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LDS8868 Datasheet(PDF) 10 Page - IXYS Corporation |
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LDS8868 Datasheet(HTML) 10 Page - IXYS Corporation |
10 / 14 page ![]() LDS8868 © 2009 IXYS Corp. 10 Doc. No. 8868_DS, Rev. N2.1 Characteristics subject to change without notice control input. By pulsing this signal according to a specific protocol, a set of internal registers can be addressed and written into allowing to configure each bank of LEDs with the desired current. There are six registers: the first five are 4 bits long and the sixth is 1 bit long. The registers are programmed by first selecting the register address and then programming data into that register. An internal counter records the number of falling edges to identify the address and data. The address is serially programmed adhering to low and high duration time delays. One down pulse corresponds to register 1 being selected. Two down pulses correspond to register 2 being selected and so on up to register 6. . tLO and tHI must be within 200ns to 100 μs. . Any pulse with less than 200 ns width may be ignored. Once the final rising edge of the address pulse is programmed, the user must wait at least 500 μs before programming the first data pulse. Any falling edge after this minimum delay will be recognised as a first data pulse. Data in a register is reset once it is selected by the address pulses. If a register is selected but no data is programmed, next pulse sequence will be recognized as data only. Do not send register address only without following data because it may disrupt normal device operation. Once the final rising edge of the data pulses is programmed, the user must wait at least 1.5ms before programming another address. If programming fails or is interrupted, the user must wait at least 2 ms (tRESETDELAY) from the last rising edge before reprogramming can commence. Upon EN/SET pin goes high the device automatically starts looking for an address. If no falling edge is detected within 100 μs, then the user must wait at least 2 ms before trying to program the device again. The device requires a minimum 10 μs delay to ensure the initialization of the internal logic at power-up. After this time delay, EN/SET pin may be set high and the device registers may be programmed adhering to the timing constraints shown in Figure 1. Register REG1 allows to set the mode and select the pairs of LEDs to be turned on. A low LED current mode exists to allow for very low current operation under 4mA per channel. If IMODE equals 1, the high current range is selected up to 32mA. If IMODE is set to 0, all currents are divided by 8. Each bank of LEDs (A, B or C) can be turned on independently by setting the respective bit ENA, ENB, ENC to 1. Register REG2 allows to set the same current for all 6 channels. REG3, REG4, REG5 allow to set the current respectively in banks C, B and A. The three banks can be programmed with independent current values. REG6 triggers a charge pump. This forces the charge pump to start from 1x mode and determine the correct mode it should be in to drive the LEDs most efficiently. If the input voltage has risen or the device has been reprogrammed to other LED values, it is recommended to trigger this reset allowing the charge pump to run in the most efficient mode. To power-down the device and turn-off all current sources, the EN/SET input should be low for at least 1.5ms (tOFF) or longer. The driver typically powers- down with a delay of about 1ms. All register data are cleared. Unused LED Channels For applications with only four or two LEDs, unused LED banks can be disabled via the enable register internally and left to float or connect to Vout. For applications requiring 1, 3, or 5 channels, the unused LED pins should be tied to VOUT (see Figure 3). If LED pin voltage is within 1 V of VOUT, then the channel is switched off and a 250 μA test current is placed in the channel to sense when the channel moves below VOUT – 1.5 V. Protection Modes The LDS8868 has follow protection modes: Figure 3. Application circuit with 5 LEDs 1. LED short to VOUT protection If LED pin is shorted to VOUT, LED burned out becomes as short circuit, or LED pin voltage is within from VOUT to (VOUT - 1.5V) range, LDS8868 recognizes this condition as “LED Short” and disables this channel. If LED pin voltage is less than (Vout – |
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