Design Ideas: July 6, 1995
In Fig 1, switch-mode-controller IC1 operates with an external inductor, two diodes, and two capacitors to produce ±6.5V. FETs Q1 and Q2 ensure start-up for the circuit by disconnecting the load until these switch-mode supply voltages are present. Q1 must be a logic-level device.
Unlike ICs designed to generate supply voltages, an interface IC generally doesn't specify how much current you can draw from its internally generated supply rails. The amount available depends almost entirely on loads connected to the driver outputs. For example, IC1 guarantees that one transmitter can drive a parallel combination of 1 kę and 1000 pF at 250 kbps, and the other two transmitters can maintain dc outputs across 3-kę loads.
To calculate the maximum output current available, superimpose the ac and dc components. Output current flows alternately from each rail as the NRZ output waveform swings between the guaranteed-minimum-output levels (±5V). Assuming the output requires one whole data period (4 µsec at 250 kbps) to slew from 5 to +5V, the ac component equals CLOAD(dV/dt)=1000 pF (10V/4 µsec)=2.5 mA. For the dc component, Ohm's law gives I=E/R=5V/3 kę=1.67 mA from one transmitter, so the three transmitters together represent a dc load of 5 mA. Adding the ac and dc components gives a conservative maximum rating of 2.5 mA+5 mA=7.5 mA.
The 3-kę load is an EIA-232 requirement, but the data rate and load capacitance are application-dependent parameters. Lower values for these parameters make more current available for external use. A remote-sensing system, for instance, might operate at 2400 bps with a load of 3 kę in parallel with 1000 pF (50 ft of cable at 20 pF/ft). For three transmitters, the load is 5 mA, and the ac load for one transmitter (72 µA) is almost negligible in this low-data-rate application. The available current in this case is calculated as 7.5 mA (5 mA+72 µA)=2.428 mA.
These calculations are conservative: With VCC at 2.7V and the three transmitters loaded with 3 kę/1000 pF while transmitting valid EIA-232 levels at 2400 bps, the circuit actually delivers 6.7 mA to an external load. As noted, Q1 and Q2 allow the circuit to start under these conditions. If you disconnect the transmitter loads, the maximum external load current that allows start-up is 11.5 mA; with Q1 and Q2 removed, the maximum current is only 5.7 mA. (DI ##1725)