Datasheet

AD8209 Data Sheet
Rev. B | Page 12 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
+V
S
A2
OUT
AD8209
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
NC = NO CONNECT
08461-026
C
F
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
+V
S
A2
OUT
AD8209
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
NC = NO CONNECT
08461-027
C
F
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
C
F
5V
–INNCGND
OVERCURRENT
DETECTION (<100ns)
OUT
V
S
+INV
REG
NC
–IN
GND
A1
A2
+IN
V
S
NC
OUT
AD8209
1
2
3
4
8
7
6
5
8765
1234
+
–
08461-028
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
+V
S
A2
OUT
AD8209
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
NC = NO CONNECT
08461-029
C
F
OUTPUT
Figure 27. Ground-Referenced Shunt Resistor