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ICM7211ALPL Datasheet(PDF) 9 Page - Intersil Corporation |
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ICM7211ALPL Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 12 page 9-14 Description Of Operation LCD Devices The LCD devices in the family (ICM7211, ICM7211A, ICM7211M, ICM7211AM) provide outputs suitable for driving conventional four-digit, seven-segment LCD displays. These devices include 28 individual segment drivers, backplane driver, and a self-contained oscillator and divider chain to generate the backplane frequency. The segment and backplane drivers each consist of a CMOS inverter, with the N-Channel and P-Channel devices ratioed to provide identical on resistances, and thus equal rise and fall times. This eliminates any DC component, which could arise from differing rise and fall times, and ensures maximum display life. The backplane output devices can be disabled by connect- ing the OSCillator input (pin 36) to VSS. This allows the 28 segment outputs to be synchronized directly to a signal input at the BP terminal (pin 5). In this manner, several slave devices may be cascaded to the backplane output of one master device, or the backplane may be derived from an external source. This allows the use of displays with charac- ters in multiples of four and a single backplane. A slave device represents a load of approximately 200pF (compara- ble to one additional segment). Thus the limitation of the number of devices that can be slaved to one master device backplane driver is the additional load represented by the larger backplane of displays of more than four digits. A good rule of thumb to observe in order to minimize power con- sumption is to keep the backplane rise and fall times less than about 5 µs. The backplane output driver should handle the backplane to a display of 16 one-half inch characters. It is recommended, if more than four devices are to be slaved together, the backplane signal be derived externally and all the ICM7211 devices be slaved to it. This external signal should be capable of driving very large capacitive loads with short (1 - 2 µs) rise and fall times. The maximum frequency for a backplane signal should be about 150Hz although this may be too fast for optimum display response at lower dis- play temperatures, depending on the display type. The onboard oscillator is designed to free run at approxi- mately 19kHz at microampere current levels. The oscillator frequency is divided by 128 to provide the backplane fre- quency, which will be approximately 150Hz with the oscillator free-running; the oscillator frequency may be reduced by connecting an external capacitor between the OSCillator ter- minal and VDD. The oscillator may also be overdriven if desired, although care must be taken to ensure that the backplane driver is not dis- abled during the negative portion of the overdriving signal (which could cause a DC component to the display). This can be done by driving the OSCillator input between the positive supply and a level out of the range where the backplane disable is sensed (about one fifth of the supply voltage above VSS). Another technique for overdriving the oscillator (with a signal swinging the full supply) is to skew the duty cycle of the over- driving signal such that the negative portion has a duration shorter than about one microsecond. The backplane disable sensing circuit will not respond to signals of this duration. LED Devices The LED device in the family (ICM7212AM) provides outputs suitable for directly driving four-digit, seven-segment common-anode LED displays. These devices include 28 individual segment drivers, each consisting of a low-leakage, current-controlled, open-drain, N-Channel transistor. The drain current of these transistors can be controlled by varying the voltage at the BRtrighTness input (pin 5). The volt- age at this pin is transferred to the gates of the output devices for “on” segments, and thus directly modulates the transistor’s “on” resistance. A brightness control can be easily imple- mented with a single potentiometer controlling the voltage at pin 5, connected as in Figure 9. The potentiometer should be a high value (100k Ω to 1MΩ) to minimize power consumption, which can be significant when the display is off. The brightness input may also be operated digitally as a dis- play enable; when high, the display is fully on, and low fully off. The display brightness may also be controlled by varying the duty cycle of a signal swinging between the two voltages at the brightness input. Note that the LED device has two connections for VSS; both of these pins should be connected. The double connection is necessary to minimize effects of bond wire resistance with the large total display currents possible. When operating LED devices at higher temperatures and/or higher supply voltages, the device power dissipation may need to be reduced to prevent excessive chip temperatures. The maximum power dissipation is 1W at 25oC, derated lin- early above 35oC to 500mW at 70oC (-15mW/oC above 35oC). Power dissipation for the device is given by: P = (VSUPP - VFLED)(lSEG)(nSEG) where VFLED is the LED forward voltage drop, ISEG is segment current, and nSEG is the number of “on” segments. It is recommended that if the device is to be operated at OSCILLATOR FREQUENCY BACKPLANE INPUT/OUTPUT OFF ON 64 CYCLES 64 CYCLES 128 CYCLES SEGMENTS SEGMENTS FIGURE 8. DISPLAY WAVEFORMS 100k Ω TO 1MΩ BRIGHTNESS PIN 5 VDD (LED ANODES) FIGURE 9. BRIGHTNESS CONTROL ICM7211, ICM7212 |
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