SLVSCK5C September   2014  – January 2020 TPS92638-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Typical Application Schematic
  5. 5 Revision History
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Switching Characteristics
    7. 7.7 Typical Characteristics
  8. 8 Parameter Measurement Information
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 LED Current Setting
      2. 9.3.2 PWM Control
      3. 9.3.3 Fault Diagnostics
        1. 9.3.3.1 Open-Load Detection
      4. 9.3.4 Thermal Foldback
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Applications
      1. 10.2.1 PWM Dimming by Bank
        1. 10.2.1.1 Design Requirements
        2. 10.2.1.2 Detailed Design Procedure
        3. 10.2.1.3 Application Performance Plots
      2. 10.2.2 Two Brightness Levels for TAIL and STOP Lights
        1. 10.2.2.1 Design Requirements
        2. 10.2.2.2 Detailed Design Procedure
      3. 10.2.3 PWM Dimming by Modulated Supply
        1. 10.2.3.1 Design Requirements
        2. 10.2.3.2 Design Procedure
      4. 10.2.4 Driving LEDs From a Single Device With Channels in Parallel
        1. 10.2.4.1 Design Requirements
        2. 10.2.4.2 Design Procedure
      5. 10.2.5 Driving LEDs From Multiple Devices With Channels in Parallel
        1. 10.2.5.1 Design Requirements
        2. 10.2.5.2 Design Procedure
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
    3. 12.3 Thermal Information
  13. 13Device and Documentation Support
    1. 13.1 Trademarks
    2. 13.2 Electrostatic Discharge Caution
    3. 13.3 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

9.3.4 Thermal Foldback

The TPS92638-Q1 device integrates thermal shutdown protection to prevent the IC from overheating. In addition, to prevent LEDs from flickering due to rapid thermal changes, the device includes a programmable thermal current foldback feature to reduce power dissipation at high junction temperatures.

The TPS92638-Q1 device reduces the LED current as the silicon junction temperature of the TPS92638-Q1 device increases (see Figure 35). Mounting the TPS92638-Q1 device on the same thermal substrate as the LEDs allows use of this feature to limit the dissipation of the LEDs. As its junction temperature increases, the TPS92638-Q1 device reduces the regulated current level, thereby reducing the dissipated power in the TPS92638-Q1 and in the LEDs. The current reduction from the 100% level is typically 2% per degree Celsius until the point where the current drops to 50% of the full value, which occurs at T(th) + 20ºC.

TPS92638-Q1 ther_fold_1_SLVSC76.gifFigure 35. Thermal Foldback

Above this temperature, the device maintains the current at the 50% current level until the junction temperature reaches the overtemperature shutdown threshold, T(shutdown). Changing the voltage on the TEMP pin adjusts the temperature at which the current reduction begins. With TEMP left open, the definition of thermal monitor activation temperature is the temperature at which the current reduction begins, T(th). The specification of T(th) in the Electrical Characteristics table is at the 90% current level. T(th) increases as the voltage at the TEMP pin, V(TEMP), decreases. Equation 3 provides an approximate calculation of T(th).

Equation 3. TPS92638-Q1 Eq16-Tth_SLVSCK5.gif
TPS92638-Q1 D004_SLVSC76.gifFigure 36. TEMP Pin Voltage vs Thermal Foldback Temperature

A resistor connected between TEMP and GND reduces V(TEMP) and increases T(th). A resistor connected between TEMP and a reference supply greater than 1 V increases V(TEMP) and reduces T(th).

TPS92638-Q1 D005_SLVSC76.gifFigure 37. Pullup and Pulldown Resistors vs T(th)

Figure 37 shows how the nominal value of the thermal monitor activation temperature varies with the voltage at TEMP and with a resistor R(TEMP), either connected to GND or pulled up to 3 V or to 5 V.

In extreme cases, if the junction temperature exceeds the overtemperature limit, T(shutdown), the device disables all regulators. Temperature monitoring continues, and the device re-activates the regulators, when the temperature drops below the specified hysteresis threshold.

Note that it is possible for the TPS92638-Q1 device to transition rapidly between thermal shutdown and normal operation. This can happen if the thermal mass attached to the exposed thermal pad is small and T(th) is too close to the shutdown temperature. The period of oscillation depends on T(th), the dissipated power, the thermal mass of any heatsink present, and the ambient temperature.