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ADP3801 Datasheet(PDF) 9 Page - Analog Devices |
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ADP3801 Datasheet(HTML) 9 Page - Analog Devices |
9 / 20 page ADP3801/ADP3802 –9– REV. 0 Adjusting the Final Battery Voltage In addition to the PROG input, the ADP3801/ADP3802 pro- vides an input (ADJ) for fine adjustment of the final battery voltage. For example, the ADJ amplifier allows the nominal 4.2 V per cell setting for Li-Ion battery cells to be adjusted to 4.1 V for certain chemistries. An internal amplifier buffers the ADJ pin and adjusts the internal reference voltage on the input to GM3. Figure 5 shows a graph of the percent change in final battery voltage vs. the ADJ voltage. The linear portion between 0.6 VREF and 1.4 VREF follows the formula below: ∆V VV V BAT AD REF REF % () = − × J 4 100 The factor of four in the denominator is due to internal scaling. When VADJ is above 2.5 V, an internal comparator switches off the ADJ amplifier, giving a 0% change in VBAT. Whenever the ADJ function is not used it should be connected to VL. The total range of adjustment is ±10%. For example, the 4.2 V final battery voltage setting can be adjusted from 3.78 V to 4.62 V. Of course, care must be taken not to adjust the final battery voltage to an unsafe charging level for Li-Ion batteries. Follow the battery manufacturers specifications for the appro- priate final battery voltage. Never charge a Li-Ion battery above the manufacturers rated maximum! Voltage Loop Accuracy The ADP3801/ADP3802 guarantees that the battery voltage be within ±0.75% of the setpoint over the specified temperature range and the specified charge current range. This inclusive specification saves the designer the time and expense of having to design-in additional high accuracy components such as a reference and precision resistors. To maintain the ±0.75% specification, the layout and design of the external circuitry must be considered. The input impedance of BATA and BATB is typically 265 k Ω, so any additional im- pedance on these inputs will cause an error. As a result, do not add external resistors to the battery inputs. Furthermore, if the output voltage is being used for other purposes, such as to sup- ply additional circuitry, the current to this circuitry should be routed separately from the sense lines to prevent voltage drops due to impedance of the PC-board traces. In general, route the sense lines as Kelvin connections as close to the positive termi- nals of the battery as possible. The same care must be given to the ground connection for the ADP3801/ADP3802. Any voltage difference between the bat- tery ground and the GND pin will cause an error in the charge voltage. This error includes the voltage drop due to the ground current of the part. Thus, the GND pin should have a thick trace or ground plane connected as close as possible to the battery’s negative terminal. Any current from additional cir- cuitry should be routed separately to the supply return and not share a trace with the GND pin. Dual Battery Operation The ADP3801/ADP3802 is designed to charge two separate battery packs. These batteries can be of different chemistries and have a different number of cells. At any given time, only one of the two batteries is being charged. To select which bat- tery is being monitored, and therefore which battery is being charged, the ADP3801/ADP3802 includes a battery selector mux. This two-channel mux is designed to be “break-before- make” to ensure that the two batteries are not shorted together momentarily when switching from one to the other. The A/B input is a standard logic input, with a logic low selecting BATA and a logic high selecting BATB. See the application in Figure 31 for more information. Overvoltage Comparator GM3 includes an overvoltage comparator. Its output bypasses the COMP node to quickly reduce the duty cycle of the PWM to 0% when an overvoltage event occurs. A second output is connected to the COMP node and, with slower response, re- duces the voltage on the COMP cap to provide a soft start re- covery. The threshold of the comparator is typically 8% above the final battery voltage. This comparator protects external circuitry from any condition that causes the output voltage to quickly increase. The most likely reason is if the battery is suddenly removed while it is being charged with high current. Figure 27 shows the transient response when the battery is removed. Notice that the output voltage increases to the com- parator trip point, but it is quickly brought under control and held at the final battery voltage. |
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