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============ SOLUTION ============

For now, I have found this solution: use sprintf_P with PSTR(...) in the same way of F(...):

sprintf_P(buf, PSTR("{\"name\":\"Intervalo de Registros\",\"value\":\"%d\", \"unit\":\"C\"},"), (int)settings.treg);

I will test more...

Thanks for all clarification!

Best Regards!

EDIT 3

As commented, all string literals are saved in the (SRAM). So, the problem is, I have a lot of strings literals spread across different functions. How to save this amount of SRAM? Note that using F() for sprintf gives compilation error.

Is there any way to put the literal in stack (release memory when function returns) and not in the global area? Is F() the only solution?

EDIT 2

I have an advance running avr-objdump -s -j .data x.elf as suggested by Edgar below. Now I see that all my string literals are in data! For example:

void myFunction(){
    ...
    sprintf(buf, "{\"name\":\"Intervalo de Registros\",\"value\":\"%d\", \"unit\":\"C\"},", (int)settings.treg);
    ...
}

avr-objdump -s -j .data x.elf shows:

 800660 2c007b22 6e616d65 223a2249 6e746572  ,.{"name":"Inter
 800670 76616c6f 20646520 52656769 7374726f  valo de Registro
 800680 73222c22 76616c75 65223a22 2564222c  s","value":"%d",
 800690 2022756e 6974223a 2243227d 2c007b22   "unit":"C"},.{"