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1. Component foundation
2. Circuit design 3. PCB design 4. Component soldering 6. Program design
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Preface
If the thermal performance is taken into consideration, the performance of the application can be further improved. The characteristic of low-dropout regulator (LDO) is to realize voltage regulation by converting excess power into heat. Therefore, this integrated circuit is very suitable for low power consumption or applications where the difference between VIN and VOUT is small. With this in mind, choosing the appropriate LDO in the appropriate package is essential to maximize application performance. This is what makes designers feel tricky, because the smallest available package does not always meet the requirements of the desired application.
1. Thermal performance information
One of the most important characteristics to consider when choosing an LDO is its thermal resistance (RθJA). RθJA shows the heat dissipation efficiency of the LDO in a specific package. The larger the value of RθJA, the lower the heat dissipation efficiency of the package, and the smaller the value, the higher the heat dissipation efficiency of the device.
The smaller the package size, the larger the RθJA value is usually. For example, TPS732 has different thermal resistance values depending on the package: the thermal resistance of the small-outline transistor (SOT)-23 (2.9mmx1.6mm) package is 205.9°C/W, while the SOT-223 (6.5mmx3.5mm) package The thermal resistance is 53.1°C/W. This means that for every 1W of power consumed by the TPS732, the temperature will rise by 205.9°C or 53.1°C. These values can be found in the "thermal performance information" section of the device data sheet, as shown in Table 1.
Table 1: Thermal resistance corresponding to different packages
2. Have you chosen a suitable package?
The recommended LDO operating junction temperature is between -40°C and 125°C; again, these values can be viewed in the device data sheet, as shown in Table 2.
Recommended operating conditions in Table 2
These recommended temperatures indicate that the device will operate as described in the "Electrical Characteristics" table in the data sheet. You can use Equation 1 to determine which package will work at the proper temperature.
Official 1TJ = TA + (RθJA × PD)
PD = ［（VIN − VOUT） × IOUT］ + （VIN × Iground）
Among them, TJ is the junction temperature, TA is the ambient temperature, RθJA is the thermal resistance (taken from the data sheet), PD is the power consumption, and Iground is the ground current (taken from the data sheet).
A simple example is given below. The TPS732 is used to reduce the voltage from 5.5V to 3V, the output current is 250mA, and it is packaged in SOT-23 and SOT-223.
PD = （（5.5V-3V） x250mA］ + （5.5Vx0.95mA） = 0.63W
SOT-23：TJ=25°C+（205.9°C/Wx0.63W）=154.72°C
SOT-223：TJ=25°C+（53.1°C/Wx0.63W）=58.45°C
Three, thermal shutdown
The device with a junction temperature of 154.72°C not only exceeds the recommended temperature specification, it is also very close to the thermal shutdown temperature. The shutdown temperature is usually 160°C; this means that when the junction temperature of the device is higher than 160°C, the thermal protection circuit inside the device will be activated. This thermal protection circuit disables the output circuit, reduces the temperature of the device, and prevents damage to the device due to overheating.
When the junction temperature of the device drops to about 140°C, the thermal protection circuit is disabled and the output circuit is re-enabled. If the ambient temperature and/or power consumption are not reduced, the device may be repeatedly turned on and off under the action of the thermal protection circuit. If the ambient temperature and/or power consumption are not reduced, the design must be changed to obtain proper performance.
A clearer design solution is to use a larger size package, because the device needs to work at the recommended temperature.
4. Here are some tips and tricks that can help minimize calories
1. Increase the size of the ground plane, VIN and VOUT contact layers
When power is dissipated, heat is dissipated from the LDO through the heat dissipation pad; therefore, increasing the size of the input layer, output layer, and ground layer in the printed circuit board (PCB) will reduce the thermal resistance. As shown in Figure 1, the ground plane is usually as large as possible, covering most of the area on the PCB that is not occupied by other circuit traces. This size design principle is because many components generate return current, and it is necessary to ensure that these components have the same reference voltage. Finally, the contact layer helps avoid pressure drops that can damage the system. The large contact layer also helps to improve heat dissipation and minimize trace resistance. Increasing the size of copper traces and expanding the heat dissipation interface can significantly improve the conduction cooling efficiency.
When designing a multi-layer PCB, it is usually a good practice to use a separate circuit board layer (including a ground layer that covers the entire circuit board). This helps to ground any components without the need for additional traces. The component pins are directly connected to the circuit board layer containing the ground plane through the holes on the circuit board.
Figure 1: PCB layout of the SOT-23 package
2. Install the radiator
The radiator will reduce RθJA, but will increase the system size and increase the system cost. When choosing a heat sink, the size of the base plate should be similar to the size of the device it is connected to. This helps to dissipate heat evenly on the surface of the heat sink. If the size of the heat sink is different from the size of the surface of the connected device, the thermal resistance will increase.
Considering the physical size of the package, packages such as SC-70 (2mm×1.25mm) and SOT-23 (2.9mm×1.6mm) are usually not used with heat sinks. On the other hand, transistor outline (TO)-220 (10.16mm×8.7mm) and TO-263 (10.16mm×9.85mm) packages can be used with heat sinks.
Figure 2 shows the differences between the four packages.
Figure 2: Packaging differences
A resistor can be connected in series on the input voltage side to share some power consumption; Figure 3 shows a related example. The goal of this technique is to use resistors to reduce the input voltage to the lowest possible level.
Figure 3: Resistors connected in series
Since the LDO needs to be in saturation for proper regulation, the lowest input voltage can be obtained by adding the required output voltage and the voltage drop. Formula 2 expresses the calculation method of these two attributes of LDO:
Official 2
VIN−［（IOUT+Iground）xRmax］=VOUT+Vdropout
Rmax=（VIN−VOUT−Vdropout）/（IOUT+Iground）
Using the conditions in the TPS732 example (output 250mA current, adjust 5.5V to 3V), you can use Equation 3 to calculate the maximum value of the resistor and the maximum power consumed by the resistor:
PD（Rmax）=（IOUT+Iground）^2xRmax
Choose a suitable resistor to ensure that it will not exceed its "rated power consumption". This rating indicates how many watts of power can be converted into heat by a resistor without damaging itself.
Therefore, if VIN=5.5V, VOUT=3V, Vdropout=0.15V (taken from the data sheet), IOUT=250mA and Iground=0.95mA (taken from the data sheet), then:
Rmax=（5.5V−3V−0.15V）/（250mA+0.95mA）=9.36Ω
PD（Rmax）=（250mA+0.95mA）2x9.36Ω=0.59W
3. Layout
If other heating devices on the PCB are very close to the LDO, these devices may affect the temperature of the LDO. To avoid temperature rise, please make sure to place the LDO as far away as possible from heating devices.
For specific applications, there are many ways to achieve a high-efficiency, appropriately-sized, and low-cost thermal solution. The key lies in the various considerations that need to be considered in the early design phase to ensure that all options are available. For heat dissipation, selecting suitable components is not a simple task, but selecting suitable components and technologies will help the design process to be successfully completed.