L9777

Functional description

Doc ID 13496 Rev 2

13/25

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.

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

t

fall

V

Δ

C

ext

⋅

()

I

lim

-----------------------------

=

RESET (V)

TIME (µs)

5

90%

Vsw

10%

Vsw

Vsw

Tfall

VRESL