LT1970CFE datasheet(21/24 Pages) LINER

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LT1970

1970fb

APPLICATIO S I FOR ATIO

Figure 13. Simple Bidirectional DC Motor Speed Controller

maintain 0V, both outputs must be equal and opposite in

polarity, thus they track each other. If one output reaches

current limit and drops in voltage, the other output follows

to maintain a symmetrical + and – voltage across a

common load. Again, the output current limit is less than

the full capability of the LT1970 due to thermal reasons.

Separate current limit indicators are used on each LT1970

because one output only sources current and the other

only sinks current. Both devices can share the same

thermal shutdown indicator, as the output flags can be

OR’ed together.

Another simple linear power amplifier circuit is shown in

Figure 13. This uses the LT1970 as a linear driver of a DC

motor with speed control. The ability to source and sink

the same amount of output current provides for bidirec-

tional rotation of the motor. Speed control is managed by

sensing the output of a tachometer built on to the motor.

A typical feedback signal of 3V/1000rpm is compared with

the desired speed-set input voltage. Because the LT1970

is unity-gain stable, it can be configured as an integrator

to force whatever voltage across the motor as necessary

to match the feedback speed signal with the set input

signal.

Additionally, the current limit of the amplifier can be

adjusted to control the torque and stall current of the

motor. For reliability, a feedback scheme similar to that

shown in Figure 4 can be used. Assuming that a stalled

rotor will generate a current limit condition, the stall

current limit can be significantly reduced to prevent exces-

sive power dissipation in the motor windings.

For motor speed control without using a tachometer, the

circuit in Figure 14 shows an approach. Using the enable

feature of the LT1970, the drive to the motor can be

removed periodically. With no drive applied, the spinning

motor presents a back EMF voltage proportional to its

rotational speed. The LT1782 is a tiny rail-to-rail amplifier

VCSRC

COMMON

49.9k

15V

–15V

REVERSE

FORWARD

10k

VEE

VCSNK

V–

FILTER

V+

15V

VCC

ISNK

ISRC

SENSE–

SENSE+

TSD

OUT

+IN

LT1970

–15V

–IN

Ω

12V DC

MOTOR

TACH

FEEDBACK

3V/1000rpm

GND

1970 F13

µF

49.9k

1.2k

OV TO 5V

TORQUE/STALL

CURRENT CONTROL

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