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Boost and LDO

Oscar , 05-22-2018, 08:03 AM
Hi everyone,

I would like to ask for help in a technical doubt regarding Boost and LDO converters. Is it a good-practice design connect two LDO's at the output of the Boost converter. I need to get +-5V and +3.3V, with a max output current of 150mA for all of them. So I decided to use the TPS65131 boost converter from Texas Instruments, this Boost converter can provide two independet output voltages: one positive output voltage and one negative output voltage. However I need very low noise in the power rails, for that reason I am using ultra low noise LDO's at the positive output and at the negative output of the boost converter. So, the idea is get +-6V from the boost converter and then with one positive LDO, get +5V and with one negative LDO get -5V. I also want to connect other positive LDO at the +6V output to get 3.3V, but I do not know if this is a good practice, it means, connect two LDO´s at the output of one Boost converter. Thanks for help.
robertferanec , 05-23-2018, 12:00 PM
- I usually do not use LDO for bigger voltage difference. For example, if you will be making 3.3V from 6V, that is 2.7V (difference) * 150mA (if 3.3v has also 150mA max current) = 0.405W loss on the LDO regulator. Especially if you will be using boost converter, that may be a lot of wasted energy.

- Double check if the LDO will work ok with negative voltage and if you can use that output in your circuit

- What is the input voltage?

I would be also probably considering other options (maybe find a way using lower voltage, not 5V ?).

PS: Generally, if possible, I try to avoid using boost converter as they can be very noisy and may cause problems during EMC/EMI certification
Oscar, 05-23-2018, 12:46 PM
What is the recommended loss in the LDO, how much difference between the input and output is rigth?.The LDO can work with negative voltage, so there is no problem.The input voltage is a 3.7V 2200mAh Li-Ion Battery.I have to supply energy to other circuit that cannot be change. So I need a very stable and low noise power rails of +-5V and +3.3V. What do you can recommend me?Thanks.
robertferanec , 05-24-2018, 12:55 PM
There is no really recommended loss on LDO, However higher power = heat. Also, if this is a battery powered device, you may want to be very careful about wasted power losses.

I would have a look what other people are using in similar applications or if there are recommended chips / connections for these applications. I do not really know much about your application, so it is hard to suggest something.
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