SNVS730C October   2011  – June 2019 LMR10520

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Descriptions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Recommended Operating Ratings
    3. 6.3 Electrical Characteristics
    4. 6.4 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 Soft-Start
      2. 7.3.2 Output Overvoltage Protection
      3. 7.3.3 Undervoltage Lockout
      4. 7.3.4 Current Limit
      5. 7.3.5 Thermal Shutdown
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Detailed Design Procedure
        1. 8.2.1.1 Custom Design With WEBENCH® Tools
        2. 8.2.1.2 Inductor Selection
        3. 8.2.1.3 Input Capacitor
        4. 8.2.1.4 Output Capacitor
        5. 8.2.1.5 Catch Diode
        6. 8.2.1.6 Output Voltage
        7. 8.2.1.7 Calculating Efficiency and Junction Temperature
      2. 8.2.2 Application Curves
      3. 8.2.3 Other System Examples
        1. 8.2.3.1 LMR10520X Design Example 1
        2. 8.2.3.2 LMR10510X Design Example 2
        3. 8.2.3.3 LMR10510Y Design Example 3
        4. 8.2.3.4 LMR10510Y Design Example 4
  9. 9 Layout
    1. 9.1 Layout Guidelines
    2. 9.2 Layout Example
    3. 9.3 Thermal Definitions
    4. 9.4 WSON Package
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Third-Party Products Disclaimer
      2. 10.1.2 Development Support
        1. 10.1.2.1 Custom Design With WEBENCH® Tools
    2. 10.2 Receiving Notification of Documentation Updates
    3. 10.3 Community Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.1.7 Calculating Efficiency and Junction Temperature

The complete LMR10520 DC/DC converter efficiency can be calculated in the following manner.

Equation 14. LMR10520 30166320.gif

Or

Equation 15. LMR10520 30166322.gif

Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are not discussed.

Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction. Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):

Equation 16. LMR10520 30166310.gif

VSW is the voltage drop across the internal PFET when it is on, and is equal to:

Equation 17. VSW = IOUT x RDSON

VD is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the voltage drop across the inductor (VDCR) is accounted for, the equation becomes:

Equation 18. LMR10520 30166321.gif

The conduction losses in the free-wheeling Schottky diode are calculated as follows:

Equation 19. PDIODE = VD x IOUT x (1-D)

Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode that has a low forward voltage drop.

Another significant external power loss is the conduction loss in the output inductor. The equation can be simplified to:

Equation 20. PIND = IOUT2 x RDCR

The LMR10520 conduction loss is mainly associated with the internal PFET:

Equation 21. LMR10520 30166372.gif

If the inductor ripple current is fairly small, the conduction losses can be simplified to:

Equation 22. PCOND = IOUT2 x RDSON x D

Switching losses are also associated with the internal PFET. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node.

Switching Power Loss is calculated as follows:

Equation 23. PSWR = 1/2(VIN x IOUT x FSW x TRISE)
Equation 24. PSWF = 1/2(VIN x IOUT x FSW x TFALL)
Equation 25. PSW = PSWR + PSWF

Another loss is the power required for operation of the internal circuitry:

Equation 26. PQ = IQ x VIN

IQ is the quiescent operating current, and is typically around 3.3 mA for the 1.6-MHz frequency option.

Typical application power losses are:

Table 1. Power Loss Tabulation

VIN 5 V
VOUT 3.3 V POUT 5.78 W
IOUT 1.75 A
VD 0.45 V PDIODE 262 mW
FSW 1.6 MHz
IQ 3.3 mA PQ 16.5 mW
TRISE 4 ns PSWR 28 mW
TFALL 4 ns PSWF 28 mW
RDS(ON) 150 mΩ PCOND 306 mW
INDDCR 50 mΩ PIND 153 mW
D 0.667 PLOSS 794 mW
η 88% PINTERNAL 379 mW
Equation 27. ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS
Equation 28. ΣPCOND + PSWF + PSWR + PQ = PINTERNAL
Equation 29. PINTERNAL = 379mW