dc/dc converter
I'm setting up a Step-Up Boost circuit for my main board using the MT3608 CI. It will be powered by a 18650 3.7v battery and need to output 5.2v @ 2A.
Reading the datasheet ( http://prom-electric.ru/media/MT3608.pdf ) of MT3608 I saw that it regulates its output through R1, R2 and the Schottky diode.
I wonder if R1 and R2 are correct and what is the value of my diode to get 5.2v output?
My Schematic:
Going from -3.7V to -32V to is a boost, a negative boost.
To make things more less confusing, here's are application notes Designing a negative boost converter from a standard positive buck converter and Positive Buck Regulator Makes Negative Boost DC/DC Converter. (The regulators in these app notes are intended to be positive bucks, but repurposed as negative boosts.)
For a simple inductive boost converter, it's advisable not to have a boost factor greater than 6. This applies to both positive and negative boost.
p.s. We’re situated roughly in the same neck of the woods, interestingly.
I think there are two approaches that must be considered;
1) Accurate current detection to simulate the Over-Current-Protection (OCP) design of the chip.
- Assuming I understand, (big assumption) it consists of a 3 Amp comparator (not sure when/if it is sampled) and a 6 bit binary counter (/64) that must complete before OCP , shuts the regulator off into hiccup mode. However any 2 consecutive clock cycles less than OCP threshold can reset the counter may have some condition that you need to avoid such as the ratio of 2.5MHz and your choice LC resonant frequency for choice of components. Confirm with the OEM tech forum how the IC actually counts within each cycle.
2) Scope critical points and observe responses in 4 channels before it fails using ideal probing methods ( Low ESL). - Then model and analyze the actual RLC component parameters for every component including avg RdsOn of switch and cross-talk and analyse in time & frequency domain for gain/phase analysis and look for unstable oscillations. Consider different LC values. ( such as those suggested in datasheet, 470nH for 3.3V)
Best Answer
The formula for setting the output voltage is given on page 6 of the data sheet, the values of R1 and R2 you've chosen will result in a very high voltage. Values of 47k and 6.2k will be closer to getting 5.2v output with a 0.6v reference.
The schottky diode is not part of the voltage regulation, it's required for the basic operation of the converter. Diode selection is also covered on page 6.
You are unlikely to get 2A out of this converter, notwithstanding its headline figure is 2A and the switch current 4A. Read the data sheet in detail, and once you've read the efficiency curves and power dissipation limits, and applied them to your specific operating point, I'm sure you'll agree. Don't forget that an 18650 is not a 3.7v battery, you will probably want to use it down to 3v, 2.7v if you're hardcore.