Determining the packaging limits range of linear regulators

Typically, engineers select linear regulators based on a few specifications listed on the front of the datasheet that outline the operating envelope of the regulator such as input voltage range, output voltage, output current and drop out voltage. Then engineers typically search for the smallest package size so that the overall solution size is minimized.

Herein lies the problem. The thermal characteristics of the packaging can impose limits on the operating envelope listed in the datasheet.

For example, the datasheet for the TPS73401 linear regulator states the input voltage can range from 2.7V to 6.5V, the output voltage can be set to any value between 1.2V and 6.3V, and the device can provide up to 250mA of output current and is packaged in a 5-pin TSOT23 (or 6-pin SON) package. Based on these specifications, the operating envelope of the linear regulator could be graphed as shown in Figure 1 below.

Figure 1. Datasheet implied operating area of a linear regulator

The X-axis in Figure 1 above is the input voltage minus the output voltage, or more simply the voltage drop across the linear regulator. The lower end of the operating window's X-axis is limited by the drop out voltage of the linear regulator, while the upper end is limited by both the maximum input voltage and minimum output voltage the regulator can support.

The Y-axis is the maximum load current at which the regulator is specified. However, this ideal operating envelope does not take into account the power dissipation and, thus, the junction temperature rise of the device at any of these operating points. A thermal analysis must be performed to fully understand the actual operating envelope.

The first step of the thermal analysis is to determine the linear regulator's power dissipation. A generic linear regulator with its pertinent voltages and currents is shown in Figure 2 below.

Figure 2. Generic linear regulator

The power dissipated by the linear regulator is a fairly straight forward calculation. There is the power dissipated by the voltage drop across the regulator, multiplied by the load current passing through the regulator.

There is an additional power loss due to the quiescent current needed to operate the internal logic and control functions. This power loss is simply the quiescent current, or ground-pin current, multiplied by the input voltage. The equation for the linear regulator's total power dissipation is shown in Equation 1 below.

Many older, standard linear regulators (non-low drop out), were implemented using bipolar transistors. Bipolar transistors require current always to be flowing into the base in order to bias the transistor on. This base current increases the quiescent current, or ground-pin current of the linear regulator. The ground-pin current of these older devices can be in the tens of milliamps range, which noticeably adds to the power dissipation of the linear regulator.

Most new LDO linear regulators are implemented using MOSFETs instead of bipolar transistors. MOSFETs do not require constant gate current, so the quiescent current typically is very small. In this case, the last term of Equation 1 above that includes the ground-pin current becomes negligible compared to the first term and can be ignored.