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LDO dropout voltage BJT vs MOSFET

Oliver Kim7
Intellectual 2680 points

Hi, I want to ask something related with the LDO dropout voltage.

I found that mostly TI LDO products consist of PMOS MOSFET since it has lower ground current and lower dropout voltage.

The fact that MOSFET has lower ground current than BJT makes sense since the gate node doesn't consume much current, on the other hand, BJT consumes base current.

However, why MOSFET has lower dropout voltage than BJT??

You know, the BJT can drive more current, since the current increases exponentially propotional to Vce. But MOSFET just propotional to Vds.

Thus, I think BJT on resistance seems to be more lower than MOSFET because it drives more current(R=V/I).

This is against to the fact PMOS is better than BJT in perspective of dropout voltage.

Help me.

  • 0
    TI__Genius 13085 points
    Hi Oliver,

    When a new device is being designed there are a lot of different things that go into the selection of the process technology used to create the device. As you mentioned the ground current in a BJT device will be significantly higher than that of a FET device and this can be a very important specification for many applications. Other considerations include the matching of transistors which is better for FETs, also BJTs are susceptible to thermal runaway where an increase in junction temperature causes an increase in current which further increases the temperature and can cause damage to the BJT.

    -Kyle
  • 0 in reply to Kyle Van Renterghem
    TI__Intellectual 2680 points
    Thanks.

    You mean that dropout voltage is not considered that much when selecting the type of LDO made with BJT vs MOSFET?
  • 0 in reply to Oliver Kim7
    TI__Genius 13085 points
    Hi Oliver,

    Dropout is certainly considered however, I wanted to point out that there are other application requirements which are also taken into account when choosing which process technology to use.