage. 2) Upon insertion of a battery, with TEMP higher than TLO and lower than THI and cell voltage higher than the UVLO voltage. RSENSE sets the fast-charge current into the battery. In fast charge, the voltage difference between the BATT- and GND pins is regulated to 250mV. DRV current increases its sink current if this voltage difference falls below 250mV, and decreases its sink current if the volt- age difference exceeds 250mV. fast-charge current (IFAST) = 0.25V / RSENSE Trickle Charge Selecting a fast-charge current (IFAST) of C/2, C, 2C, or 4C ensures a C/16 trickle-charge current. Other fast- charge rates can be used, but the trickle-charge current will not be exactly C/16. BATT- X V+ OPEN REF BATT- 1 0 0 0 0 8 512 256 128 64 CURRENT-SENSE AMPLIFIER PGM3 FAST_CHARGE Av 1.25V V+ DC IN GND DRV GND CC BATT- RSENSE D1 REF VLIMIT CELL_VOLTAGE BATT- BATT- IN_REGULATION C2 Figure 6. Current and Voltage Regulator (linear mode) |
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