LT6105

17

6105fa

TYPICAL APPLICATIONS

50ms/DIV

5V/DIV

2V/DIV

10V/DIV

6105 F05

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

24V, 3W

SOLENOID

200Ω

1%

1N5818

2N7000

–IN

+IN

4.99k

1%

200Ω

1%

0V/OFF

5V/ON

1Ω

1%

6105 F06

LT6105

2k

1%

2k

1%

24V, 3W

SOLENOID

200Ω

1%

1N5818

TP0610L

1N914

–IN

+IN

4.99k

1%

200Ω

1%

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 Amplifier Inputs From

Falling Outside of Their Accurate Input Range