The following tutorial nugget comes courtesy of Greg Waterfall of Texas Instruments, one of our experts.
In the low-dropout (LDO) regulator, the pass element (Figure 1) can be a single PNP bipolar transistor (Figure 2) or a MOSFET (Figure 3). The big advantage of the LDO is that the PNP pass transistor can maintain output regulation with very little voltage drop across it.
Figure 1
The LDO pass element. VOUT provides feedback, which is compared to the internal voltage reference. The error amp senses the VOUT change and controls the Pass Element to maintain a constant output voltage. Power that is not delivered to the load gets dissipated in the pass element, which generates
heat and reduces efficiency.
heat and reduces efficiency.
VDROP = VSAT (LDO Regulator)
Figure 2
A PNP LDO regulator.
Figure 3
When a MOSFET is used as the pass element, the low ON resistance (RDSON) enables
very low dropout voltages.
very low dropout voltages.
With the bipolar PNP transistor, full-load dropout voltages of less than 500 mV are typical. At light loads, dropout voltages can fall as low as 10-20 mV. Using a MOSFET pass element, in some cases, will bring dropout voltages an order of magnitude lower than a bipolar transistor pass element.
One of the advantages of using a NMOS transistor as pass transistor is the easy driving of the gate terminal.
I guess, The considerable fact here is the resistance of the switch while it is ON.
Well I think the resistance is good since that might prevent internal damages and sparks. It can cause some serious issues too
@etnapowers: What about the transition time period for it ? Does it take a long period ?
that's a interesting question. The transition time usually is very small. A good design of driving circuitry might help to avoid a reverse flow of power from the output to the input, that might create a discharge of the output capacitor, that is usually utilized to keep the output voltage stable, and hence, to avoid any load regulation spikes.
A period is long when it is compared with the period of the overall circuit, but I can say that generally a NMOS pass transistor does not take a long period to switch from its off state to its on state and viceversa. the switching time is related to the time useful to deplete o fill the gate capacitance, and it is usually a short time frame.
Another basic point i want to add here is drain current will be induced based on voltages applied at the gate and drain of the transistor.
@samicksha, that's correct, as the voltage applied between gate and source reaches the threshold voltage the MOS is on and the pass element acts as a resistor, which value can be tuned by regulating the voltage applied.
I agree you @etnapowers, Every NMOS transistor contains a threshold voltage which is constant and unique for each transistor.
@samicksha, the low resistance is an important factor too to avoid voltage drops, but the easy driving of the gate is important too to avoid, for example, that the electric current returns to the input voltage source, due to a slow switching off of the NMOS.
@chirshadblog: Resistance is really important to guarantee the correct behaviour of the IC, it has to be reliable and easy to drive at the same time, this will enhance reliability on the long term, as you correctly said.
@samicksha: I add that the threshold depends on temperature and on the charges trapped inside the gate oxide, so the threshold voltage has to be carefully tuned depending on the application circuit to be utilized.
@chirshadblog: the transition time period is not long compared to the period of switching, the designer of the application has to individuate an alternative path for the current during the transition time.