Undershoot

AN3060

8/23

Doc ID 16253 Rev 2

3

Undershoot

3.1

Can undershoot affect operation of my real-time clock?

How? What can be done to guard against it?

Inherent in all field effect devices - all CMOS devices - are parasitic diodes resulting from the

intersection of P and N type regions in the device. Every PN junction forms a diode which

will conduct under any condition which biases the P region above the N region.

current flow to cause some disturbance in the real-time clock registers and hence can

adversely affect timekeeping. Therefore, undershoot should be avoided.

Figure 3.

Typical parasitic diode

undershoot to 0.3 V below ground.

Some supplies ramp very quickly when power cycling with the result that ringing can occur

which produces an undershoot condition that can cause problems for real-time clocks.

In the rare situations where this occurs, the best protection is a back biased Schottky diode

for undershoot problems.

Figure 4.

Undershoot protection

N

N+

N+

P

D

S

G

VCC

AM04721v1

SDA

RTC

SCL

XI

XO

AM04722v1

1N5817 (through hole)

BAT54J (surface mount)