Designing a Charge Pump Voltage Inverter Circuit with the Microchip TC7662ACPA

In modern electronic design, the need to generate a negative voltage rail from a single positive supply is a common challenge, particularly for powering operational amplifiers, sensors, and interface circuits. While switched-mode power supplies (SMPS) offer high efficiency, they can introduce significant complexity and noise. A charge pump voltage inverter provides an elegant, cost-effective, and simple alternative for applications with moderate current requirements. The Microchip TC7662ACPA is a quintessential integrated circuit specifically designed for this purpose, offering a robust and straightforward solution.

The TC7662ACPA is a monolithic, CMOS-based voltage converter capable of converting a positive input voltage in the range of +1.5V to +12V to a corresponding negative output (-1.5V to -12V). Its primary operating principle is that of a switched capacitor charge pump. Internally, an oscillator controls four MOS switches that charge an external "flying" capacitor during one half-cycle and then reconfigure the circuit to dump that charge into an external reservoir capacitor during the next half-cycle, effectively inverting the input voltage.

A typical application circuit for inverting a voltage is remarkably simple, requiring only two external components. For a basic voltage inverter configuration:

1. Flying Capacitor (C1): A capacitor, typically in the range of 10µF, is connected between pins 2 (CAP+) and 4 (CAP-). This capacitor is crucial for the charge transfer process.

2. Output Reservoir Capacitor (C2): A second capacitor, also typically 10µF, is connected from the output pin (pin 5, VOUT) to ground. This capacitor smooths the output, reducing the inherent ripple voltage from the switching process.

The input voltage is applied between V+ (pin 8) and GND (pin 3), and the resulting inverted output (VOUT = -VIN) is available at pin 5. The oscillator frequency is set internally to approximately 10kHz, but it can be boosted by connecting pin 1 (LV) to GND or overridden by an external clock applied to pin 6 (OSC).

Key design considerations for optimizing performance with the TC7662ACPA include:

Output Impedance and Current Capability: The output impedance, typically around 50Ω, is a critical parameter. It directly limits the available output current and causes the output voltage to droop as load current increases. Designers must ensure the load current does not exceed the device's maximum rating (typically 20mA for the 'A' grade version) and that the resulting voltage drop is acceptable for the application.

Capacitor Selection: While standard aluminum electrolytic or tantalum capacitors were common historically, using low-ESR ceramic capacitors is now the preferred practice for both C1 and C2. This minimizes losses and improves overall efficiency and ripple performance.

Output Ripple: The charge pump operation generates a switching ripple on the output voltage. The magnitude of this ripple is a function of the switching frequency, the flying capacitance value, and the load current. Increasing the value of C1 and C2 or adding a small LC filter at the output can significantly reduce ripple for noise-sensitive circuits.

LV Pin Functionality: For operation with an input voltage below 3.5V, the LV (Low Voltage) pin (pin 1) must be connected to GND. This ensures proper internal MOS switch biasing. For VIN > 3.5V, it can be left unconnected.

Beyond simple inversion, the TC7662ACPA's architecture allows for other configurations, such as voltage doubling (using two devices) or even higher multiplication ladder circuits. Its simplicity, low external component count, and wide operating voltage range make it an indispensable tool for the electronics designer's toolkit when a small, localized negative voltage is required.

ICGOOODFIND: The Microchip TC7662ACPA stands out as an exceptionally versatile and easy-to-use IC for generating negative voltages. Its minimal external component count, primarily just two capacitors, allows for a highly compact and cost-effective PCB footprint. The device's ability to operate efficiently from a wide input range and its flexibility for both voltage inversion and doubling solidify its status as a fundamental building block for power management in space-constrained and cost-sensitive designs.

Keywords: Charge Pump, Voltage Inverter, Negative Voltage Generator, Switched Capacitor Converter, Output Impedance.