UT232A

Preliminary

CMOS

UNISONICTECHNOLOGIESCO.,LTD

4 of 5

www.unisonic.com.tw

QW-R502-272.d

FUNCTION DESCRIPTION

Driver/Transmitter

The drivers are inverting transmitters, which accept TTL or CMOS inputs and output the RS-232 signals with an

inverted sense relative to the input logic levels. Typically the RS-232 output voltage swing is ±9V. Even under worst

case loading conditions of 3kOhms and 2500pF, the output is guaranteed to be ±5V, which is consistent with the

RS-232 standard specifications. The transmitter outputs are protected against infinite short-circuits to ground without

degradation in reliability.

The instantaneous slew rate of the transmitter output is internally limited to a maximum of 30V/µs. However, the

transition region slew rate of these enhanced products is typically 10V/µs. The smooth transition of the loaded output

from VOL to VOH clearly meets the monotonicity requirements.

Receivers

The receivers convert RS-232 input signals to inverted TTL signals. Since the input is usually from a transmission

line, where long cable lengths and system interference can degrade the signal, the inputs have a typical hysteresis

margin of 0.5V.

This ensures that the receiver is virtually immune to noisy transmission lines. The input thresholds are 0.8V

minimum and 2.4V maximum, again well within the ±3V RS-232 requirements. The receiver inputs are also protected

against voltages up to ±30V. Should an input be left unconnected, a 5kOhm pull down resistor to ground will commit

the output of the receiver to a high state.

In actual system applications, it is quite possible for signals to be applied to the receiver inputs before power is

applied to the receiver circuitry.

This occurs, for example, when a PC user attempts to print, only to realize the printer wasn’t turned on. In this

case an RS-232 signal from the PC will appear on the receiver input at the printer. When the printer power is turned

on, the receiver will operate normally. All of these enhanced devices are fully protected.

Charge Pump

The charge pump section of the these devices allows the circuit to operate from a single +5V±10% power supply

by generating the required operating voltages internal to the devices. The charge pump consists of a voltage doubler

and a voltage inverter. As shown in Figure 1, an internal oscillator triggers the charge accumulation and voltage

During the next transition of the oscillator this charge is boot-strapped to transfer charge to capacitor C3. The voltage

In the inverter section as shown in Figure 2, the voltage across C3 is transferred to C2 forcing a range of 0V to

V+ across C2. Boot-strapping of C2 will then transfer charge to C4 to generate V-. One of the significant

enhancements over previous products of this type is that the values of the capacitors are no longer critical and have

been decreased in size considerably to 0.1mF. Because the charge pump runs at a much higher frequency, the

0.1uF capacitors are sufficient to transfer and sustain charges to the two transmitters.