L292 データシートの表示（PDF） - STMicroelectronics

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L292

SWITCH-MODE DRIVER FOR DC MOTORS

STMicroelectronics 

L292

Gmo

=

---I--M-----

VTH

(DC transfer function from the input of the comparator (VTH) to the motor current (IM)).

s=0

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

Gmo

=

---1----

RM

2----V----s- where

VR

: VS = supply voltage VR = 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 -L----M--- (pole cancellation)

(2)

RM

from which RC = -L----M--- LM

RM

(Note that in practice R must greater than 5.6 KΩ)

The transfer function is then,

I--M---

VI

(s)

=

-R----2---R-----4-

R1R3

Gmo

---------------------------1-----+-----s---R-----F---C-----F----------------------------

GmoRs + sR4C + s2RFCFR4C

(3)

In DC condition, this is reduced to

I--M---

VI

(s)

=

-R----2---R-----4-

R1R3

⋅

--1----

Rs

=

0----.-4----4--

Rs

A---

V

(4)

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

Aβ

=

-------1--------

sRFC

⋅

Gmo

-R----s-

R4

-----------R-----F------------

1 + sRFCF

=

G-----m-----o---R-----s -----------------1-------------------

R4C s(1 + sRFCF)

(5)

In order to achieve good stability, the phase margin must be greater than 45° when | Aβ | = 1.

That means that, at fF =

----------1------------

2 π RF CF

must be | Aβ | < 1 (see fig. 7), that is :

Aβ f

=

----------1------------

2πRFCF

=

-G----m-----o---R----s-

R4C

R-----F---C-----F- < 1

2

8/13

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