Linear Regulator Part 11 - Thermal Management

Linear Regulator Thermal management:

Low dropout (LDO) regulators are very easy to use. Only one external filter capacitor is necessary for operation, so the electrical design effort is minimal. In many cases, thermal design is also quite simple, aided by the heat sink assembly.

LDOs operate with dropout voltages of 300mV–often less. The resulting Voltage × Current power loss can be quite small even with moderate output current. At higher currents and/or higher input-to-output voltage differentials, however, selecting the correct heat sink is an essential “chore”.

Thermal Parameters:

 Power dissipation in a linear regulator is:

PD  = Power dissipation

VIN  = Input voltage applied to the regulator

VOUT  = Regulator output voltage

IOUT= Regulator output current

IGND  = Regulator biasing currents

Calculating Maximum Allowable Thermal Resistance

Given the power dissipation, ambient operating temperature, and the maximum junction  temperature of a regulator, the maximum allowable thermal resistance is readily calculated.

We calculate the thermal resistance (θSA) required of the heat sink using the following formula:

Heat Sink Selection

With this information we may specify a heat sink.The worst case is still air (natural convection). The heat sink should be mounted so that at least 0.25 inches (about 6mm) of separation exists between the sides and top of the sink and other components or the system case. Thermal properties are maximized when the heat sink is mounted so that natural vertical motion of warm air is directed along the long axis of the sink fins.

If we are fortunate enough to have some forced airflow, reductions in heat sink cost and space are possible by characterizing air speed–even a slow air stream significantly assists cooling. As with natural convection, a small gap allowing the air stream to pass is necessary. Fins should be located to maximize airflow along them. Orientation with respect to vertical is not very important, as airflow cooling dominates the natural convection.

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