Understanding the No-active Current Specifications of RS232 Devices
System designers often fail to find the active power consumption mentioned in the typical product specification. Many RS-232 interface device product specifications only specify the supply current under no-load and shutdown settings. However, RS-232 devices are only suitable for communication when connected to remote RS-232 devices. The capacitance of the data line and the resistance of the remote receiver can increase the load on the local RS-232 device, thereby increasing power consumption. Although most new RS-232 devices will have at least one active current or power specification, many RS-232 devices do not have such specifications.
Active power consumption is the sum of the power consumed by the load and the power lost in the device. The first step is to calculate the load power for the receiver resistor and cable capacitance. Expression 1 is the remote receiver resistor power formula expressed as the number of channels multiplied by the square of the driver voltage divided by the receiver resistance.
N × V2/R (1)
Expression 2 is the power consumption of the data line, expressed as the square of the peak-to-peak voltage of the driver multiplied by frequency and capacitance. The number of local RS-232 drives has no effect on this because only one drive is connected at a time.
F × C ×(2 × V)2 (2)
The maximum frequency of a continuously connected RS-232 stream is half the baud rate. The frequency of any data stream is 30% of the baud rate.
The total load power is the sum of resistive power (Expression 1) and capacitive power (Expression 2).
P = N × V2/R + F × C × (2 × V)2
For devices without a capacitor charge pump or an inductive charge pump, the load requires the same supply current as the load current. The power supply is directly proportional to the output current, similar to linear regulators.
Therefore, you need to convert the load power to load current. The load current is equal to the load power divided by the driver voltage.
I = (N × V2/R + F × C × (2 × V) 2) / V
This can be simplified to Equation 3 (representing the active load current).
I = N × V/R + 4 × F × C × V (3)
Use the active load current (which is the same as the supply current) to calculate the power generated by the load. With no-load power, the total system device power can be derived.
Here are two examples of how to calculate active power.
Example 1
The GD75232 transceiver has three drivers and five receivers. VDD = 9V, VSS = -9V and VCC = 5V. The maximum supply current is 15mA, 15mA and 30mA, respectively. The maximum no-load power is 9V × 15mA + -9V × -15mA + 5V × 30mA = 420mW. This is static (idle) power.
The data stream is an alternating bit pattern of 120kBaud with a cable capacitance of 2500pF and a remote receiver with a resistance of 3kΩ (kΩ). The RS-232 driver voltage is 7.5V.
Substitute the example parameters into Equation 3 to calculate the load current.
I = 3 channels × 7.5V/3000kΩ + 4 × 120kbps/2 × 2500pF × 7.5V = 12mA
Since this current is from VDD or VSS, the power required to support this load is 9V × 12mA = 108mW.
The total power is equal to the no-load (static) power (420mW) plus active power (108mW) - the calculated total power is 528mW.
Example 2
The TRS3232E multi-channel RS-232 line driver/receiver has two drivers and two receivers. VCC = 5V. The maximum value of no-load ICC is 1mA.
The device has a capacitor charge pump that can both boost the voltage and invert the voltage by consuming current. If the product specification has two rated load values, then you can calculate the efficiency of the charge pump, but there is no such specification in the product specification. In this case, you must measure active current based on experience. Power consumption varies with the data flow frequency and cable capacitance and receiver input resistance. See the following example:
Figure 1: Power consumption and frequency of the TRS3232E (resistance per channel is 3kΩ)
Previously TTL-based RS-232 devices had no active current specifications - this didn't matter much because active power was easily calculated in a three-supply topology. Later, the single-supply charge pump RS-232 device continued the idea of ​​“no active power specifications on the product specificationâ€. However, this idea is wrong, because you cannot calculate the power based on the product specification alone. No matter where the product specification does not provide enough information, you can visit the TI E2ETM Community RS-232 Forum for help.
50Hz Diesel Generator,50Hz Yuchai Power Generator Set,Kosta Small Power Genset,Yuchai Power Generator Set
Shanghai Kosta Electric Co., Ltd. , https://www.ksdgenerator.com