UT232A(2010) Ver la hoja de datos (PDF) - Unisonic Technologies
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UT232A Datasheet PDF : 5 Pages
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UT232A
FUNCTION DESCRIPTION
Preliminary
CMOS
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
inversion. The voltage doubler momentarily stores a charge on capacitor C1 equal to VCC, referenced to ground.
During the next transition of the oscillator this charge is boot-strapped to transfer charge to capacitor C3. The voltage
across C3 is now from VCC to V+.
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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R502-272.d
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