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Use thermal analysis to predict an IC's transient behavior and avoid overheating

• MAX16815/MAX16828 HBLED drivers.
  
  Both LED drivers are suitable in automotive applications such as side lighting, automotive exterior rear comb电感器厂家ination lights, backlighting, and indicators. The MAX16828 can dissipate considerable heat if the internal MOSFET sees high current combined with a large dropout voltage. (The MOSFET does this when the LED string's forward voltage is low.) The voltage across RSENSE is regulated to 200mV ±3.5%, which allows that resistor to set the LED current. The chip's DIM input provides a wide range of PWM dimming for the LEDs and, because it also withstands high voltages, it can connect directly to the IN pin.
  
  To obtain a direct indication of the die temperature, we measure the forward-bias voltage of an internal ESD diode connected between the DIM and IN pins. This diode is biased at ~100µA, causing its forward voltage to vary 2mV/K. (This can be confirmed by heating the part in a temperature-controlled oven.) Figure 7 shows the setup for these experiments. The 5V source and 56kΩ resistor provide 100µA for forward biasing the ESD diode. The driver is programmed to deliver 200mA of output current for the 工字电感器LEDs.
  
  
  Figure 7. The test setup shown lets you measure transient die temperatures using an on-chip ESD diode. *EP indicates an exposed pad.
  
  In this state the part carries a lot of current and our ESD diode measurement is in the path of that measurement. Consequently, we will get some error due to the parasitic resistance of the bond wire and internal metallization. From the internal layout and calculation of the length of bond wire, the worst-case parasitic resistance is estimated to be 50mΩ. With 200mA, this parasitic resistance will cause an error of around ±10mV (max) in our diode reading. Our accuracy error will be larger than ±5°C. Additionally, the ESD diode on the die is placed adjacent to the on-chip power MOSFET device and 电感器厂家thermal-shutdown circuitry. This configuration makes the diode the best indicator of that region's temperature.
  
  

  System definition 1

  This next section describes how you can use a test setup to capture transient-thermal diode voltages for共模电感 use in the system-definition equations presented above in Equations 7 and 21.
  
  To calculate kA and θJA (for substitution in Equation 11), we heat the chip using a hot-air gun. The chip should be powered off because we do not want to generate internal heat. Heating the part with a hot-air gun causes the temperature of the package and die to rise together. You can monitor the die's temperature change by measuring the diode voltage on a scope (Figure 8).
  
  
  Figure 8. This diode-voltage transient includes exponential curves that represent heating with an external heat gun (falling curve) and cooling by removal of the heat gun (rising curve).
  
  When the chip is heated, the diode voltage decreases with an exponential rate of change as the equation predicts. Near the center of the curve the hot-air gun is switched off, causing the package and die to begin cooling. The diode voltage rises, again following an exponential curve.
  
  We do not know exactly how much heat is imparted from the heat gun to the chip. Therefore, to eliminate that unknown we first adjust Equation 28 to fit only the rising (cooling) part of the curve (Figure 8). This curve-fitting exercise lets us estimate the best value for kA. With no heat power transferred to the package during cooling, the package is simply cooling down with P = 0. Equation 28, therefore, simplifies t

  VDB = VDA + (VDi - VDA)e-kAt

  (Eq. 34)

  
  

  We know the values for VDA (643mV from the initial measurement at room temperature) and VDi (the reading for t = 0 reference). To determine kA, we must just adjust the equation so that it includes a couple of readings on the rising curve. Thi工字电感器s exercise yields kA = -0.0175. A graph of the readings (diode voltages in mV, with respect to time in seconds) and Equation 34 with the above kA is shown in Figure 9.
  
  

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