8.3.2.2.2 Cell Amplifier Headroom Under BAT Voltage Drop

Voltage differences between BAT and the top cell potential come from two sources as shown in Figure 11: V3P3 regulator current that flows through the R_BAT filter resistor, and the voltage drop in the series diode D_BAT of the hold-up circuit. These effects cause BAT to be less than the top-cell voltage measured by the cell amplifier.

Figure 11. Sources of Voltage Drop Affecting the BAT Pin

The top-cell amplifier (VC6 – VC5) is designed to measure an input voltage down to 1.4 V with a difference between the BAT and VC6 pin up to 1.2 V (that is, BAT can be 1.2 V lower than VC6). However, in applications with fewer than 6 cells, the upper-cell inputs are typically shorted to the top-cell input. For example, in a 5-cell application VC6 and VC5 would be shorted together and the (VC5 – VC4) amplifier would measure the top-cell voltage. The case is similar for 4-cell and 3-cell applications.

For these cases when using the (VC5 – VC4), (VC4 –VC3), or (VC3 – VC2) amplifier to measure the top cell, the difference between BAT and the top-cell amplifier must be less than 240 mV in order to measure cell voltages down to 1.4 V. Note that at higher-cell input voltages the top amplifier tolerates a greater difference. For example, in a 5-cell configuration (VC6 and VC5 tied together) the (VC5 – VC4) amplifier is able to measure down to a 1.7 V input with a 600-mV difference between VC5 and BAT.

Accordingly, in systems with fewer than 6 cells, it is important in system design to minimize R_BAT and to use a Schottky type diode for D_BAT with a low forward voltage. If it is not possible to reduce the drop at BAT to an acceptable level, then for 4-cell and 5-cell configurations, the (VC6 – VC5) amplifier may be used as the top cell amplifier as shown in Table 1, which allows up to a 1.2 V difference between BAT and the top cell.