TC7MB3257CFT/FK/FTG
2014-03-01 7
Rise and Fall Times (tr / tf) of the TC7MB3257C I/O Signals
The tr(out) and tf(out) values of the output signals are affected by the CR time constant of the input, which consists of
the switch terminal capacitance (C
I/O
) and the on-resistance (R
ON
) of the input.
In practice, the tr(out) and tf(out) values are also affected by the circuit’s capacitance and resistance components other
than those of the TC7MB3257C.
The tr(out) / tf(out) values can be approximated as follows. (Figure 3 shows the test circuit.)
tr(out) / tf(out) (approx) = - ( C
I/O
+ C
L
) ･ (R
DRIVE+
R
ON
) ･ ln ( ( ( V
OH
– V
OL
) – V
M
) / ( V
OH
– V
OL
) )
where, R
DRIVE
is the output impedance of the previous-stage circuit.
Calculation example:
tr(out) (approx) = - ( 8.5
+ 15
)E-12 ･ ( 120 + 3) ･ ln ( ( ( 4.5 - 0
) – 2.25 ) / ( 4.5 – 0
) )
≈ 2.0 ns
Calculation conditions:
V
CC
= 4.5 V , C
L
= 15 pF , R
DRIVE
= 120 Ω (output impedance of the previous IC), V
M
= 2.25 V (V
CC
/ 2)
Output of the previous IC = digital (i.e., high-level voltage = V
CC
; low-level voltage = GND)
V
CC
Parameter
5.0 ± 0.5 V
V
M
V
CC
/ 2
Figure 3 Test Circuit
Measure
C
L
A/Bn
C
I/O
(Switch On)
TC7MB3257C
R
DRIVE
Previous IC
Bn/A
R
DRIVE
= output impedance of the previous IC
=
R
ON
V
OL
V
OH
V
M
tr(out)
V
M
Measure
C
L
R
DRIVE
Previous IC
R
DRIVE
= output impedance of the previous IC
V
CC
GND
Waveform generated without the TC7MB3257C
tf(out)
Waveform generated with the TC7MB3257C added
