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What is the meaning of input voltage range for the WLED boost regulator specified in the datasheet as 2.7-5.8 V?. As an ordinary boost WLED regulator it has SW and FB pins rated at 44V, ISINK pins rated at 20V and ISET rated at 7V. To what part of the WLED boost regulator 2.7-5.8 V limitation is applied? To the internal power circuit? If so then what is in this spec for the end user?
The question arises since I am contemplating committing to a project with AM335x being powered from a 24VDC rail. A buck converter at the input will bring the 24VDC input voltage to 3.3VDC to power TPS65217 and additional circuitry (eMMC, WiLink Wi-Fi+Bluetooth module, 2 uSB ports using TPS2505, 3G module, 8.4'' LCD screen, etc).
The problem is with the LCD backlight circuit. It has 2 x 100 mA strings of 10 WLEDs. Bringing 75 mA more WLED current additionally to 25 mA provided by TPS65217 is not a big deal (with some additional external circuit), but boosting 3.3VDC back isn't an option due to TPS65217's switching current limitation. The idea is to apply 24VDC directly to the inductor at the WLED regulator's input. Also the higher input voltage will decrease the converter's duty factor and bring its switching current back within the range specified. Therefore there is a question - since the voltage applied to the inductor is obviously the input voltage for the WLED boost regulator then what is the datasheet's input voltage range for?
As you have pointed out there are no physical device limitations preventing 24V from being used as the input to the WLED driver. The 2.7 - 5.8V spec represents the range over which the WLED driver has been tested and we guarantee proper operation. The WLED driver has not been tested with a 24V input, and we cannot guarantee proper operation in such a configuration.
Integrated Power - Systems and Applications
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In reply to Gregory Watkins:
In order to implement additional sinking of 75 mA the most straightforward way is to use the voltage across an ISET resistor as a control voltage for an external current sinking op amp. But from the Block Diagram of WLED Driver (page 35 of SLVSB64F) it is not clear what is the internal schematic of LED current sinks. Is voltage across (current through) ISET resistors being PWM modulated or it is constant with actual modulation taking place somewhere behind internal current mirrors? Most probably the latter but it is worth to ask.
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