Obsolete Product(s) - Obsolete Product(s)
L292
10/13
Figure 8. Small Signal Step Response (normalized amplitude vs. t / R
F
C
F
).
V
7
= 200 mV/div.
I
M
= 100 mA/div.
t = 100 µs/div. with V
I
= 1.5 Vp.
It is possible to verify that the L292 works in "closed-loop" conditions during the entire motor current rise-
time: the voltage at pin 7 inverting input of the error amplifier) is locked to the reference voltage VR,
present at the non-inverting input of the same amplifier.
The previous linear analysis is correct for this example.
Descresing the ξ value, the rise-time of the current decreases. But for a good stability, from relationship
(6), the maximum value of ξ is: (phase margin = 45°)
b) Large signal reponse
The large step signal response is limited by slew-rate and inductive load.
In this case, during the rise-time of the motor current, The L292 works is open-loop condition.
CLOSED LOOP SYSTEM BANDWIDTH.
A good choice for x is the value 1 / Ö2. In this case :
(8)
The module of the transfer function is :
(9)
The cutoff frequency is derived by the expression (9) by putting = 0.707 · (-3 dB), from which:
ξmin
1
2
4
2
--------------=
I
M
V
I
-----
s()
0.044
R
s
---------------
1sR
F
C
F
+
12sR
F
C
F
2s
2
R
F
2
C
F
2
++
----------------------------------------------------------------------
=
I
M
V
I
-----
0.044
R
s
---------------
21 ω
2
R
F
2
C
F
2
+
12ωR
F
C
F
+()
2
1+[]12–ωR
F
C
F
()
2
1+[]⋅
----------------------------------------------------------------------------------------------------------------------
=
I
M
V
I
-----
0.044
R
s
---------------
ω
T
0.9
R
F
C
F
---------------
f
T
0.9
2πR
F
C
F
-----------------------==
