Data Sheet AD8209A
Rev. A | Page 11 of 15
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 pulse-width modulation (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.
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 AD8209A 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 AD8209A
operates down to −2 V, providing an accurate current measurement.
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 AD8209A 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.
Figure 26. Battery Referenced Shunt Resistor
LOW-SIDE CURRENT SENSING
In systems where low-side current sensing is preferable, the
AD8209A provides a simple, high accuracy, integrated solution. In
this configuration, the AD8209A rejects ground noise and offers
high input to output linearity, regardless of the differential input
voltage.
Figure 27. Ground Referenced Shunt Resistor
GND
DNC
–IN
+IN
A1
V
S
A2
OUT
AD8209A
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
DNC = DO NOT CONNECT
C
F
OUTPUT
+
–
145
11-026
GND
DNC
–IN
+IN
A1
V
S
A2
OUT
AD8209A
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BA
TTERY
DNC = DO NOT CONNECT
C
F
OUTPUT
+
–
14511-027
AD8214
INDUCTIVE
LOAD
SWITCH
CLAMP
DIODE
BATTERY
SHUNT
C
F
5V
–IN
NCGND
OVERCURRENT
DETECTION (<100ns)
OUT
V
S
+IN
V
REG
NC
–IN
GND
A1
A2
+IN
V
S
DNC
OUT
AD8209A
1
2
3
4
8
7
6
5
8765
12
34
+
–
14511-028
DNC = DO NOT CONNECT
GND
DNC
–IN
+IN
A1
V
S
A2
OUT
AD8209A
5V
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
DNC = DO NOT CONNECT
C
F
OUTPUT
14511-029