LT6105
17
6105fa
TYPICAL APPLICATIONS
50ms/DIV
5V/DIV
2V/DIV
10V/DIV
6105 F05
VBAT = 3.6V
ICPO = 200μA
CCPO = 2.2ΩF
Figure 5. Current Measurement Waveforms. The Top Trace Is the
MOSFET Gate with High On. The Middle Trace Is the Bottom of
the Solenoid/ Inductor. The Bottom Trace Is the LT6105 Output,
Representing Solenoid Current at 80mA / DIV. Glitches Are Useful
Indicators of Solenoid Plunger Movement
Solenoid Monitor
The large input common mode range of the LT6105
makes it suitable for monitoring currents in quarter,
half and full bridge inductive load driving applications.
Figure 4 shows an example of a quarter bridge. The
MOSFET pulls down on the bottom of the solenoid to
increase solenoid current. It lets go to decrease current,
and the solenoid voltage freewheels around the Schottky
diode. Current measurement waveforms are shown in
Figure 5. The small glitches occur due to the action of
the solenoid plunger, and this provides an opportunity for
mechanical system monitoring without an independent
sensor or limit switch.
Figure 6 shows another solenoid driver circuit, this time
with one end of the solenoid grounded and a P-channel
MOSFET pulling up on the other end. In this case, the
inductor freewheels around ground, imposing a negative
input common mode voltage of one Schottky diode drop.
This voltage may exceed the input range of the LT6105.
This does not endanger the device, but it severely degrades
its accuracy. In order to avoid violating the input range,
pull-up resistors may be used as shown.
6105 F04
LT6105
V–
V+
24V, 3W
SOLENOID
200Ω
1%
1N5818
2N7000
–IN
+IN
4.99k
1%
200Ω
1%
VOUT = 25mV/mA
VOUT
5VDC
24VDC
0V/OFF
5V/ON
–
+
1Ω
1%
6105 F06
LT6105
V–
V+
2k
1%
2k
1%
24V, 3W
SOLENOID
200Ω
1%
1N5818
TP0610L
1N914
–IN
+IN
4.99k
1%
200Ω
1%
VOUT = 25mV/mA
VOUT
5VDC
24VDC
19V/ON
24V/OFF
–
+
1Ω
1%
Figure 4. Simplest Form of a Solenoid Driver. The LT6105
Monitors the Current in Both On and Freewheel States. The
Lowest Common Mode Voltage Is 0V, While the Highest Is
24V Plus the Forward Voltage of the Schottky Diode
Figure 6. A Similar Circuit to Figure 4, but with Solenoid
Grounded, so Freewheeling Forces Inputs Negative.
Providing Resistive Pull-Ups Keeps Amplier Inputs From
Falling Outside of Their Accurate Input Range