A brief discussion of charge pump architecture and design considerations
There are two categories of switching power supplies: inductive switching power supplies and capacitive switching power supplies. The key components for energy storage in power supplies are inductors and capacitors. The capacitor switching power supply is a charge pump. The charge pump can realize voltage boost, step down, and inversion in the power supply.
1. Understanding the charge pump architecture
The core of the charge pump is based on the energy storage characteristics of the capacitor, changing the series-parallel relationship between the capacitors through switches, thereby changing the output voltage. The process of energy storage and transfer by the charge pump can be understood from the figure below. Core device switches and flying capacitors. Charge pumps are no longer limited to low-power (tens to hundreds of mA) solutions. There are already high-power solutions such as the ADI LTC7821 solution.
2. Charge pump classification
Charge pumps can be divided into unregulated charge pumps and regulated charge pumps.
A. From the architecture in Figure 1, we can see that the output voltage doubles and inverts the input voltage, and the output voltage changes with the input voltage. This structure can achieve double voltage, half voltage, inverting and fractional ratio voltage requirements, but the specifics depend on the internal topology of the chip.
B. A stable output voltage is usually required in a power supply, so the internal architecture control requires a closed loop.
Figure 2 - The internal view of ADI LTC3265 is a charge pump architecture + LDO architecture, which achieves stable positive and negative power output.
Figure 3 - The internal architecture of ADI ADP3605 is a charge pump architecture that regulates stable output by increasing the on-resistance of the output feedback adjustment loop control switch.
Overall, the charge pump with an unregulated architecture is more efficient than the two stable output voltage charge pumps.
3. Design considerations
Overall, the charge pump peripheral circuit is relatively simple, small in size, and high in efficiency. The core components include switches and flying capacitors. In order to optimize power supply performance, everything from device selection to PCB layout must be considered. For example, whether the switching device is built-in or external, its corresponding switching speed and switching loss; the type, capacitance, loss, size and other common parameters of the capacitor.
Flying capacitor: Since the polarity of the capacitor needs to be switched during switching, ceramic capacitors are preferred. X5R or X7R ceramic capacitors maintain good performance at -40°C-85°C.
Input and output capacitors: In order to improve efficiency while reducing output noise and ripple, it is recommended to use low-ESR ceramic capacitors. To increase the charge capacity required for transients, tantalum capacitors and electrolytic capacitors can be connected in parallel with them. Ripple voltage is often closely related to switching frequency, output current, capacitance value and ESR and needs to be considered comprehensively.
Switching MOSFET: The switching frequency of the charge pump is usually tens of KHz to several MHz. Some high-power charge pumps have external switching tubes. In addition to the drive of the tube, switching losses and conduction losses must also be considered.
4. Application scenarios
Charge pumps can be used in high-power fast charging scenarios for mobile devices
Charge pumps can be used in portable devices with low current requirements of less than 200mA.
Charge pumps can be used to drive LCDs and LEDs
In our commonly used RS232 level conversion and some power supplies, hot swap and other chips, we can also see the use of charge pumps as MOS tube gate drivers.
In scenarios where negative voltage and small current are required, a charge pump is also a very good choice.
Overall, the charge pump circuit is simple, the volume and efficiency solutions have certain advantages, and there are more and more usage scenarios.
#discussion #charge #pump #architecture #design #considerations
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