Serial Port Complete - Latest Microcontroller projects

Inside RS-232

for power might be a solution if you’re using the port to control a synchronous

interface and using flow-control lines for the clock and data.


 

: 

Adding a high-efficiency regulator results in a steady output voltage with little

wasted power. Figure 4-5B shows a regulated 5V output using the MAX667

low-dropout, linear regulator. The input can be as high as 16.5V. For an output

current of 3 mA, typical operating characteristics specify an input voltage just

10 mV greater than the output, so the circuit will work with most ports. The

regulator’s quiescent current is under 100 μΑ with a load of several mA.

For a circuit that will work even if the RS-232 output drops below 5V, use the

MAX770, a switching regulator that has a 5V output with an input between 2V

and 16V. The MAX770 requires several external components, including an

inductor and output transistor.

Another option for port power is to use a lower regulated voltage, either by con-

necting voltage-divider resistors between Vout, Set, and Gnd as described in the

MAX667’s data sheet or by using a regulator with a lower fixed output such as

the 3V MAX689.

Figure 4-5: You can use spare flow-control outputs as a power source. Diodes enable

safely drawing current from multiple outputs. For a regulated voltage, use a

high-efficiency regulator like the Max667.