DocID025506 Rev 1

3/19

AN4391

Description

19

1

Description

P-channel MOSFETs don't play a relevant role in high-current applications, where a low

R

DS(on)

is required to minimize the system conduction losses. In fact, with the same die-

size, the P-channel device’s R

DS(on)

is around 2.5/3 times higher than N-channel one, in

other words, a bigger die-size for P-channel structures is needed to achieve the same on-

state performance. This is a serious drawback in terms of overall system cost, efficiency and

thermal management when the system works at a high switching frequency. If the die-size is

bigger, device’s intrinsic capacitances and switching losses are higher. However, there are

several application segments where P-channel FETs can be used with a good performance,

thanks to their electrical features. First of all, in low power DC-DC converters (buck

converter, with maximum load current in the range of 2 - 3 A), a P-channel device can be

used as high-side switch, without any additional external gate driving circuitry (i.e. charge

pump), simplifying the overall circuit complexity. Secondly, in boost converters with low input

voltages, a P-channel device can be used as an output synchronous rectifier, replacing a

low-VF diode and improving the converter efficiency thanks to its figure-of-merit (FOM =

R

DS(on)

* Q

G

). Finally, one of the most common P-channel MOSFET applications is the load

switch, a pass element connecting a power source (battery, adapter) to a given load

(display, ASIC…); by shutting off the load switch, the loads can be temporarily disconnected

improving battery autonomy. Load switches are gaining ever-increasing importance

because battery life is becoming one of the most important requirements for modern

handheld devices.