Datasheet

LT1970
21
1970fc
APPLICATIONS INFORMATION
range all the way to 0V, an LTC1046 charge pump inverter
is used to develop a –5V supply. This produces a negative
rail for the LT1970 which has to sink only the quiescent
current of the amplifi er, typically 7mA.
Using a second LT1970, a 0V to ±12V dual tracking power
supply is shown in Figure 12. The midpoint of two 10k
resistors connected between the + and – outputs is held
at 0V by the LT1881 dual op amp servo feedback loop. To
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 fol-
lows 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 fl ags can be
OR’ed together.
Another simple linear power amplifi er 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 confi gured 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 amplifi er can be ad-
justed 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 signifi cantly reduced to prevent excessive power
dissipation in the motor windings.
Figure 13. Simple Bidirectional DC Motor Speed Controller
VC
SRC
COMMON
R4
49.9k
15V
–15V
REVERSE
FORWARD
R2
10k
V
EE
VC
SNK
V
–
FILTER
V
+
15V
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
+IN
LT1970
–15V
–IN
R
S
1Ω
12V DC
MOTOR
TACH
FEEDBACK
3V/1000rpm
GND
1970 F13
C1
1µF
R5
49.9k
R3
1.2k
R1
1.2k
OV TO 5V
TORQUE/STALL
CURRENT CONTROL