How to calculate the power loss of high-speed RS-485 transceiver?

 Calculating the power loss of a high-speed RS-485 transceiver is crucial for selecting power supplies, designing PCB traces, and ensuring thermal reliability. The total power loss is the sum of several components, not just the quiescent current.

Here is a detailed breakdown of how to calculate it.

The Three Main Components of Power Loss

The total power dissipation ($P_{total}$) of an RS-485 transceiver is the sum of three primary components:

1. Quiescent Power ($P_Q$): Power consumed by the IC's internal circuitry when it is powered on but not actively transmitting or receiving (i.e., in idle mode).

2. Driver Power (Transmitter Power, $P_{DRV}$): Power lost when the IC is driving a differential signal onto the bus.

3. Receiver Power ($P_{RCV}$): Power consumed by the receiver circuitry when it is enabled and monitoring the bus.

The formula is:
$P_{Total}=P_Q+P_{DRV}+P_{RCV}$

Let's analyze each component.

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1. Quiescent Power ($P_Q$)

This is the simplest to calculate. It's the power consumed by the device's internal bias circuits, logic, and pre-drivers when the drivers and receivers are inactive.

• How to find it: Look in the datasheet for $I_{CC}$ (Supply Current) and $I_{I\mathrm{/}O}$ (Input/Output Current, if separate). Sometimes it's given as a single "No-Load" supply current.

• Formula:
  $P_Q=V_{CC}\times I_{CC}$

• Example: If $V_{CC}=3.3V$ and the datasheet specifies $I_{CC}=2.5mA$ (max) with drivers and receivers disabled, then:
  $P_Q=3.3V\times 2.5mA=8.25mW$

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2. Driver Power ($P_{DRV}$)

This is the most complex part and has two sub-components: Internal Power and Load Power. The total driver power is the sum of these two.

$P_{DRV}=P_{INT}+P_{LOAD}$

A. Internal Power ($P_{INT}$)

This is the power dissipated inside the IC due to the driver's output stage. A significant portion of this comes from the "shoot-through" or "crossover" current that flows momentarily from $V_{CC}$ to GND through both the high-side and low-side FETs in the driver's output stage when it switches. This current spike is large, especially at high data rates.

• How to find it: This is the trickiest to calculate. Many modern datasheets provide a plot of "Power Dissipation vs. Data Rate" or "Supply Current vs. Data Rate". This curve inherently includes both $P_Q$ and $P_{INT}$.

• Formula (if no plot is available): An older, more general formula is:
  $P_{INT}=V_{CC}\times (I_{SH}\times t_{SH}\times F_{DATA})$

  • $I_{SH}$: Peak shoot-through current (rarely specified, often must be estimated).

  • $t_{SH}$: Duration of the shoot-through current (rise/fall time of the output signal).

  • $F_{DATA}$: Data rate (bits per second). Crucially, for a random data pattern like NRZ, use the fundamental frequency, which is $F_{DATA}\mathrm{/}2$ (e.g., 10 Mbps data rate has a 5 MHz fundamental).

Because this is so difficult, the best practice is to use the graph in the datasheet.

B. Load Power ($P_{LOAD}$)

This is the power delivered to and dissipated in the external load (the termination resistors and the cable).

• How to find it: The driver's output is a differential voltage ($V_{OD}$) across a load ($R_L$), which is typically the parallel combination of two termination resistors.

• Formula:
  $P_{LOAD}=\frac{V_{OD(RMS)}^2}{R_L}$

• Calculating $R_L$: In a standard RS-485 network, there are two 120Ω termination resistors at each end of the bus, but for a single transceiver's perspective, the equivalent load is typically 60Ω (because the two 120Ω resistors are in parallel: (120Ω || 120Ω) = 60Ω).

• Calculating $V_{OD(RMS)}$: You need the RMS value of the differential output voltage.

  • For a worst-case (maximum power) calculation, assume the driver is constantly switching with a 50% duty cycle square wave (e.g., a clock signal).

