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LM2623 2.5V in and 12V/0.1A out
With my last problem regarding simple switchers being solved I'm now facing a new one.
After reading the datasheet several times I decided to try out the LM2623 in an application where the voltage input is limited to 2.5V and where the output preferrably would reach 12V at a 100mA load.
I did numerous tries to get correct C3 value with the suggested method in the datasheet, trial and error that is, without any luck. Only conclusion I could get was that with low value C3 (3.3pF -> 4.7pF) the current drawn by the circuit sky-rocketed to ~2A. And with higher value C3 (10pF > 33pF) there was less current drawn but no way near 12V out, closer to 7-8V. All tests were done with voltage divider calculated according to datasheet, 82k resistor for switch frequency and with a 4.7uH/3.2A inductor.
My question is if it's even possible to get 12V out from the LM2623 with the said input and if so, any pointers in how to choose the correct C3? Also I can see that the datasheet states output as high as 14V but I have no idea if it can deliver that from such a low input as 2.5V.
LM2623 is a gated oscillator Boost converter. It has a fixed duty cycle part and it is designed to provide high efficiency for applications around the 17% fixed Duty cycle. From the input and output voltage values you provided, your D is 79% and it would be challenging to shift the duty cycle by that much using the C3 Capacitor.
What is the source of your 2.5V input? is the input voltage changing over time? if Vin is not changing, you can use LM2621 that has a duty cycle rabge up to 80%
Using our webench tool, you can see possible solution for your application by entering your Vin, Vout and Iout on the bottom left corner. www.ti.com
The idea is to get this particular application to work with both 2xAA batteries as well as a supercap which will make the Vin range between 1.8V to 5.0V. And the output needs to be around 12V (+-1V) in order to power a small solenoid (~100mA). Webench Tool was the one guiding me towards the LM2623 switcher however it cannot display any possible solutions, only recommend the part.
Since the process of sizing the C3 cap is based on trial and error, I will have to power up a board with your specific input and output voltages.
LM2623 were recommended on Webench because it is the only Boost in webench library that can startup at lower than 2.2Vin. However, There are other solution to consider if you set your minimum Vin to 2.2V. Once the part starts up and regulating, the Vin can drop to 1.8V if you bias the IC with the output voltage. This solution will be more efficient in discharging the superCap on the input.
You can not use the schematic in the application note to generate 12V (by only changing the resistor divider and C3, etc). The absolute maximum ratings for supply pins (BOOT, VDD, EN) is 10V, so even if you succeed to raise the Vout to 12, you will most probable damage the IC. Some diodes to separate the out from the pins must be inserted in the schematic.
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