SNVS070D March   2000  – September 2016 LM2765

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 Typical Characteristics
  7. Parameter Measurement Information
    1. 7.1 Test Circuit
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Circuit Description
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Voltage Doubler
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Requirements
          1. 9.2.1.2.1 Positive Voltage Doubler
          2. 9.2.1.2.2 Capacitor Selection
          3. 9.2.1.2.3 Paralleling Devices
          4. 9.2.1.2.4 Cascading Devices
          5. 9.2.1.2.5 Regulating VOUT
        3. 9.2.1.3 Application Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers must validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LM2765 provides a simple and efficient means of creating an output voltage level equal to twice that of the input voltage. Without the need of an inductor, the application solution size can be reduced versus the magnetic DC-DC converter solution.

9.2 Typical Applications

9.2.1 Voltage Doubler

The main application of the LM2765 is to double the input voltage. The range of the input supply voltage is 1.8 V to 5.5 V.

LM2765 10128101.png Figure 10. Voltage Doubler

9.2.1.1 Design Requirements

Example requirements for LM2765 device applications:

DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 1.8 V to 5.5 V
Output current 0 mA to 20 mA
Boost switching frequency 20 kHz

9.2.1.2 Detailed Design Requirements

9.2.1.2.1 Positive Voltage Doubler

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals 2 V+. The output resistance ROUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and equivalent series resistance (ESR) of C1 and C2. Since the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 will be 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 when in the output resistance. A good approximation of ROUT is:

Equation 1. LM2765 10004915.gif

where

  • RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 9. RSW is typically 8 Ω for the LM2765.

The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C2:

Equation 2. LM2765 10004916.gif

High capacitance, low-ESR capacitors can reduce both the output resistance and the voltage ripple.

The Schottky diode D1 is only needed for start-up. The internal oscillator circuit uses the OUT pin and the GND pin. Voltage across OUT and GND must be larger than 1.8 V to insure the operation of the oscillator. During start-up, D1 is used to charge up the voltage at the OUT pin to start the oscillator; also, it protects the device from turning-on its own parasitic diode and potentially latching-up. Therefore, the Schottky diode D1 must have enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning-on. A Schottky diode such as 1N5817 can be used for most applications. If the input voltage ramp is less than 10 V/ms, a smaller Schottky diode such as MBR0520LT1 can be used to reduce the circuit size.

9.2.1.2.2 Capacitor Selection

As discussed in Positive Voltage Doubler, the output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is:

Equation 3. LM2765 10004917.gif

where

  • IQ(V+) is the quiescent power loss of the device; and
  • IL2Rout is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs.

The selection of capacitors is based on the specifications of the dropout voltage (which equals IOUT ROUT), the output voltage ripple, and the converter efficiency. Low ESR capacitors are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple.

9.2.1.2.3 Paralleling Devices

Any number of LM2765 devices can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C1, while only one output capacitor, COUT, is required as shown in Figure 11. The composite output resistance is:

Equation 4. ROUT = ROUT of each LM2765 / number of devices
LM2765 10128119.png Figure 11. Lowering Output Resistance by Paralleling Devices

9.2.1.2.4 Cascading Devices

Cascading the LM2765 devices is an easy way to produce a greater voltage (a two-stage cascade circuit is shown in Figure 12).

The effective output resistance is equal to the weighted sum of each individual device, shown in Equation 5:

Equation 5. ROUT = 1.5 ROUT_1 + ROUT_2

Note that the increasing of the number of cascading stages is practically limited since it significantly reduces the efficiency, increases the output resistance and output voltage ripple.

LM2765 10128120.png Figure 12. Increasing Output Voltage by Cascading Devices

9.2.1.2.5 Regulating VOUT

It is possible to regulate the output of the LM2765 by use of a low dropout regulator (such as LP2980-5.0). The whole converter is depicted in Figure 13.

A different output voltage is possible by use of LP2980-3.3, LP2980-3.0, or LP2980-ADJ.

Note that the following conditions must be satisfied simultaneously for worst-case design:

Equation 6. 2Vin_min > Vout_min + Vdrop_max (LP2980) + Iout_max × Rout_max (LM2765)
Equation 7. 2Vin_max < Vout_max + Vdrop_min (LP2980) + Iout_min × Rout_min (LM2765)
LM2765 10128121.png Figure 13. Generating a Regulated +5-V From +3-V Input Voltage

9.2.1.3 Application Curve

LM2765 10128109.png
Figure 14. Efficiency vs Load Current