Generating Negative Rails with LMC7660: A Deep Dive into CMOS Voltage Conversion

Executive Summary: What is the LMC7660?

The LMC7660 is a CMOS switched-capacitor voltage converter designed to invert a positive input voltage ranging from +1.5V to +10V into a corresponding negative output (-1.5V to -10V). It serves as a highly efficient, pin-compatible upgrade to the industry-standard 7660 series.

• Market Position: A low-cost, high-efficiency industry standard for low-power voltage inversion.

• Top Features: 97% voltage conversion efficiency, ultra-low 200 µA supply current, and operation without external diodes.

• Primary Audience: Ideal for design engineers building handheld instruments, IoT sensors, and analog signal chains requiring split-supply rails.

• Supply Status: Active and widely available from major distributors like Texas Instruments.

1. Technical Specifications & Performance Analysis

1.1 Core Architecture (Charge Pump)

The LMC7660 utilizes a charge-pump or "switched-capacitor" architecture. Unlike inductive switching regulators, it uses internal CMOS switches to charge and discharge external capacitors to "pump" the voltage to a negative potential. This eliminates the need for bulky inductors, reducing both Bill of Materials (BOM) costs and electromagnetic interference (EMI).

1.2 Key Electrical Characteristics

Engineers must note the LMC7660's balance between efficiency and power constraints: 

- Input Voltage Range: 1.5V to 10V (Note: Do not exceed 10V to avoid CMOS latch-up). 

- Power Efficiency: 95% typical, making it excellent for battery-operated devices. 

- Supply Current: Only 200 µA maximum, significantly lower than bipolar alternatives. 

- Oscillator Frequency: Operates at a nominal 10 kHz, which can be adjusted or overdriven if necessary.

1.3 Interfaces and Connectivity

As a power component, the LMC7660 features a simple 8-pin interface. It is designed to work seamlessly with analog components like Op-Amps and Data Converters that require a negative bias from a single-ended positive supply (e.g., a 5V USB rail).

2. Pinout, Package, and Configuration

2.1 Pin Configuration Guide

The LMC7660 is typically housed in an 8-pin format. Key pins include: 

- Pin 2 & 4 (CAP+ / CAP-): Connection points for the external charge-pump capacitor. 

- Pin 3 (GND): System ground. 

- Pin 5 (Vout): The inverted negative voltage output. 

- Pin 6 (LV): Low Voltage pin. This must be grounded if the input voltage is below 3.5V to prevent internal malfunctions. 

- Pin 8 (V+): Positive supply input.

2.2 Naming Convention & Ordering Codes

When sourcing the LMC7660, procurement managers should look for suffixes that denote package and temperature: 

- LMC7660IN/NOPB: 8-pin PDIP (Plastic Dual In-Line Package), through-hole. 

- LMC7660IM/NOPB: 8-pin SOIC (Small Outline Integrated Circuit), surface mount. 

- NOPB: Indicates "No Pb" (Lead-Free/RoHS compliant).

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

Pro Tip: For the best performance, use Low-ESR (Equivalent Series Resistance) capacitors to minimize output voltage ripple.

3.1 Hardware Implementation

• Bypass Capacitors: Use a 10 µF electrolytic or tantalum capacitor for both the pump (C1) and the reservoir (C2).

• PCB Layout: Keep the capacitors as close to the IC pins as possible to reduce parasitic inductance.

• Thermal Management: The LMC7660 is high-efficiency and rarely requires heat sinking, but ensure the ambient temperature stays within the -40°C to +85°C range for industrial grades.

3.2 Common Design Challenges

• Issue: Output Voltage Drop: The LMC7660 has an internal resistance of roughly 55Ω. As you draw more current, the output voltage will "droop."

• Fix: Limit load current to <20mA. If higher current is needed, consider the LM2660.

• Issue: Switching Noise: The 10 kHz ripple may bleed into sensitive audio or precision DC circuits.

• Fix: Add a low-dropout (LDO) negative regulator (like the LM337) after the LMC7660 to provide a clean, stable rail.

4. Typical Applications & Use Cases

🎬 Watch Tutorial: LMC7660

4.1 Real-World Example: Split-Supply Op-Amp Rail

In many data acquisition systems, an Op-Amp requires +5V and -5V to process AC signals centered around ground. If your system only has a +5V battery, the LMC7660 creates the -5V rail instantly using just two capacitors, saving the cost of a complex DC-DC converter.

5. Alternatives and Cross-Reference Guide

If the LMC7660 is out of stock or does not meet your current requirements, consider these equivalents:

• Direct Replacements:

• Renesas ICL7660S: Includes a "Boost" pin to increase switching frequency above the audio range.

• Microchip TC7660: A reliable, cost-effective alternative.

• High-Performance Upgrade:

• Maxim MAX1044: Popular in guitar pedals for its high-frequency mode.

• Texas Instruments LM2660: Choose this if you need up to 100mA of output current.

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between LMC7660 and the original ICL7660?

• A: The LMC7660 is a CMOS version that does not require an external diode between the output and the load for high-voltage operation, simplifying the design.

• Q: Can LMC7660 be used in automotive applications?

• A: While it works up to 10V, automotive rails often spike to 14V or higher. A pre-regulator or a higher-voltage rated part is required for automotive safety.

• Q: Is LMC7660 suitable for battery-operated devices?

• A: Yes, its 200 µA quiescent current is among the lowest in its class, preserving battery life in "always-on" sensors.

• Q: Why is my output voltage lower than my input voltage?

• A: This is due to the internal output resistance (~55Ω). If your load is too heavy, the voltage drop becomes significant.

7. Resources

• Datasheet: Available via the Texas Instruments official website.

• Simulation Tools: Compatible with TI Spice and Proteus for circuit verification.

• Evaluation Boards: Look for "Switched Capacitor Conversion" breakout boards for rapid prototyping.