SLVAE30E February   2021  – March 2021 TPS1H000-Q1 , TPS1H100-Q1 , TPS1H200A-Q1 , TPS1HA08-Q1 , TPS25200-Q1 , TPS27S100 , TPS2H000-Q1 , TPS2H160-Q1 , TPS2HB16-Q1 , TPS2HB35-Q1 , TPS2HB50-Q1 , TPS4H000-Q1 , TPS4H160-Q1

 

  1.   Trademarks
  2. 1Introduction
  3. 2Driving Resistive Loads
    1. 2.1 Background
    2. 2.2 Application Example
    3. 2.3 Why Use a Smart High Side Switch?
      1. 2.3.1 Accurate Current Sensing
      2. 2.3.2 Adjustable Current Limiting
    4. 2.4 Selecting the Right Smart High Side Switch
      1. 2.4.1 Power Dissipation Calculation
      2. 2.4.2 PWM and Switching Loss
  4. 3Driving Capacitive Loads
    1. 3.1 Background
    2. 3.2 Application Examples
    3. 3.3 Why Use a Smart High Side Switch?
      1. 3.3.1 Capacitive Load Charging
      2. 3.3.2 Inrush Current Mitigation
        1. 3.3.2.1 Capacitor Charging Time
      3. 3.3.3 Thermal Dissipation
      4. 3.3.4 Junction Temperature During Capacitive Inrush
      5. 3.3.5 Over Temperature Shutdown
      6. 3.3.6 Selecting the Correct Smart High Side Switch
  5. 4Driving Inductive Loads
    1. 4.1 Background
    2. 4.2 Application Examples
    3. 4.3 Why Use a Smart High Side Switch?
    4. 4.4 Turn-On Phase
    5. 4.5 Turn-Off Phase
      1. 4.5.1 Demagnetization Time
      2. 4.5.2 Instantaneous Power Losses During Demagnetization
      3. 4.5.3 Total Energy Dissipated During Demagnetization
      4. 4.5.4 Measurement Accuracy
      5. 4.5.5 Application Example
      6. 4.5.6 Calculations
      7. 4.5.7 Measurements
    6. 4.6 Selecting the Correct Smart High Side Switch
  6. 5Driving LED Loads
    1. 5.1 Background
    2. 5.2 Application Examples
    3. 5.3 LED Direct Drive
    4. 5.4 LED Modules
    5. 5.5 Why Use a Smart High Side Switch?
    6. 5.6 Open Load Detection
    7. 5.7 Load Current Sensing
    8. 5.8 Constant Current Source
      1. 5.8.1 Selecting the Correct Smart High Side Switch
  7. 6Appendix
    1. 6.1 Transient Thermal Impedance Data
    2. 6.2 Demagnitization Energy Capability Data
  8. 7References
  9. 8Revision History

5.8 Constant Current Source

For the best LED performance it is best to use a constant current source, however often in systems with no LED drive module the only source available is a constant voltage source. Using a TI Smart High Side Switch a designer is able to create a constant current drive mode by forcing the device into current limit regulation mode. The current limiting feature is discussed more in depth in the capacitive driving section of this document in Section 3 as well as in the Adjustable Current Limit of Smart High Side Switches application note. The current limit threshold is set by an external resistor from the CL pin and should be chosen as equal to nominal LED current. Doing this ensures that the Smart High Side Switch will regulate the input voltage to provide a constant current source as long as the supply is capable of providing the power required. Figure 5-4 shows a switch in constant voltage mode on the left and a switch in constant current mode on the right.

GUID-9690407F-C9CA-48E7-A677-4246DF2FF9F2-low.gifFigure 5-4 Driving LED in Constant Output Voltage and Constant Current Mode

On the left, the LED is nominally operated at 100 mA, but because it is regulated by the resistor that current will increase with an increasing supply voltage. On the right, if the supply voltage increases the switch will increase its resistance to maintain a 100 mA output current.

To operate the switch in constant current mode it must be ensured that the switch has enough room to regulate the source. For example, if the LED string consists of five LED's each with a 0.7-V drop and require 100-mA current (typical for a LED automotive interior light) then the total voltage over the string will be 3.5 V. If the minimum supply is 8 V then the resistor R2 should be chosen so that the supply can always supply 100 mA. In this case if R2 is less than 45 ohm then even at the minimum input supply voltage the LED string will still maintain 100 mA. When the supply is nominally at its value of 13.5 V the switch and the resistor will dissipate more heat to regulate the current flow as calculated in Equation 60. In this case the total DC dissipation should not cause thermal issues so the LEDs will be driven by the Smart High Side Switch creating a constant current source.

Equation 60. GUID-20200811-CA0I-RPLR-KBJC-MXJGML3QF5NS-low.png
Equation 61. GUID-20200729-CA0I-18MV-ZS22-PXGMB1HDZJQF-low.png