L292

8/13

Neglecting the VCEsat of the bridge transistors and the VBE of the diodes:

where

(1)

DC TRANSFER FUNCTION

In order to be sure that the current loop is stable the following condition is imposed :

1 + sRC = 1 + s

(pole cancellation)

(2)

from which RC =

LM

(Note that in practice R must greater than 5.6 K

Ω)

The transfer function is then,

(3)

In DC condition, this is reduced to

(4)

OPEN-LOOP GAIN AND STABILITY CRITERION For RC = LM / RM, the open loop gain is:

(5)

In order to achieve good stability, the phase margin must be greater than 45° when | A

β | = 1.

must be | A

β | < 1 (see fig. 7), that is :

G

mo

I

M

V

TH

-----------

s0

=

=

G

mo

1

R

M

--------

2V

s

V

R

----------

=

L

M

R

M

--------

L

M

R

M

--------

I

M

V

I

-----

s

()

R

R

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

G

mo

1sR

+

G

sR

2

R

++

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

=

I

M

V

I

-----

s

()

R

R

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

1

R

s

-------

⋅

0.44

R

s

-----------

A

V

----

==

A

β

1

sR

--------------- G

mo

R

s

R

4

-------

R

F

1sR

+

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

⋅

G

R

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

1

s1

sR

+

()

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

==

1

2

πR

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

A

β f

1

2

πR

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

G

R

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

R

2

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

1

<

==