Technical Information Site of Power Supply Design

• TECH INFO
• Factors that Determine the Actual Output Current

Product Key Points

New lineup of LDO linear regulators, covering all 91 products!

# Factors that Determine the Actual Output Current

Keyword Output currentThermal resistanceTjTj maxAllowable power dissipationHeat sink

The G, H, and I series of new LDO linear regulators encompass a total of 91 products. In terms of output voltages, the adjustable type covers 0.8 V to 13 V, and the fixed type 1 V to 12 V, for a total of 14 different voltages, with a high output accuracy of 1%. In output current, these products provide four levels, from 0.3 A to 1.5 A, in a matrix of different current levels.

The series comprises the following product offerings and specifications:

G Series H Series I Series
Input supply voltage range 4.5V～14.0V 4.5V～8.0V 2.4V～5.5V
Maximum output current 0.3A, 0.5A, 1.0A 0.3A, 0.5A, 1.0A, 1.5A 0.5A, 1.0A
Output voltage range (adjustable type) 1.5V～13.0V 1.5V～7.0V 0.8V～4.5V
Output voltage (fixed type) 1.5V, 1.8V, 2.5V, 3.0V,
3.3V, 5.0V, 6.0V, 7.0V, 8.0V,
9.0V, 10V, 12V
1.5V, 1.8V, 2.5V, 3.0V,
3.3V, 5.0V, 6.0V, 7.0V
1.0V, 1.2V, 1.5V, 1.8V,
2.5V, 3.0V, 3.3V

The table indicates that the product that yields the greatest output current is the 1.5 A device in the H-series. A 7 V device, for example, yields a 10.5 W output power.

In actuality, however, you may have noticed that things do not turn out that rosy, depending upon prevailing conditions. Basically, ICs that handle large power, such as in power ICs, the amount of power that can be obtained is limited by heat. To be precise, since the Tj, representing a chip temperature, must not exceed Tj max, which is the maximum rating, the device must be used at Tj max or lower. In the case of a linear regulator, Tj is calculated as follows:

Tj ＝ self-heating ＋Ta
＝（thermal resistance θja × power dissipation W）＋Ta
＝｛θja×（input/output voltage difference × output current）W｝＋ Ta

For the sake of simplicity, the power consumption in this equation does not include self-power dissipation. In strict terms, a power factor equal to input voltage x self-consumption current must be added. In situations where self-consumption current is small and the output current is large, however, the output current is predominant.

In the actual use of the device, the output voltage must be equal to a specified voltage. Therefore, the issue to be addressed is the question of whether a desired output current, that is, the required load current, can be attained. Before calculating this factor, we need to consider the fact that there are other parameters that can be adjusted or that are difficult to adjust. Obviously the quantity Tj max cannot be changed.

The input/output voltage difference is a large-impact factor. That said, the voltage that can be received (the input voltage) is more or less fixed. Normally it is not common practice to provide an input voltage source that optimizes conditions for the use of a given linear regulator. Thus, the input/output voltage difference is also a parameter that cannot be changed under normal circumstances.

Because Ta is subject to the temperature specifications for the equipment being designed, for fixed temperature specifications for the equipment, such as 0℃ to 50℃, the maximum limit on Ta is either 50℃, or by taking into consideration the fact that the device is housed in an encasing unit and that the temperature inside the case rises due to heat dissipation, the maximum limit is calculated in terms of an increased Ta temperature level. In situations where cooling can be provided by means of a fan, the Ta level under that condition can be employed. Be that as it may, it would be safe to assume that the Ta is a component amenable to not much adjustment.

If the desired output current cannot be attained due to thermal constraints as a result, the best approach would be to reduce thermal resistance. There may be several methods for reducing thermal resistance, such as using an IC packaged in low thermal resistance, and a mounting board with superior thermal characteristics such as a multi-layer structure, or attaching a heat sink. However, given the recent demand for space savings, including form factors, there may be cases where it is difficult to attach a heat sink. In such cases, heat dissipation in terms of packaging and board needs to be considered.

In the new series of linear regulators, in all cases we adopted an HTSOP-J8 package (4.9×6.0×1.0 mm) with an exposed thermal pad on the backside. Thermal resistance can be improved substantially by soldering the thermal pad for the package to a PCB designed for extensive heat dissipation. The figure below shows an allowable power dissipation graph for the HTSOP-J8 package.

For a Tj max of 150℃, let us calculate the output power dissipation, assuming an input voltage of 12V for the previously mentioned 7V /1.5A output:

Output power dissipation = （12V－7V）×1.5A　＝7.5W

Given that the allowable loss is 3.76W（under condition ⑤, θja＝33.3W/℃） according to the graph, it is immediately clear that the linear regulator is unsuitable for this output current. In this case, the self-heating is 7.5W×33.3℃ = approximately 250℃, which is out of the question before we can even discuss Ta.

Assuming a Ta value of 50℃, the allowable power under condition ⑤ with the lowest thermal resistance is 3W. Conversely, it is clear that 0.6A represents the limit under this condition. If thermal resistance can be reduced further, calculating back we get （Tj max 150℃－Ta 50℃）÷7.5W＝13.3℃/℃. If this can be achieved, we are home free at 1.5A.

Roughly speaking, the actual factor that determines the output current is Tj, that is, heating and the ambient temperature. Under these conditions, whether you use a 1.5A product or a 1A one, the result is the same, and nothing changes.