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HCMS-3917 Datasheet(PDF) 20 Page - AVAGO TECHNOLOGIES LIMITED |
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HCMS-3917 Datasheet(HTML) 20 Page - AVAGO TECHNOLOGIES LIMITED |
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20 / 22 page  Appendix D. Refresh Circuitry This display driver consists of 20 one-of-eight column decoders and 20 constant current sources, 1 one-of-eight row decoder and eight row sinks, a pulse width modulation control block, a peak current control block, and the circuit to refresh the LEDs. The refresh counters and oscillator are used to synchronize the columns and rows. The 160 bits are organized as 20 columns by 8 rows. The IC illuminates the display by sequentially turning ON each of the 8 row-drivers. To refresh the display once takes 512 oscillator cycles. Because there are eight row drivers, each row driver is selected for 64 (512/8) oscillator cycles. Four cycles are used to briefly blank the display before the following row is switched on. Thus, each row is ON for 60 oscillator cycles out of a possible 64. This corresponds to the maximum LED on time. The temperature of the display will also affect the LED bright- ness as shown in Figure 10. connections are used, the LED ground can vary from -0.3 V to +0.3 V with respect to the logic ground. Voltages below -0.3 V can cause all the dots to be ON. Voltage above +0.3 V can cause dimming and dot mismatch. Using a decoupling capacitor between the power supply and ground will help prevent any supply noise in the frequency range greater than that of the functioning display from interfering with the display’s internal circuitry. The value of the capacitor depends on the series resistance from the ground back to the power supply and the range of frequencies that need to be suppressed. It is also advantageous to use the largest ground plane possible. Electrostatic Discharge The inputs to the ICs are protected against static discharge and input current latch up. However, for best results, standard CMOS handling precautions should be used. Before use, the HCMS-39XX should be stored in antistatic tubes or in conductive material. During assembly, a grounded conductive work area should be used and assembly personnel should wear conductive wrist straps. Lab coats made of synthetic material should be avoided since they are prone to static buildup. Input current latch up is caused when the CMOS inputs are subjected to either a voltage below ground (VIN < ground) or to a voltage higher than VLOGIC (VIN >VLOGIC) and when a high current is forced into the input. To prevent input current latch up and ESD damage, unused inputs should be connected to either ground or VLOGIC. Voltages should not be applied to the inputs until VLOGIC has been applied to the display. Appendix C. Oscillator The oscillator provides the internal refresh circuitry with a signal that is used to synchronize the columns and rows. This ensures that the right data is in the dot drivers for that row. This signal can be supplied from either an external source or the internal source. A display refresh rate of 100 Hz or faster ensures flicker-free operation. Thus, for an external oscillator the frequency should be greater than or equal to 512 x 100 Hz = 51.2 kHz. Operation above 1 MHz without the prescaler or 8 MHz with the prescaler may cause noticeable pixel-to-pixel mismatch. |
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