L292_03 STMICROELECTRONICS [STMicroelectronics], L292_03 Datasheet - Page 8

• Manufacturer Part Number: L292_03
• Description: SWITCH-MODE DRIVER FOR DC MOTORS
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L292
Neglecting the VCEsat of the bridge transistors and the VBE of the diodes:
DC TRANSFER FUNCTION
In order to be sure that the current loop is stable the following condition is imposed :
1 + sRC = 1 + s
from which RC =
The transfer function is then,
In DC condition, this is reduced to
OPEN-LOOP GAIN AND STABILITY CRITERION For RC = LM / RM, the open loop gain is:
In order to achieve good stability, the phase margin must be greater than 45° when | A | = 1.
That means that, at f
8/13
G
G
I
-----
V
I
-----
V
A
A f
M
M
I
I
m o
m o
=
s
s
=
=
=
--------------- G
sR
---------- -
V
------- -
R
=
=
---------------------- -
2 R
1
I
1
F
TH
M
M
R
-------------- -
R
R
-------------- -
R
C
2
1
2
1
1
F
s
R
R
R
R
2V
--------- -
V
C
=
4
3
4
3
R
------- -
R
L
m o
F
0
s
M
------- -
R
M
L
(DC transfer function from the input of the comparator (V
=
------ -
R
where
M
R
------ -
R
G
M
1
(pole cancellation)
s
F
4
s
G
------------------ -
mo
LM
--------------------------- -
1
=
R
=
m o
+
4
0.44
---------- -
---------------------- -
2 R
C
--------------------------------------------------------------------------------- -
G
R
sR
R
R
m o
s
s
F
: V
F
1
R
-------------- -
F
R
C
(Note that in practice R must greater than 5.6 K )
C
F
S
s
F
C
2
= supply voltage V
F
+
A
--- -
V
F
=
sR
must be | A | < 1 (see fig. 7), that is :
1
G
------------------ -
+
1
R
4
m o
sR
C
4
C
R
+
F
s
C
s
------------------------------------ -
s 1
2
F
R
F
+
C
sR
1
F
R
R
F
= 8 V (reference voltage)
4
C
C
F
TH
) to the motor current (I
(1)
M
(4)
(3)
(2)
(5)
)).