http://www.union-ic.com Rev.01 Mar.2015

16/22

UM3243

RS-232 Transmitters

The transmitters are inverting level translators that convert CMOS-logic levels to EIA/TIA-232 levels.

They guarantee a 250kbps data rate with worst-case loads of 3kΩ in parallel with 1000pF, providing

compatibility with PC-to-PC communication software. Transmitters can be paralleled to drive

multiple receivers. In Power-Off mode (See Table 1), the transmitters are disabled and the outputs are

forced into a high-impedance state.

Dual Charge Pump Voltage Converter and Capacitor Selection

The UM3243’s internal power supply consists of a regulated dual charge pump that provides output

voltages of +6V (doubling charge pump) and -6V (inverting charge pump), over the +3.0V to +5.5V

charge pump is enabled; if the output voltage exceeds 6V, the charge pump is disabled. The charge

pump requires a flying capacitor (C1, C2) and a reservoir capacitor (C3, C4) to generate the V+ and

V- supplies.

The capacitor type used for C1–C4 is not critical for proper operation; either polarized or

non-polarized capacitors may be used. The charge pump requires 0.1μF capacitors for 3.3V operation.

For other supply voltages, refer to Table 4 for required capacitor values. Do not use values smaller

than those listed in Table 4. Increasing the capacitor values (e.g., by a factor of 2) reduces ripple on

the transmitter outputs and slightly reduces power consumption. When using the minimum required

capacitor values, make sure the capacitor value does not degrade excessively with temperature. If in

doubt, use capacitors with a larger nominal value. The capacitor’s equivalent series resistance (ESR)

usually rises at low temperatures and influences the amount of ripple on V+ and V-.

Table 4. Required Capacitor Values

C1, C2, C3, C4 (μF)

3.15 to 3.6

0.1

4.5 to 5.5

0.1

3.0 to 5.5

0.47

2.4 to 3.0

0.47

High Data Rates

The UM3243 maintains the RS-232 ±5.0V minimum transmitter output voltage even at high data rates.

Figure 10 shows a transmitter loopback test circuit. Figure 11 shows a loopback test result at 120kbps,

and Figure 12 shows the same test at 250kbps. For Figure 11, all three transmitters were driven

simultaneously at 120kbps into RS-232 loads in parallel with 1000pF. For Figure 12, a single

transmitter was driven at 250kbps, and all three transmitters were loaded with an RS-232 receiver in

parallel with 1000pF.