L9777
Functional description
3.3
NMI and RESET driver delay
NMI and RESET pins are driven by bipolar transistor with a maximum current capability
internally limited of value respectively INMIL and IRESL.
For this reason, when the drivers are activated, the capacitors present on pin NMI or RESET
are discharged with constant current. The waveform on output pin is a voltage ramp with a
slope linearly dependent on external capacitance.
The fall time needed by drivers to discharge external capacitor can be calculated in first
approximation using this expression.
( Δ
V
⋅
C
ext
)
t
fall
= ----------------------------
-
I
lim
Where
ΔV
is the voltage difference between 90% and 10% of total voltage swing of the
transition, Cext is the total pin capacitance and Ilim is the current limitation of the driver
(IRESL and INMIL).
Figure 10. RESET and NMI drivers fall time
RESET (V)
5
90%
Vsw
Vsw
10%
Vsw
VRESL
TIME (µs)
Tfall
Doc ID 13496 Rev 2
13/25
STMICROELECTRONICS [ STMICROELECTRONICS ]
