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TC7117ACPL Datasheet(PDF) 9 Page - Microchip Technology |
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TC7117ACPL Datasheet(HTML) 9 Page - Microchip Technology |
9 / 22 page © 2002 Microchip Technology Inc. DS21457B-page 9 TC7116/A/TC7117/A Analog commonisalsousedasVIN- return during auto-zero and de-integrate. If VIN- is different from ana- log common, a Common mode voltage exists in the system and is taken care of by the excellent CMRR of the converter. However, in some applications, VIN-will be set at a fixed, known voltage (power supply common for instance). In this application, analog common should be tied to the same point, thus removing the Common mode voltage from the converter. The same holds true for the reference voltage; if it can be conve- niently referenced to analog common, it should be. This removes the Common mode voltage from the reference system. Within the IC, analog common is tied to an N-channel FET, that can sink 30mA or more of current to hold the voltage 3V below the positive supply (when a load is trying to pull the analog common line positive). How- ever, there is only 10 µA of source current, so analog common may easily be tied to a more negative voltage, thus overriding the internal reference. 3.1.7 TEST The TEST pin serves two functions. On the TC7117/ TC7117A, it is coupled to the internally generated digi- tal supply through a 500 Ω resistor. Thus, it can be used as a negative supply for externally generated segment drivers, such as decimal points, or any other presenta- tion the user may want to include on the LCD. (Figure 3-3 and Figure 3-4 show such an application.) No more than a 1mA load should be applied. The second function is a "lamp test." When TEST is pulled HIGH (to V+), all segments will be turned ON and the display should read -1888. The TEST pin will sink about 10mA under these conditions. FIGURE 3-3: SIMPLE INVERTER FOR FIXED DECIMAL POINT FIGURE 3-4: EXCLUSIVE “OR” GATE FOR DECIMAL POINT DRIVE 3.2 Digital Section Figure 3-5 and Figure 3-6 show the digital section for TC7116/TC7116A and TC7117/TC7117A, respectively. For the TC7116/TC7116A (Figure 3-5), an internal dig- ital ground is generated from a 6V zener diode and a large P-channel source follower. This supply is made stiff to absorb the relative large capacitive currents when the backplane (BP) voltage is switched. The BP frequency is the clock frequency 4800. For 3 readings per second, this is a 60Hz square wave with a nominal amplitude of 5V. The segments are driven at the same frequency and amplitude, and are in phase with BP when OFF, but out of phase when ON. In all cases, negligible DC voltage exists across the segments. Figure 3-6 is the digital section of the TC7117/ TC7117A. It is identical to the TC7116/TC7116A, except that the regulated supply and BP drive have been eliminated, and the segment drive is typically 8mA. The 1000's output (Pin 19) sinks current from two LED segments, and has a 16mA drive capability. The TC7117/TC7117A are designed to drive common anode LED displays. In both devices, the polarity indication is ON for analog inputs. If VIN- and VIN+ are reversed, this indication can be reversed also, if desired. TC7116 TC7116A BP TEST 37 21 V+ V+ GND To LCD Decimal Point To LCD Backplane 4049 TC7116 TC7116A Decimal Point Select V+ V+ TEST GND 4030 To LCD Decimal Point BP |
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