Data Sheet

AD8417-2

APPLICATIONS INFORMATION

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Rev. 0 | 14 of 15

SOLENOID CONTROL

High-Side Current Sense with a Low-Side

Switch

In the case of a high-side current sense with a low-side switch,

the PWM control switch is ground referenced. Tie an inductive load

(solenoid) to a power supply and place a resistive shunt between

the switch and the load (see Figure 34). An advantage of placing

the shunt on the high side is that the entire current, including the

recirculation current, is measurable because the shunt remains

in the loop when the switch is off. In addition, diagnostics are

enhanced because shorts to ground are detected with the shunt on

the high side.

In this circuit configuration, when the switch is closed, the common-

mode voltage decreases to near the negative rail. When the switch

is open, the voltage reversal across the inductive load causes the

common-mode voltage to be held one diode drop above the battery

by the clamp diode.

Figure 34. Low-Side Switch

High-Side Current Sense with a High-Side

Switch

The high-side current sense with a high-side switch configuration

minimizes the possibility of unexpected solenoid activation and

excessive corrosion (see Figure 35). In this case, both the switch

and the shunt are on the high side. When the switch is off, the

battery is removed from the load, which prevents damage from po-

tential shorts to ground while still allowing the recirculating current

to be measured and to provide diagnostics. Removing the power

supply from the load for the majority of the time that the switch

is open minimizes the corrosive effects that can be caused by the

differential voltage between the load and ground.

When using a high-side switch, the battery voltage is connected

to the load when the switch is closed, causing the common-mode

voltage to increase to the battery voltage. In this case, when

the switch is open, the voltage reversal across the inductive load

causes the common-mode voltage to be held one diode drop below

ground by the clamp diode.

Figure 35. High-Side Switch

High Rail Current Sensing

In the high rail current sensing configuration, the shunt resistor is

referenced to the battery. High voltage is present at the inputs of

the current sense amplifier. When the shunt is battery referenced,

the AD8417-2 produces a linear ground referenced analog output.

Additionally, the AD8214 provides an overcurrent detection signal

in as little as 100 ns (see Figure 36). This feature is useful in high

current systems where fast shutdown in overcurrent conditions is

essential.

Figure 36. High Rail Current Sensing