Microchip TC7660HEOA: A Comprehensive Technical Overview and Application Guide
The Microchip TC7660HEOA is a monolithic, CMOS-based voltage converter IC renowned for its ability to efficiently generate a negative output voltage from a single positive input supply. This device, which functions as a voltage doubler and an inverting charge pump, is an essential component in countless electronic systems where a localized negative bias rail is required without the complexity and cost of an inductive switching regulator.
Core Architecture and Operating Principle
At its heart, the TC7660HEOA utilizes a sophisticated charge-pump architecture. The internal oscillator, which typically operates at a nominal frequency of 10 kHz, drives a set of four CMOS switches. These switches first charge an external "flying" capacitor to the input voltage (V+). In the next phase of the clock cycle, the switches reconfigure to connect this charged capacitor in series with the input supply and the output, effectively transferring the charge to an external reservoir capacitor. This action produces an output voltage of approximately -V+, minus the small voltage drops across the internal switches.
A key feature of this device is its wide operating voltage range, from 1.5V to 12V. This flexibility allows it to be used in both low-voltage modern microcontroller-based systems and older legacy 12V systems. The H-grade variant specified in the part number (HEOA) guarantees an extended operational temperature range of -40°C to +125°C, making it suitable for automotive, industrial, and harsh environment applications.
Key Performance Characteristics and Advantages
High Voltage Conversion Efficiency: Typically greater than 99%, as it relies on capacitive charge transfer rather than magnetic induction, minimizing energy loss.
Minimal External Components: Requires only two inexpensive capacitors (a flying capacitor and a reservoir capacitor) to function, leading to a compact and low-cost Bill of Materials (BOM).
Low Power Consumption: Inherited from its CMOS design, the device draws very low quiescent current, which is critical for battery-powered applications.
Simple Frequency Boosting: For applications requiring reduced output ripple or smaller capacitor values, the oscillator frequency can be easily increased by adding an external capacitor between the OSC pin and ground.
No Inductors: Eliminates concerns related to electromagnetic interference (EMI), magnetic field generation, and inductor saturation.
Primary Application Circuits
1. Voltage Inverter: This is the most straightforward application. By connecting the flying capacitor (C1) between pins 2 and 4, and the reservoir capacitor (C2) from VOUT to ground, the IC generates a negative voltage rail from the positive input.
2. Voltage Doubler: By slightly modifying the external connections (LV pin to ground, C1 from V+ to pin 2, C2 from VOUT to ground), the TC7660 can double the input voltage, providing an output of approximately 2x V+.
3. Supply Rail Splitter: A combination of a voltage inverter and a linear regulator can be used to create symmetric positive and negative supplies (e.g., ±5V) from a single positive source (e.g., +10V).
Design Considerations and Best Practices
Capacitor Selection: Low-ESR ceramic capacitors are highly recommended for both the flying and reservoir capacitors. Values typically range from 10µF to 100µF, with higher values reducing output ripple.
Output Impedance: The output impedance, typically around 50Ω, limits the amount of current the device can source. For higher output currents, consider using a larger value flying capacitor or selecting a charge pump IC with a lower inherent impedance.
Output Ripple: The inherent switching action causes output ripple. This can be mitigated by using larger reservoir capacitors, increasing the oscillator frequency, or adding a small low-pass filter (e.g., an LC or RC filter) at the output.
LV Pin Configuration: For operation below 3.5V, the LV (Low Voltage) pin must be connected to ground to ensure proper internal MOSFET biasing. For inputs above 3.5V, it should be left floating.
The Microchip TC7660HEOA stands as a remarkably versatile and robust solution for generating negative voltages. Its unparalleled simplicity, high efficiency, and minimal component count make it an indispensable tool for engineers. It is the ideal choice for applications demanding a low-noise, low-current auxiliary rail, such as biasing op-amps, sensors, LCD displays, and data acquisition systems, where the size, cost, and EMI of a switching regulator are undesirable.
Keywords: Charge Pump, Voltage Inverter, Negative Voltage Generator, CMOS, Microchip
