9 Power Supply Recommendations

Power supply requirements for the BQ34Z100-R2 are simplified due to the presence of the internal LDO voltage regulation. The REGIN pin accepts any voltage level between 2.7 V and 4.5 V, which is optimum for a single-cell Li-ion application. For higher battery voltage applications, a simple pre-regulator can be provided to power the bq34Z100-R2 and any optional LEDs. Decoupling the REGIN pin should be done with a 0.1-µF 10% ceramic X5R capacitor placed close to the device. While the pre-regulator circuit is not critical, special attention should be paid to its quiescent current and power dissipation. The input voltage should handle the maximum battery stack voltage. The output voltage can be centered within the 2.7-V to 4.5-V range as recommended for the REGIN pin.

For high stack count applications, a commercially available LDO is often the best quality solution, but comes with a cost tradeoff. To lower the BOM cost, the following approaches are recommended.

In Figure 9-1, Q1 is used to drop the battery stack voltage to roughly 4 V to power the BQ34Z100-R2 REGIN pin and also to feed the anode of any LEDs used in the application. To avoid unwanted quiescent current consumption, R1 should be set as high as is practical. It is recommended to use a low-current Zener diode.

Figure 9-1 Q1 Dropping Battery Stack Voltage to 4 V

Alternatively, if the range of a high-voltage battery stack can be well defined, a simple source follower based on a resistive divider can be used to lower the BOM cost and the quiescent current. For example:

Figure 9-2 Source Follower on a Resistive Divider

Power dissipation of the linear pre-regulator may become an important design decision when multiple LEDs are employed in the application. For example, the BQ34Z100-R2 EVM uses a pair of FETs in parallel to inexpensively dissipate enough power for 10-LED evaluation.