SLUAB16 March   2025 LMR51403 , TPS629203

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Input and Output Ripple of Buck Converter in DCM
  6. 3TPS629203 Design Consideration
  7. 4Experimental Results
    1. 4.1 VIN and VOUT Ripple Results
    2.     Efficiency
  8. 5Summary
  9. 6References

1 Introduction

Sensor Transmitters with HART Protocol

Sensor transmitters are widely utilized in factory automation and process control. Sensor transmitters need sensing front-ends, microcontrollers and processors. This includes a wide variety of data transmission interfaces, protocols and communication design, such as 4-20mA HART.

The HART Communication Protocol is a hybrid analog and digital industrial automation open protocol. The most notable advantage is that HART can communicate over a 4–20 mA analog current loop with a shared pair of wires. HART can support two operational modes: point-to-point and multi-drop. Point-to-point is where the 4–20 mA current and the digital signal are valid signaling protocols between the controller and measuring instrument or final control element. Multi-drop is where the analog loop current is fixed at 4mA and is possible to have more than one instrument on a signal loop. Generally, the analog signal transmits some measure of level, flow, temperature, and pressure, and so forth. The HART protocol is a backward-compatible enhancement to 4-20mA, to send command or return standardized responses which can communicate device status and diagnostics. Data can also include the device measurement digital values, and other information about the remote transmitter. Therefore, HART modifies the 4-20 mA system from only sending primary variable as a current value to adding digital communication with more functionality and flexibility. This technology is backwards compatible and can be used with existed infrastructure so is easy to adopt and cost-effective.

Safety Requirement of Explosive Atmospheres

In electrical and safety engineering, explosive atmospheres are places where fire or explosion hazards exist. Under these conditions, flammable substances in the form of gases, vapors or dust, in the mixture with air, cause combustion spread to the entire unburned mixture after ignition. Some international standards like IEC 60079 have specified the construction and testing of intrinsically safe circuits for use in an explosive atmosphere. The key feature for a power supply in such an intrinsically safe circuits is the input and output capacitance. This is because permitted capacitance needs to be strictly restricted based on the node voltage according to the standards, to limit the spark and energy generated when capacitors get failed. Table 1-1 shows the permitted capacitance corresponds to voltage based on the equipment group.

Table 1-1 Permitted Capacitance Corresponds To Voltage
Voltage, V Permitted Capacitance, μF (for Group IIC Apparatus as an Example)
1 Factor of Safety 1.5 Factor of Safety
5.0 100
6.0

600

40
7.0

175

15.7
8.0

69

8.4
9.0

40

4.9

10.0

20

3

20.0

0.90

0.22

30.0

0.22

0.066

Simplified Block Diagram

Figure 1-1 shows a simplified block diagram of a sensor transmitter with HART protocol in explosive environments. The input power comes from a 4-20mA current loop. To meet the stringent capacitance requirements in Table 1-1, there is a pre-regulator that outputs 10V. A buck converter is then used to output 3.3V to power the MCU, HART, and DAC. For the 4-20mA current loop, higher power efficiency is the goal, so a low IQ buck converter operating in pulse frequency modulation (PFM) mode is an excellent choice. However, the input and output voltage ripples of the buck converter can be large due to the discontinuous conduction mode (DCM) at light loads. The HART communication protocol uses a modulation frequency to represent the digital signal, making the signal highly sensitive to large voltage ripples. These ripples cannot be reduced simply by increasing the input and output capacitance of the buck converter due to the allowed capacitance limit. Therefore, designing a power supply that considers low ripple and high efficiency is necessary.

Simplified Block Diagram Figure 1-1 Simplified Block Diagram

This application note based on the low - IQ buck converter TPS629203, which features PFM and COT control. This design meets the requirements of a typical HART system as shown in the Table 1-2 and serves as a guide for engineers.

Table 1-2 Typical Power Rail of Sensor Transmitter System
Parameter
Input voltage (VIN) 10V
Output voltage (VOUT) 3.3V
Rated load (IO) 4mA-20mA
Input capacitance (CIN) 1uF
Output capacitance (COUT) 10uF
Input ripple target (ΔVIN) 0.2%⋅ VIN
Output ripple target (ΔVOUT) 0.2%⋅ VOUT