When to Use the Integrated Charge Pump in a Microcontroller (MCU)
You can use an integrated charge pump to improve the analog performance of an MCU at low operating voltages.
What is a Charge Pump?
Before discussing the charge pump in 8-bit MCUs, let’s quickly recap what a charge pump is. A charge pump is a power supply topology that moves charge through a capacitor, which is electrically switched between the input and output. Charge is placed on this capacitor on one side, then removed on the other. Depending on the specific objective of the power supply, this can be used to double the input voltage or to generate a negative power supply.
Discrete Charge Pump Regulators
Discrete charge pump regulators are commonly used in low-power applications due to their high conversion efficiencies. Additionally, charge pump regulators usually have a low Bill of Materials (BOM) cost to implement—usually only three capacitors are needed to implement, and in many cases, the same capacitor values may be used elsewhere in the system.
Integrated Charge Pump on 8-bit MCUs
However, for this blog post, I will be referring to the integrated charge pump found on some 8-bit MCUs. These charge pumps do not require additional external components and are fully integrated into the device. Returning to the original question—when should you use the integrated charge pump in the MCU?
You can use the integrated charge pump(s) to improve the analog performance of the MCU at low operating voltages. As the operating voltage of the MCU decreases, the internal transistors don’t switch as well, which increases the internal resistance for analog signals. To compensate, MCUs that run at low voltages can switch on an internal charge pump to create a higher switching voltage for the transistors that are responsible for the analog circuits. This improves the analog performance, but it increases the quiescent current of the microcontroller.
MCUs can integrate charge pumps in different ways. Some peripherals may have a dedicated charge pump, like the Operational Amplifier (OPA or OPAMP), while other peripherals may share a common charge pump. For instance, on the PIC16F17146 family of MCUs, the integrated charge pump is shared by the Analog-to-Digital Converter with Computation (ADCC), Digital-to-Analog Converter (DAC) and Comparator (CMP).
The charge pump can be set to always-on or always-off, but since the charge pump is only needed at low operating voltages, it can also be configured to automatically start when the operating voltage is below a fixed threshold in the device. A variant of this mode operates nearly identically, but additionally requires an analog peripheral to request the charge pump. This mode is useful in applications that switch peripherals on and off to save power. When the peripherals are off, the charge pump is off.
Another way to reduce the operating power of the charge pump is via the oscillator selection. The charge pump can be set to be always running from an internal oscillator for maximum performance, or alternatively, it can switch between the slower ADCRC clock and Fosc when the ADCC is running to maximize analog performance during conversions. When the ADCC is idle, the charge pump is still active, but at a reduced performance level.
The main drawback of the integrated charge pump in the MCU is a higher quiescent power draw. However, in applications that require high performance analog at low operating voltages, it is an essential feature. For more information about the charge pump, please download PIC16F17126/46 family device data sheet.