Other Parts Discussed in Thread: DLP4500
Hello,
I am trying to design a circuit that provides the Voffset, Vbias and Vreset voltages of 8.50, 16.0 and -10.0 for the DLP4500 DMD using the TPS65145. I am a bit confused about the negative charge pump. In the datasheet I see that the most negative voltage that can be generated is
VOUTmin = –(VO1 – 2 VF – IO (2 × Rds(on)Q8 + 2 × Rds(on)Q9 + Xcfly))
Note that there is no reference to the Vin voltage.
This formula suggests that the voltage magnitude of the negative charge pump will always be smaller than that of the boost converter. This implies that you cannot generate 8.5V from the boost converter and -10V from the negative charge pump.
However, looking at the schematic of the Lightcrafter 4500, (tidr157a) page 24, it is shown that the TPS65145 is in fact used to generate these voltages. However, there is a connection from the SW nodes to the charge pump part of the schematic. It seems some clever engineering is used to generate a larger (more negative) voltage with the charge pump). I don't understand how the circuit from the reference design works. I would like to use the same technique, however in my board the Vin voltage is 3.3 instead of 5V. Therefore, I am not sure if I can use the same circuit from a 3.3V rail. I would like to avoid adding a 5V rail to my circuit as all other components use the 3.3 rail.
So, my questions are:
- What is the name for the circuit that is used in the Lightcrafter 4500 schematic?
- Is is possible to use this circuit when Vin in 3.3V instead of 5V?
Looking forward to your reply,
Jim