LINEAR Current Sense Amplifier Inputs: -0.3V to 44V Independent of Supply Design Note 451 Data Manual User Guide

Current Sense Amp Inputs Work from –0.3V to 44V Independent of Supply Design Note 451 Glen Brisebois Introduction Solenoid Monitoring Monitoring current fl ow in electrical and electromechani- The large input common mode range of the LT6105 cal systems is commonly used to provide feedback to makes it suitable for monitoring currents in quarter, improve system operation, accelerate fault detection half and full bridge inductive load driving applications. and diagnosis, and raise effi ciency. A current monitoring Figure 2 shows an example of a quarter bridge. The circuit usually involves placing a sense resistor in series MOSFET pulls down on the bottom of the solenoid to with the monitored conductor and determining the voltage increase solenoid current. It lets go to decrease current, across the sense resistor. To minimize power loss in the and the solenoid current freewheels through the Schottky sense resistor it is kept as small as possible, resulting diode. Current measurement waveforms are shown in in a small differential voltage that must be monitored on Figure 3. The small glitches occur due to the action of top of what may be a fairly large varying common mode the solenoid plunger, and this provides an opportunity for voltage. The LT®6105 is an ideal current sense amplifi er mechanical system monitoring without an independent for this application. Just give it any reasonable supply sensor or limit switch. voltage, say 3V, and its inputs can monitor small sense

Figure 4 shows another solenoid driver circuit, this time

voltages at common modes of –0.3V to 44V and anything

with one end of the solenoid grounded and a P-channel

in between. The accuracy of the LT6105 over this range

MOSFET pulling up on the other end. In this case, the

is displayed in Figure 1.

inductor current freewheels around ground, imposing a

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negative input common mode voltage of one Schottky

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24VDC

1Ω SOLENOID

VSENSE = 50m V RIN = 100Ω

AV = 50V

TA = –40°C LT6105

–IN +IN

TA = 25°C 5V/ON

0V/OFF

TA = 85°CTA = 125°C 5VDC V+

–4 VOUT = 25m V/m AVOUT

+ INPUT VOLTAGE (V)

VS DN451 F02 4.99k DN451 F01 1%

Figure 1. Gain Error vs Input Common Mode Figure 2. Solenoid is Pulled Low, Freewheels High.

Input Travels from 0V to 24.3V