4.1 Thermal Design

Linear regulators are simple to use. The most complicated design parameters to consider are thermal characteristics. Thermal design requires the following application-specific parameters:

• Maximum ambient temperature (T_A(MAX))
• Output current (I_OUT)
• Output voltage (V_OUT)
• Input voltage (V_IN)
• Ground current (I_GND)

First, calculate the power dissipation of the regulator from these numbers and the device parameters from this data sheet.

$P_D = \left(V_{IN} – V_{OUT}\right) \times I_{OUT} + V_{IN} \times I_{GND}$

In the equation above, the ground current is approximated by using numbers from the Electrical Characteristics table or Typical Characteristics section. Then the heat sink thermal resistance is determined with this formula:

${\theta }_{SA} = \left(\left(T_{J\left(MAX\right)} - T_A\right)/P_D\right) - \left({\theta }_{JC} + {\theta }_{CS}\right)$

The heat sink may be significantly reduced in applications where the minimum input voltage is known and is large compared with the dropout voltage. Use a series input resistor to drop excessive voltage and distribute the heat between this resistor and the regulator. The low dropout properties of Microchip Super βeta PNP® regulators allow significant reductions in regulator power dissipation and the associated heat sink without compromising performance. When this technique is employed, a capacitor of at least 1.0μF is needed directly between the input and regulator ground. Refer to Micrel Application Note 9 for further details and examples on thermal design and heat sink applications.