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. 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

Capacitive Load Charging

When a voltage is applied to an uncharged capacitor the capacitor will sink current until it’s voltage is equal to the supply voltage. The magnitude of the inrush current is directly proportional to the rate at which the voltage across the capacitor changes with time. The resulting inrush current can be calculated by Equation 16 and be seen in Figure 3-3.

Equation 16. GUID-20200811-CA0I-FXZ2-BMH1-XV9ZDQ6LPVH2-low.png
GUID-B8722808-9B7A-4738-B88A-670C155354BB-low.gifFigure 3-3 Capacitive Load Charging Diagram

When the switch is closed and the voltage is first applied to the capacitor, dV/dT is determined by the rate at which the switch in Figure 3-3 ramps up the output voltage. Depending on this rate the inrush can be very high and would only be limited by the parasitic resistance and inductance present in the routing between the switch output and the capacitor. Without anything limiting IINRUSH, these high currents can lead to a voltage supply droop at the input voltage supply which could collapse due to the high level of power required. This can be seen in Figure 3-4, where charging a capacitor with a high dV/dT leads to inrush currents up to a peak of 40 A and causes a noticeable drop on the yellow input supply voltage.

GUID-E66FE3AA-AB45-48D0-A94D-1259D569CF04-low.gifFigure 3-4 Inrush Supply Droop Example

This input supply drop means that any other systems connected to the same voltage supply must be able to operate without any variation even with an unstable supply. Additionally, the 40 A of current itself causes problems as the system must now be analyzed to make sure that there won’t be any harm caused by the excessive current flow through the cables and connectors. This means more complex and expensive systems in the form of:

  • Larger traces and connectors to accommodate the large current
  • More powerful supply to prevent the supply droop
  • Increased bulk capacitors at the input of downstream systems to enable continued device operation

To prevent these system considerations it is necessary to have a solution in place to let the system drive the capacitor and charge it at a controlled rate without allowing it to sink high levels of inrush current. In the next section we will show how this can be done with an adjustable current-limiting Smart High Side Switch.