HIN232 - HIN241 series RS-232 transmitter/receiver interface circuits meet all E1ARS-232E and V.28 specifications, and are especially suitable for applications where ± 12V is not available. They require a single +5V power supply (except the HIN239) and feature an onboard charge pump voltage converter which generates +10V and 5V power and -10V power. This series of devices offers a wide variety of RS-232 transmitter/receiver combinations to suit different applications. The driver features true TTL/CMOS compatible inputs, slew rate limited outputs, and 300 Ω off-supply impedance. The receiver can handle up to ±30V and has an input impedance of 3kΩ to 7kΩ. The receiver also has hysteresis that greatly improves noise rejection.

The HIN232-HIN241 series RS-232 transmitter/receiver is powered by a single +5V power supply (except HIN239), has low power consumption, and meets all E1ARS-232C and V.28 specifications. The circuit is divided into three parts: charge pump, transmitter and receiver.

Figure 1 Charge Pump

charge pump

The equivalent circuit of the charge pump is shown in Figure 1. The charge pump consists of two parts: the voltage doubler and the voltage source inverter. Each section is driven by a two-phase, internally generated clock to generate +10V and -10V. The nominal clock frequency is 16kHz. During one of the first phase clocks, capacitor C1 is charged to VCC. In Phase 2, the voltage on C1 is applied to VCC, resulting in a signal across C3 equal to twice VCC. In the first stage, C2 is also denoted 2VCC, then in the second stage, it is inverted with respect to ground to produce a signal across C4 of -2VCC. The charge pump accepts input voltages up to 5.5V. The output impedance (V+) of the voltage multiplier section is about 200 Ω, and the output impedance of the voltage inverter section (V-) is about 450 Ω. A typical application uses 1µF capacitors C1 to C4, however, this value is not critical. Increasing the values of C1 and C2 will reduce the output impedance of the voltage multiplier and inverter, and increasing the storage capacitors, C3 and C4 will reduce the ripple on the V+ and V- supplies. In shutdown mode (HIN236, HIN240 and HIN241) when the shutdown control line is set to logic "1", the charge pump is off, V+ is pulled down to VCC, V is pulled up to GND, and the supply current is reduced to less than 10μA. The transmitter output is disabled and the receiver output is placed in a high impedance state.

Figure 2. Transmitter

transmitter

The transmitter is a TTL/CMOS compatible inverter which translates the input to the RS-232 output. The input logic threshold is about 26% of VCC or 1.3V of VCC = 5V. A logic 1 input results in a voltage between -5V and V- at the output, and a logic 0 results in a voltage between +5V and (V+-0.6V). Each transmitter input has an internal 400kΩ pull-up resistor so that any unused input can be left floating and its output remains in its low state. The output voltage swing meets the ±5V minimum RS-232C specification with a worst-case 3kΩ minimum load driven by all transmitters: conditional impedance, VCC=4.5V, and maximum allowable operating temperature. The transmitter has an internally limited output slew rate that is less than 30V/µs. The output is a short circuit protection function, which can be shorted to ground. The power-down output impedance is 300Ω minimum with ±2V applied to the output and fifth CC=0V.

Figure 3. Receiver

Figure 4. Propagation Delay Definition

receiver

The receiver input accepts up to ±30V while presenting the required 3kΩ to 7kΩ input impedance even when the power is off (VCC=0V). The receiver has a typical input threshold of 1.3V which is in the range of ±3V, known as the transition-specific region in RS-232 connections. The receiver output is 0V to VCC. The output will be low when the input is greater than 2.4V and high as long as the input is floating or driven between +0.8V and -30V. The receiver features 0.5V hysteresis to improve noise rejection. The receiver enable line EN, when set to logic "1", (HIN236, HIN239, HIN240, and HIN241) disables the receiver outputs, placing them in high impedance mode. The receiver outputs are also placed into a high impedance state when in shutdown mode.