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IW7032 Datasheet(PDF) 12 Page - Dialog Semiconductor |
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IW7032 Datasheet(HTML) 12 Page - Dialog Semiconductor |
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12 / 31 page  iW7032 32-Channel LED Driver for LCD Panel Backlighting Rev. 1.3 iW7032 Page 12 to settle at its highest possible setting to start out the calibration sequence. At this point, after the Boost/Buck voltage is ramped up, all the channels should indicate that they are in regulation through their individual LDOs FORCE_ON signal. There is a regulation detector on the LDO regulation loop that monitors multiple voltage potentials to ensure that the regulation is operating with enough loop gain for guaranteeing regulation and best efficiency. When the loop gain falls below the lower-bound limit threshold, this indicates that the channel is about to go out of regulation due to not having enough Boost/ Buck voltage head room for that channel. With the Boost/ Buck voltage at the maximum level, all channels should be in regulation, and therefore this regulation detector output called FORCE_ON should not be true. For each group, the VDAC is increased 4 LSB at a time at 2 millisecond intervals by default, in a ramp decreasing the Boost/Buck voltage. This occurs until one channel within the Boost/Buck group indicates that it is coming out of regulation. This is sequentially done for every Boost/Buck group. Then the VDAC output is decreased which increases the Boost/Buck voltage at 1 LSB per 2 millisecond interval, by default, until the FORCE_ON comparator output deactivates. This is also accomplished for each Boost/Buck group. With 2 Boost/Buck groups and 256 possible steps for each one, the maximum time for this operation is just over 0.25 seconds, but will most often be shorter since each VDAC does not need to ramp the full distance. The above estimate is based on SISET being larger than or equal to ISET. If this is not true, the time can, under some circumstances, be a little longer. After completing this function with the Scanning Mode ISET current setting, the whole process of VDAC calibration is repeated using the Local Dimming Mode current settings. Once the VDAC calibration is completed, a similar function occurs with each IDAC. IDAC is the digital value (4-bits) that drives the DAC for each channel controlling the LDO current for the adaptive switching operation. This DAC is seen for each channel in figure 9.1. Initially all of the IDAC settings for the individual LDO’s are set to 0, meaning that each channel is set up to operate at the current specified by the ISET bits in the SPI interface. Now that the Boost/Buck voltage is set to its ideal point where the weakest channel within the Boost/Buck group is in regulation. One-by-one, each channel will be adjusted within the Boost/Buck group by incrementing the IDAC value one bit at a time at 1 bit per 4.5 microseconds while again monitoring the FORCE_ON comparator output to determine when the LDO transistor moves deeper inside the triode region indicating that it is about to go out of regulation due to the current demand exceeding that which is feasible with the previously adjusted boost voltage. After the FORCE_ON comparator activates, then the IDAC value is reduced again by one step at a time until it deactivates again. The calibration can be bypassed by setting cali_cfg to be HIGH. If calibration is bypassed, 2-set internal VDACs for LD & SC are automatically loaded from VDAC_LD and VDAC_SC. The control flow to bypass calibration is shown below. IC Analog/Digital / Wakeup Software Initializes IC control registers (include cali_cfg bit) and vdac pre-load registers through SPI interface IC installs VDAC external calibration value (for LD/SC) into internal registers init_done System Enable ICs Initialization Pre-calculation Enter regualar PWM mode Firmware updates bri, OTF adjustment performs idac is updated, re-kick off pre- calculation Real-time calculation Regular PWM Cali_done Figure 9.7. Control Flow to Bypass Calibration 9.3 Channel Grouping Channel grouping allows multiple LED ports on the iW7032 IC to be connected together to one string of LED’s at a higher current rating at the cost of a reduction in number of strings controlled by a single iW7032 IC. For example, if the LED string current is to operate at 240 mA per string, the channel grouping can be set to 2 while the LDO Iset |
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