Electronics > Projects, Designs, and Technical Stuff
LTC3786 3V to 18V boost converter
Hamed_ta:
Hi,
I'm designing a li-po battery booster with LPC3786 which is able to convert 3-4.2V battery voltage to 18V 5A, for this purpose I tried to customized one of the sample circuits from the datasheet, (3786 F13a ) I have changed a few of components and entered the values in the LTpowerCAD project file and it seems to work, the only problem I can see is that as the original sample circuit's output voltage is 5V the output of the circuit is connected to the output pin of the LTC1754 pin (through the body diode of mosfet Q), which has the maximum rate 6V of the pin voltage so now with the output voltage of 18V is there any other solution for that ? maybe just replacing mosfet with a simple diod ?
is there any other problem you can mention in the circuts ?
SiliconWizard:
--- Quote from: Hamed_ta on November 14, 2019, 04:28:30 am ---the only problem I can see is that as the original sample circuit's output voltage is 5V the output of the circuit is connected to the output pin of the LTC1754 pin (through the body diode of mosfet Q), which has the maximum rate 6V of the pin voltage so now with the output voltage of 18V is there any other solution for that ? maybe just replacing mosfet with a simple diod ?
--- End quote ---
Good catch! You could have just overlooked that and be in for some nice magic smoke. ;D
There was no problem in the original design as the output voltage was 5V and the output of the LTC1754 -5 was also 5V.
The issue here is that the VBIAS pin requires a min. voltage of 4.5V, which is why they added a charge pump to generate 5V from the battery. Directly connecting VIN (thus the battery) to VBIAS, which is the simplest way of using this converter if VIN >= 4.5V, and what is suggested in the datasheet, was not an option here.
I'm just having a quick look, but am I right thinking that the whole idea around D2 and the PMOS controlled by PGOOD is for bootstrapping (meaning the VBIAS will be taken from VOUT through D2 once the LTC3786 has reached regulation)? I'm not sure I see a big benefit power-consumption-wise, as the charge pump will still be running all the time? So couldn't you just 1/ remove D2 and 2/ directly connect the VOUT from the LTC1754 to the VBIAS pin, remove the PMOS, and call it a day?
Edit: after reading a bit more closely the datasheet, it appears there can be significant current drawn from VBIAS (whereas I had assumed it was negligible), thus this explains why they added this switching over circuit. (I still thnk it could be further improved by automatically shutting down the external charge pump when VOUT is regulated, but I see the point.)
So yeah, you have two options here: you can either replace the PMOS with a diode (VOUT of LTC1754 to VBIAS), which I think would be good enough for bootstrapping as long as you use a schottky diode (remember you must provide at least 4.5V, thus you're allowed MAX 0.5V drop with the diode...). Not the most efficient, but since it would only provide current while starting, I don't think it matters. The other option would be to replace the single PMOS with two PMOS back-to-back for a true bidirectional switch.
As I said above, it could be a bonus idea to find a way to shut down the external charge pump (/SHDN pin) using the PGOOD output of the converter, so you'd save some power once the converter runs regulated (always a good idea when running off batteries!) (Admittedly, with a typ. operating current of 11µA, it may not matter much, depends on your overall power consumption and the battery's capacity of course!)
Miti:
You may be dangerously close to the Vdss max of the power transistors. It is only 20V.
Hamed_ta:
Thank you for your analyzing it was awesome ;D,
I've already tested the schottky diode instead of PMOS but something strange happened, still I have about 13 volt on LTC1754 out pin! which I don't know where it comes from !? maybe it the leakage current of diod? do you think it can be harmful for the LTC1754 ?
and in general do you think this circuit will work ?
Hamed_ta:
--- Quote from: Miti on November 14, 2019, 04:20:22 pm ---You may be dangerously close to the Vdss max of the power transistors. It is only 20V.
--- End quote ---
yes thank you but
I'm going to limit the output voltage to 18 volt do you think still it can be dangerous ?
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