AD8209

Rev. 0 | Page 11 of 16

APPLICATIONS INFORMATION

HIGH-SIDE CURRENT SENSING

WITH A LOW-SIDE SWITCH

In load control configurations for high-side current sensing with a

low-side switch, the PWM-controlled switch is ground referenced.

An inductive load (solenoid) connects to a power supply/battery.

A resistive shunt is placed between the switch and the load (see

Figure 24). An advantage of placing the shunt on the high side

is that the entire current, including the recirculation current, is

monitored because the shunt remains in the loop when the switch

is off. In addition, shorts to ground can be detected with the shunt

on the high side, enhancing the diagnostics of the control loop. In

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

mode voltage moves down to near the negative rail. When the

switch is opened, 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.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

INDUCTIVE

LOAD

SWITCH

SHUNT

CLAMP

DIODE

BATTERY

NC = NO CONNECT

OUTPUT

+

–

Figure 24. Low-Side Switch

In cases where a high-side switch is used for PWM control of the

load current in an application, the AD8209 can be used as shown

in Figure 25. The recirculation current through the freewheeling

diode (clamp diode) is monitored through the shunt resistor. In

this configuration, the common-mode voltage in the application

drops below GND when the FET is switched off. The AD8209

operates down to −2 V, providing an accurate current measurement.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

INDUCTIVE

LOAD

SWITCH

SHUNT

CLAMP

DIODE

BATTERY

NC = NO CONNECT

OUTPUT

+

–

Figure 25. 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 AD8209 produces a linear ground-referenced analog output.

Additionally, the AD8214 can be used to provide an overcurrent

detection signal in as little as 100 ns (see Figure 26). This feature is

useful in high current systems where fast shutdown in overcurrent

conditions is essential.

AD8214

INDUCTIVE

LOAD

SWITCH

CLAMP

DIODE

BATTERY

SHUNT

5V

–IN

NC

GND

OVERCURRENT

DETECTION (<100ns)

OUT

+IN

NC

–IN

GND

A1

A2

+IN

NC

OUT

AD8209

1

2

3

4

8

7

6

5

8

7

6

5

1

2

3

4

+

–

Figure 26. Battery-Referenced Shunt Resistor

LOW-SIDE CURRENT SENSING

In systems where low-side current sensing is preferable, the

AD8209 provides a simple, high accuracy, integrated solution. In

this configuration, the AD8209 rejects ground noise and offers high

input to output linearity, regardless of the differential input voltage.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

INDUCTIVE

LOAD

SWITCH

SHUNT

CLAMP

DIODE

BATTERY

NC = NO CONNECT

OUTPUT

Figure 27. Ground-Referenced Shunt Resistor