What are the following core features of reset and supervisor ICs used for?

TN0941

8/16

DocID023775 Rev 1

5.5

Push-pull and open drain reset outputs

A push-pull output consists of a pair of complementary MOSFETs. The reset output goes

high when the PMOS transistor (Tp) turns on and the NMOS transistor (Tn) turns off.

Similarly, the reset output goes low when Tp turns off and Tn turns on (see Figure 2).

For open drain outputs, the transistor is turned OFF in normal mode. Hence, the reset

microprocessor has a low signal and the microprocessor is reset. Conversely, active high

resets (a minority) are designed with a P-channel transistor (see Figure 2).

Figure 2. Reset output block diagram for

(1) Push-pull and (2) Open drain reset outputs

Table 1 gives a comparison of a push-pull versus an open drain reset output.

Table 1. Comparison of a push-pull versus an open drain reset output

Advantages

Drawbacks

Push-pull

– Fast rise time

– No current is drained at high and low state -

current is drained only during transitions.

– Almost rail-to-rail response both from low to

high and high to low.

– Capability to source or sink current

– Not possible to connect to several resets in

parallel

Open drain

– Wired-OR capability - several resets can be

connected in parallel.

– Slow rise time - charge or discharge of an RC

circuit.

– Current flows in the pull-up (or pull-down)

resistance (mA) when the reset is activated.