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

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

  Abstract:This article presents a method for predicting thermal behavior in ICs. This information will be especially helpful for the PMICs (power-management ICs) used in automotive applications and other high-temperature environments. After characterizing thermal behavior, we formulate a mathematical model that simulates transient temperatures within the chip. We introduce physical laws governing thermal behavior 一体成型电感器and evaluate them for use in the thermal-body models defined for an IC. Based on that analysis, we then propose an equivalent passive RC network for modeling an IC's transient thermal behavior. To illustrate an application for the proposed analysis, we devise an RC network for an LED driver (the MAX16828). We conclude with insights on the use and usefulness of this approach, and suggest ways to speed the creation of the RC models.
  
  <-- =======================================================================电感器厂家 --><-- CONTENT: DB HTML --><-- ======================================================================= -->This article was also featured in Maxim's Engineering Journal, vol. 68 (PDF, 2.72MB).
  
  A similar article appeared in EDN Magazine in January 2010.
  
  Designers often need to know the thermal behavior of an IC, especially for the PMICs (power-management ICs) used in automotive applications. When a particular IC operates at a high temperature (such as +125°C), does it trigger the thermal-shutdow绕行电感n circuitry or exceed the product's safe operating temperature? Without a definite method of analysis, we cannot offer a reliable answer. The绕行电感器refore, when defining a new IC, we need a way to predict thermal shutdown or excessive die temperature based on complex internal functions.
  
  For operation in DC mode, you can often determine the junction temperature using data-sheet parameters such as θJA (thermal resistance) and θJC (thermal junction temperature).1 However, to predict how high the junction temperature will peak for modes other than DC (such as a power MOSFET driven by a PWM signal to control LEDs or a switching regulator), you need transient thermal data. Although useful, that data is not typically found in data sheets. You might also ask how long the chip can operate at a given power-dissipation level before encountering trouble? That question is also difficult to answer.
  
  This article derives equations that use power dissipation and the ambient temperature to predict the junction temperature of a chip as a function of time. The article begins by introducing the physical laws upon which the analysis is based. The discussion continues by defining an IC system as a complex, layered thermal body. The thermal-body model is then analyzed theoretically, and equations to govern transient thermal behavior are derived. Based on these equations, the article proposes an equivalent RC passive network that represents the IC's thermal characteristics. Finally, to demonstrate the usefulness and accuracy of this analysis, experimental results are shown for a high-voltage, linear HB LED (high-brightness LED) driver with PWM dimming, the MAX16828.
  
  

  Laws of thermal dynamics

  For any object we can derive the required relations for temperature vs. time by using two principal laws.
  
  Newton's law of cooling:

  (Eq. 1)

  
  

  Where:
  TB is body temperature.
  TA is ambient temperature.
  kA is a constant of proportionality (> 0).
  t is time.
  
  Law of conservation of nonlatent energy:

  (Eq. 2)

  
  

  Where:
  P is constant power generated or imparted to the body.
  m is the mass of the body.
  c is the specific heat capacity of the body.
  
  Combining these laws, we have:

  (Eq. 3)

  
  

  The data sheet for an IC normally lists thermal data for the package, such as θJA. That data lets us analyze the steady-state thermal equilibrium for a package to see if it agrees with Equation 3:

  at the steady state

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