LT1970A
20
1970afc
For more information www.linear.com/LT1970A
VC
SRC
COMMON
R4
49.9k
15V
REVERSE
FORWARD
R2
10k
V
EE
VC
SNK
V
–
FILTER
V
+
15V
EN
V
CC
ISNK
ISRC
SENSE
–
SENSE
+
TSD
OUT
+IN
LT1970A
–15V
–IN
R
S
1Ω
12V DC
TACH
FEEDBACK
3V/1000rpm
GND
1970A F11
C1
1µF
R5
49.9k
R3
R1
TORQUE/STALL
Figure 11. Simple Bidirectional DC Motor Speed Controller
applicaTions inForMaTion
Another simple linear power amplifier circuit is shown in
Figure 11. This uses the LT1970A 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 bidirectional
rotation of the motor. Speed control is managed by sensing
the output of a tachometer built on to the motor. A typi
-
cal feedback signal of 3V/1000rpm is compared with the
desired speed-set input voltage. Because the LT1970A 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 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 significantly reduced to prevent excessive power
dissipation in the motor windings.
For motor speed control without using a tachometer, the
circuit in Figure 12 shows an approach. Using the enable
feature of the LT1970A, 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
with a shutdown pin. The amplifier is enabled during this
interval to sample the back EMF voltage across the motor.
This voltage is then buffered by one-half of an LT1638 dual
op amp and used to provide the feedback to the LT1970A
integrator. When re-enabled the LT1970A will adjust the
drive to the motor until the speed feedback voltage, com
-
pared to the speed-set input voltage, settles the output to
a fixed value. A 0V to 5V signal for the motor speed input
controls both rotational speed and direction.
The other half of the LT1638 is used as a simple pulse
oscillator to control the periodic sampling of the motor
back EMF.
Figure 13 shows how easy it is to boost the output current
of the LT1970A. This ±5A power stage uses complemen
-
tary external N- and P
-channel MOSFETs to provide the
additional current. The output stage power supply inputs,
V
+
and V
–
, are used to provide gate drive as needed. With
higher output currents, the sense resistor R
CS
, is reduced
in value to maintain the same easy current limit control.
This Class B power stage is intended for DC and low
frequency, <1kHz, applications as crossover distortion
becomes evident at higher frequencies.
Figure 13 shows some optional resistor dividers between
the output connections and the current sense inputs. They
are required only if the load of this power stage is removed
or at a very low current level. Large power devices with
no load on them can saturate and pull the output voltage
very close to the power supply rails. The current sense
amplifiers operate properly with input voltages at least
1V away from the V
CC
and V
EE
supply rails. In boosted
current applications, it may be necessary to attenuate the
maximum output voltage levels by 1V before connecting
to the sense input pins. This only slightly deceases the
current limit thresholds.
