L292 Datasheet L292 | P7-P9

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L292

APPLICATION INFORMATION

This section has been added in order to help the designer for the best choise of the values of external

components.

Figure 5. L292 Block Diagram.

The schematic diagram used for the Laplace analysis of the system is shown in fig. 6.

Figure 6.

R

S1

= R

S2

= R

S

(sensing resistors)

= 2.5 · 10-3 W (current sensing amplifier transconductance)

L

M

= Motor inductance, R

M

= Motor resistance, I

M

= Motor current

1

R

4

-------

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L292

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(DC transfer function from the input of the comparator (V

TH

) to the motor current (I

M

)).

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

where : V

S

= supply voltage V

R

= 8 V (reference voltage) (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 KΩ)

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 | Aβ | = 1.

That means that, at f

F

= must be | Aβ | < 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

2

R

4

R

1

R

3

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

G

mo

1sR

F

C

F

+

G

mo

R

s

sR

4

Cs

2

R

F

C

F

R

4

C++

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

=

I

M

V

I

-----

s()

R

2

R

4

R

1

R

3

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

1

R

s

-------

⋅

0.44

R

s

-----------

A

V

----

==

Aβ

1

sR

F

C

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

G

mo

R

s

R

4

-------

R

F

1sR

F

C

F

+

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

⋅

G

mo

R

s

R

4

C

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

1

s1 sR

F

C

F

+()

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

==

1

2πR

F

C

F

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

A β f

1

2 π R

F

C

F

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

G

mo

R

s

R

4

C

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

R

F

C

F

2

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

1<==

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L292

Figure 7. Open Loop Frequency Response

CLOSED-LOOP SYSTEM STEP RESPONSE

a) Small - signals analysis.

The transfer function (3) can be written as follows:

(7)

where wo = is the cutoff frequency

is the dumping factor

By choosing the ξ value, it is possible to determine the system response to an input step signal.

Examples :

1) ξ = 1 from which t

(where V

i

is the amplitude of the input step).

2) ξ = from which

I

M

V

I

-----

s()

0.044

R

s

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

1

s

2ξω

o

--------------+

1

2 ξ ss

2

+

ω

o

ω

o

2

----------------------+

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

=

G

mo

R

s

R

4

CR

F

C

F

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

ξ

R

4

C

4R

F

C

F

G

mo

R

s

---------------------------------------=

I

M

t()

0.044

R

S

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

1e

t

2R

F

C

F

-------------------–

1

t

4R

F

C

F

-------------------+– V

i

⋅=

1

2

-------

I

M

t()

0.044

R

s

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

1

t

2R

F

C

F

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

e

t

2R

F

C

F

-------------------–

cos– V

i

=

L292

L292

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