Charge Pump Voltage Converter Design Using the Microchip TC7660CPA
Voltage inversion and doubling are fundamental requirements in many electronic systems, particularly where dual-rail power supplies are needed from a single source. While inductive switching regulators are a common solution, they introduce complexity, cost, and potential electromagnetic interference (EMI). The charge pump voltage converter presents a compelling alternative, offering a simple, cost-effective, and inductorless method for generating negative or doubled voltages. The Microchip TC7660CPA is a quintessential component that exemplifies the elegance and utility of this approach.
The TC7660CPA is a monolithic, CMOS voltage converter IC capable of both voltage inversion and multiplication. Housed in an 8-pin PDIP package, its primary function is to convert a positive input voltage in the range of +1.5V to +12V to a corresponding negative output (e.g., +5V in becomes -5V out). With minor external component modifications, it can also be configured to double an input voltage. Its key advantages include high power conversion efficiency (typically 99%), low power consumption, and only two external capacitors required for operation, making it exceptionally easy to implement.
The core of its operation lies in a switched-capacitor charge pump circuit. Internally, an oscillator drives a set of CMOS switches that control the charging and discharging of external capacitors. In the standard voltage inverter configuration:
1. During the first half of the oscillator cycle, the flying capacitor (C1) is charged to the input voltage (V+).
2. During the second half-cycle, the charged capacitor is effectively flipped and connected in series with the input voltage and the output capacitor (C2). This action transfers the charge from C1 to C2, resulting in an output voltage of approximately -V+.
The design process is remarkably straightforward. The essential components are:
The TC7660CPA IC itself.
A flying capacitor (C1): A value of 10µF is standard for most applications, though this can be adjusted; lower values increase output impedance, while higher values can supply more current.
An output capacitor (C2): Typically also 10µF, it smooths the output voltage and determines the level of output ripple.
For optimal performance, several design considerations must be addressed. Output impedance is a critical parameter, typically around 50Ω at 5V. This limits the available output current; if too much current is drawn, the output voltage will sag. The inherent switching action causes a small output ripple, which can be mitigated by increasing the value of C1 and C2 or by adding a small LC filter at the output. Furthermore, the oscillator frequency can be boosted from the default 10kHz by connecting an external capacitor between the OSC pin and ground, which allows for the use of smaller-valued capacitors and can reduce output ripple.
A significant enhancement to the basic circuit involves adding diode pumping for improved negative voltage regulation. By connecting the anode of a signal diode to the LV pin and the cathode to ground, the internal oscillator's swing is increased. This simple modification lowers the output impedance significantly, enhancing the load regulation and ensuring the output voltage remains stable under varying load conditions.
In conclusion, the TC7660CPA provides an incredibly efficient and simple solution for generating low-power, negative, or doubled supply rails. Its inductorless design eliminates associated magnetic costs and EMI, making it ideal for noise-sensitive applications like audio circuits, operational amplifier biasing, and sensor interfaces. Its minimal component count and ease of use make it a favorite among engineers for rapid prototyping and final designs alike.
ICGOODFIND: The Microchip TC7660CPA is an outstanding choice for designers seeking a no-fuss, highly reliable, and cost-effective charge pump solution for voltage conversion tasks.
Keywords: Charge Pump, Voltage Inverter, Switched Capacitor, Output Impedance, Voltage Doubler
