L292 Просмотр технического описания (PDF) - STMicroelectronics

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L292

SWITCH-MODE DRIVER FOR DC MOTORS

STMicroelectronics 

L292

Figure 8. Small Signal Step Response (normalized amplitude vs. t / RFCF).

V7 = 200 mV/div.

IM = 100 mA/div.

t = 100 µs/div. with VI = 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: ξmin = ------1------- (phase margin = 45°)

24 2

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 :

I--M---

VI

(

s

)

=

0----.-0----4---4-- ---------------------1-----+-----s----R----F----C----F----------------------

Rs 1 + 2sRFCF + 2s2RF2CF2

(8)

The module of the transfer function is :

-I-M---

VI

=

0----.-0----4---4-- ------------------------------------2--------1-----+-----ω----2---R-----F---2---C-----F---2-------------------------------------

Rs [(1 + 2ωRFCF)2 + 1] ⋅ [(1–2ωRFCF)2 + 1]

(9)

The cutoff frequency is derived by the expression (9) by putting

I--M---

VI

=

0.707

·

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

Rs

(-3 dB),

from which:

ωT

=

----0---.--9-----

RFCF

fT

=

--------0---.--9---------

2πRFCF

10/13

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