In applications where the load is in short load pulses, such as Bluetooth® LE System on Chip (SoC), and the battery has a high internal resistance, like a coin cell, the selection of capacitance on the boost regulator input and output affects the overall battery life.
Typically, increasing the output capacitance on a DC/DC regulator improves load transient performance and in a hysteretic operating mode it improves efficiency as well. This is because fewer refresh cycles are needed to maintain output voltage regulation. However, for batteries with high internal resistance, adding capacitance on the input side of the regulator can be more beneficial. The capacitance that is parallel to the battery helps reduce the peak current drawn from the battery and maintains a more stable battery voltage.
The battery's equivalent series resistance (ESR) losses, represented by the formula
P=I2xR,
decrease significantly if the peak current is reduced as the loss is proportional to the square
of the current. Increasing output capacitance improves load transient performance but also
increases the maximum current peaks from the battery. This increase leads to higher ESR losses,
which reduces the benefits of the added capacitance. With a weak battery, these high current
pulses can cause the battery voltage to collapse during a boost refresh.
In actual applications, it is essential to consider cost and solution size. Optimizing the placement of capacitors can significantly impact overall performance and battery life.
To demonstrate the boost regulator's performance with different input and output capacitors, a test was set up using the nPM2100 Evaluation Kit (EK) with a CR2032 battery and nRF52840 with a +8 dBm load profile simulating one Bluetooth LE advertising (RX/TX) event. The target output voltage (VOUT) was programmed to 3.3 V, and the boost regulator was blocked from entering High Power mode (NOHP). The default capacitor configuration for the nPM2100 EK included the following:
- Input capacitor (CVBAT): 10 µF + 1 nF
- Output capacitor (COUT) and boost output capacitor (CVINT): 10 µF + 2.2 µF + 1 nF
The following cases were compared:
- Baseline battery discharge test conducted without additional capacitors
- A 100 µF capacitor added to the boost regulator output (VINT)
- A 100 µF capacitor added to the input (VBAT)
- A 100 µF added to both input (VBAT) and output (VINT)
The tests show that adding capacitance to the output of the boost regulator improves the load transient response and efficiency but causes a higher drop in input voltage and increases peak current, which leads to higher ESR losses. Adding capacitance at the input reduces the ESR losses and keeps the battery voltage more stable. The best setup for extending battery life and improving load transient performance was achieved by adding 100 µF capacitance to both the input and output.
The following table shows the battery life metrics which are calculated as how many Bluetooth LE events can be drawn from the battery.
| Configuration | Battery life difference |
|---|---|
| Default capacitors | Baseline |
| +100 µF at VINT | +2.7% |
| +100 µF at VBAT | +11.6% |
| +100 µF at VINT and VBAT | +17.9% |
The following figures show the response to a Bluetooth LE event for the different test cases.