Datasheet

LT1970
22
1970fe
For more information www.linear.com/LT1970
applicaTions inForMaTion
For motor speed control without using a tachometer, the
circuit in Figure 14 shows an approach. Using the en-
able 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
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 LT1970
integrator. When re-enabled the LT1970 will adjust the drive
to the motor until the speed feedback voltage, compared
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 15 shows how easy it is to boost the output current
of the LT1970. This ±5A power stage uses complementary
external N- and P-channel MOSFETs to provide the ad
-
ditional 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 15 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.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.