SNVSA56B May   2015  – February 2017 LM2776

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
      2.      Output Impedance vs Input Voltage IOUT = 100 mA
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Input Current Limit
      2. 7.3.2 PFM Operation
      3. 7.3.3 Output Discharge
      4. 7.3.4 Thermal Shutdown
      5. 7.3.5 Undervoltage Lockout
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Enable Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application - Voltage Inverter
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Requirements
        1. 8.2.2.1 Efficiency
        2. 8.2.2.2 Power Dissipation
        3. 8.2.2.3 Capacitor Selection
        4. 8.2.2.4 Output Capacitor and Output Voltage Ripple
        5. 8.2.2.5 Input Capacitor
        6. 8.2.2.6 Flying Capacitor
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.2.2 Detailed Design Requirements

The main application of LM2776 is to generate a negative supply voltage. The voltage inverter circuit uses only three external capacitors with an range of the input supply voltage from 2.7 V to 5.5 V.

The LM2776 contains four large CMOS switches which are switched in a sequence to invert the input supply voltage. Energy transfer and storage are provided by external capacitors. Figure 19 shows the voltage conversion scheme. When S1 and S3 are closed, C1 charges to the supply voltage VIN. During this time interval, switches S2 and S4 are open. In the second time interval, S1 and S3 are open; at the same time, S2 and S4 are closed, C1 is charging C2. After a number of cycles, the voltage across C2 is pumped to VIN. Because the anode of C2 is connected to ground, the output at the cathode of C2 equals −(VIN) when there is no load current. The output voltage drop when a load is added is determined by the parasitic resistance (Rds(on) of the MOSFET switches and the equivalent series resistance (ESR) of the capacitors) and the charge transfer loss between capacitors.

LM2776 switches.gifFigure 19. Voltage Inverting Principle

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals − (VIN). The output resistance ROUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C1 and C2. Because the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The output capacitor C2 is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the output resistance. A good approximation of ROUT is:

Equation 1. ROUT = (2 × RSW) + [1 / (ƒSW × C)] + (4 × ESRC1) + ESRCOUT

where

  • RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 19.

High-capacitance, low-ESR ceramic capacitors reduce the output resistance.