  • The peak differential voltage is $V_{OD}$ (from datasheet, e.g., ±1.5V min, ±5V max).

  • The RMS value of a square wave is equal to its peak amplitude.

  • Therefore, $V_{OD(RMS)}=V_{OD}$

Final $P_{LOAD}$ Formula:
$P_{LOAD}=\frac{V_{OD}^2}{R_L}$

• Example: A driver with $V_{OD}=2V$ (typical) driving a 60Ω load:
  $P_{LOAD}=\frac{(2V{)}^2}{60\mathrm{\Omega }}=\frac{4}{60}\approx 67mW$

Important: This power is dissipated in the external resistors, not inside the IC. However, the IC's driver must sourcing the current for this power, which is why it's part of the total system power calculation.

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3. Receiver Power ($P_{RCV}$))

This is the power consumed by the receiver's input comparators and circuitry when they are enabled.

• How to find it: Look in the datasheet for $I_{RXD}$ or Receiver Supply Current. It is often merged with $I_{CC}$ but sometimes specified separately for when the receiver is active.

• Formula:
  $P_{RCV}=V_{CC}\times I_{RXD}$

• Example: If $V_{CC}=3.3V$ and $I_{RXD}=1.5mA$, then:
  $P_{RCV}=3.3V\times 1.5mA=4.95mW$

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Summary and Practical Calculation Walkthrough

Given:

• Transceiver: TI THVD1550 (3.3V, 50 Mbps)

• $V_{CC}=3.3V$

• Data Rate: 20 Mbps (using a 50% duty cycle clock pattern for worst-case)

• Load: Standard 60Ω differential

• $V_{OD}=2V$ (typical)

Step 1: Find $P_Q$

• From datasheet, Icc (No load, DE = high, RE = high) = 2.4 mA (max)

• $P_Q=3.3V\times 2.4mA=7.92mW$

Step 2: Find $P_{DRV}$

1. $P_{LOAD}$: $P_{LOAD}=\frac{(2V{)}^2}{60\mathrm{\Omega }}=66.7mW$

2. $P_{INT}$: The THVD1550 datasheet has a graph "Supply Current vs. Data Rate". For 20 Mbps data rate (10 MHz fundamental), the graph shows Icc is approximately 5.5 mA for the driver portion. (You must subtract the quiescent portion from the graph's total).

   • $P_{INT}=V_{CC}\times (I_{CC(DRV\mathrm{\_}TOTAL)}-I_{CC(Q)})=3.3V\times (5.5mA-2.4mA)=3.3V\times 3.1mA=10.23mW$

3. Total $P_{DRV}$: $P_{DRV}=P_{INT}+P_{LOAD}=10.23mW+66.7mW=76.93mW$

Step 3: Find $P_{RCV}$

• Assume the receiver is also enabled. From datasheet, this is included in the main Icc spec, so we won't double-count it. $P_{RCV}\approx 0mW$ for this calculation.

Step 4: Calculate $P_{Total}$

• $P_{Total}=P_Q+P_{DRV}+P_{RCV}=7.92mW+76.93mW+0mW=84.85mW$

Conclusion: The transceiver will dissipate approximately 85 mW under these worst-case conditions. The majority of the power (~67mW) is burned in the external termination resistors, while the IC itself dissipates about ~18mW (from $P_Q+P_{INT}$).

Key Takeaways:

1. Always check the datasheet for a "Supply Current vs. Data Rate" graph. It is the most accurate way to account for internal driver losses ($P_{INT}$).

2. The load power ($P_{LOAD}$) is often the largest component and is highly dependent on your output voltage and termination resistance.

3. For thermal design, focus on the power dissipated in the IC: $P_{IC}=P_{Total}-P_{LOAD}=P_Q+P_{INT}+P_{RCV}$. This is the power that generates heat inside the package.

4. Worst-case scenario is a high-speed clock signal, not random data. Use the maximum data rate and a 50% duty cycle for your calculations to ensure a safe design.