How to calculate the power loss of RS-485 transceiver THVD1550

The RS-485 interface is widely used in industrial applications due to its robustness and long-distance communication capabilities. Since the RS-485 standard was introduced to the market in 1998, electronic systems have continued to increase in size and complexity. Many end devices (such as motor drives, PLCs, and industrial PCs) now require high-speed (>>10Mbps) communications. TI's THVD1550 is the new TI RS-485 transceiver that supports data rates up to 50Mbps. In this article I will show you how to evaluate the power dissipation of the THVD1550 at this high speed. This way, you can understand the power consumption and evaluate the thermal performance of the system.

To calculate power loss, you can divide the power into its components. After each section has been successfully evaluated, the total power is obtained by adding all sections together. When the device is powered up without an external load, the integrated circuit (IC) itself dissipates power. If you add a load to its output pin, the device provides the power to drive the load. Since RS-485 has differential signaling, the load is usually placed between the A and B pins.

Looking further into loads, you can divide loads into two types: resistive and capacitive. A resistive load itself consumes some power, while a capacitive load only transfers energy between its plates without any energy loss. Because each type has different characteristics, power is calculated differently.

Figure 1 is a diagram of the three types of power dissipation.

Figure 1: Current flow in a typical RS-485 transceiver

PDic (blue) is the power consumed by the device itself. PDdc (red) is the power from the resistive load and PDac (green) is the power from the capacitive load. Total power is the sum of all three components and is calculated using Equation 1.

To calculate PDic you can use the quiescent supply current value Icc from the datasheet. Typical value in THVD1550 is 700µA. (In this article, I will use typical values; for example, Vcc is 5V.) Based on the voltage and current values, the device power can be calculated using Equation 2.

If you place a resistive load on the bus, the driver will develop a voltage (Vod) across the resistive load. The RS-485 standard requires a minimum Vod of 1.5V across a 54Ω resistor. In typical cases, the THVD1550's driver produces 2.7V at 54Ω, which means the device current is 50mA.

The important thing here is that you need to differentiate between the power dissipation of the load and the power of the device. To understand the thermal performance of a device, just focus on the power of the device. Therefore, Equation 3 subtracts the load power (I2*R) from the total power supply (Vcc*I):

Next, let's look at switching power, the power required to drive a capacitive load. To simplify calculations, split the differential capacitor into two single-ended capacitors connected to ground (Figure 2).

Figure 2: Equivalent capacitive loading of a typical RS-485 transceiver

The signal amplitude on each capacitor is Vod and the frequency is half the data rate (Figure 3). Even though the voltages on the A and B pins are opposite, they consume the same power.

Figure 3: Signals at the RS-485 transceiver output pins

Here, you also need to consider the parasitic capacitance of the device. In some datasheets it is called Cod. We take the typical value of SN65HVD82 as 8pF. Assume the load capacitance is 50pF. Now, the capacitance of each capacitor C in Figure 2 is the sum of 50pF + 8pF. Switching power is related to frequency and amplitude. During each charge, the energy consumed from the power source is equal to Equation 4:

where Vc is the voltage across the capacitor.

At each moment, the instantaneous current flowing through the capacitor is C (dVc/dt). You can integrate the instantaneous power over a period of time to find the energy required to charge the capacitor. Therefore, the power required to drive a capacitive load can be calculated using Equation 5:

After finding the power dissipated by all three components, Equation 6 calculates the total power as:

In Section 7.5 of the THVD1550 datasheet, the table shows test data for the device's power dissipation with a supply voltage of 5.5V, a temperature of 125°C, and a data rate of 50Mbps, which is a worst-case scenario. These values ​​include some non-idealities such as shoot-through current and therefore can be used to evaluate the maximum power dissipation of the device in a system design.