Smart transmitter power supply design
With the maturity of microcontroller MCU(or microcontroller) technology, the original analog transmitter is gradually replaced by the intelligent transmitter with microcontroller as the data processing and control core. The intelligent transmitter extends the function of the analog transmitter, not only improves the measurement accuracy and working reliability, but also easily realizes the functions of linearization, temperature compensation, automatic zero and range adjustment and digital communication. In the development of low-power intelligent two-wire transmitter, the design of micro-power supply inside the instrument is very important. First of all, an intelligent transmitter with A microprocessor to meet the power supply of the microcontroller, A/D, D/A and communication circuits requires greater power than the original analog transmitter, and requires a higher power supply efficiency from the internal power supply. In addition, for capacitive sensors and thermocouples, but also consider the grounding or sensor may touch the case (grounding), the design of the transmitter circuit must be isolated from the input and output, so as to ensure the normal operation of the subsequent control system and the ability to resist common mode interference.
Integral design
LT1934 series chip is a PWM controlled CMOS step-down DC/DC controller composed of reference voltage source, oscillation circuit and error amplifier. Main indicators: Input voltage 3.3? 34V, output voltage 1.5? 6.0V, 0.1V can be set as an advanced unit; Low static current 12μA maximum, maximum output current 300mA. Figure 4 shows the basic circuit of LT1934.
In the design of the circuit, the device selection and S-8251 is basically the same, in the PCB layout design should pay attention to the capacitor C2 and chip LT1934 distance can not be too far, as far as possible to use thick lines, it is best to use the ground plane, otherwise it will cause self-excited oscillation. Inductance L1 plays a decisive role in the conversion efficiency of DC/DC. If L1 is too small, the conversion efficiency of the circuit will be reduced, the starting current will be increased, and even it can not be started. If L1 is too large, the output capacity will be reduced, and the DC/DC circuit will likely oscillate.