I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 458

V DD

Primary
ESD diodes

Secondary
ESD diodes

Input pad

Current-limiting resistor

Figure 11-15 CMOS receiver with ESD protection.

does not require explicit modeling. Nonlinear models must take into account
the current-voltage relationship of the diode, which is given by the ideal diode
equation :

= i S (e v diode /φ T i diode − 1 ) (11-4)

where i diode is the current through the diode; i S , the diode saturation current, is
proportional to the diode area; v diode is the bias voltage across the diode; and φ T
is the thermal voltage (26 mV at room temperature).
Figure 11-16 presents an example current-voltage characteristic calculated
using equation (11-4) with φ T equal to 26mV and a saturation current of 10 pA.
We can further approximate the diode behavior for manual analysis by setting
the diode current to zero below the “turn-on” voltage (approximately 0.6 V in the
figure) and allowing it to approach infinity at the turn-on voltage. This is shown

100

90

80

70

60

50

40

30

Quasilinear
model

20

Ideal diode
equation

10

0

− 10

0 0.5 1

− 2.5 − 2 − 1.5 − 1 − 0.5

v D (V)

Figure 11-16 Example diode current-voltage characteristic and quasilinear appro-
ximation.