ad8205yc-pt Analog Devices, Inc., ad8205yc-pt Datasheet - Page 11

• Manufacturer Part Number: ad8205yc-pt
• Description: Single-supply 42 V System Difference Amplifier
• Manufacturer: Analog Devices, Inc.

APPLICATIONS
A typical application for the AD8205 is high-side measurement
of a current through a solenoid for PWM control of the sole-
noid opening. Typical applications include hydraulic transmis-
sion control and diesel injection control.
Two typical circuit configurations are used for this type of
application.
HIGH-SIDE CURRENT SENSE WITH A LOW-SIDE
SWITCH
In this case, the PWM control switch is ground referenced. An
inductive load (solenoid) is tied to a power supply. A resistive
shunt is placed between the switch and the load (see Figure 19).
An advantage of placing the shunt on the high side is that the
entire current, including the re-circulation current, can be
measured since the shunt remains in the loop when the switch
is off. In addition, diagnostics can be enhanced because shorts
to ground can be detected with the shunt on the high side.
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.
HIGH-SIDE CURRENT SENSE WITH A HIGH-SIDE
SWITCH
This configuration minimizes the possibility of unexpected
solenoid activation and excessive corrosion (see Figure 20). In
this case, both the switch and the shunt are on the high side.
When the switch is off, this removes the battery from the load,
which prevents damage from potential shorts to ground, while
still allowing the recirculating current to be measured and pro-
viding for diagnostics. Removing the power supply from the
load for the majority of the time minimizes the corrosive effects
that could be caused by the differential voltage between the load
and ground.
BATTERY
42V
SWITCH
CLAMP
DIODE
SHUNT
Figure 19. Low-Side Switch
INDUCTIVE
LOAD
+IN V
–IN
GND V
AD8205
REF
NC = NO CONNECT
1 +V
REF
5V
S
2 NC
OUT
Rev. 0 | Page 11 of 12
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 opened, the voltage reversal across the
inductive load causes the common-mode voltage to be held one
diode drop below ground by the clamp diode.
Another typical application for the AD8205 is as part of the
control loop in H-bridge motor control. In this case, the
AD8205 is placed in the middle of the H-bridge (see Figure 21)
so that it can accurately measure current in both directions by
using the shunt available at the motor. This is a better solution
than a ground referenced op amp because ground is not typi-
cally a stable reference voltage in this type of application. This
instability in the ground reference causes the measurements
that could be made with a simple ground referenced op amp to
be inaccurate.
The AD8205 measures current in both directions as the
H-bridge switches and the motor changes direction. The output
of the AD8205 is configured in an external reference bidirec-
tional mode, see the Output Offset Adjustment section.
BATTERY
MOTOR
42V
SHUNT
SWITCH
Figure 21. Motor Control Application
CLAMP
DIODE
Figure 20. High-Side Switch
SHUNT
+IN V
–IN
INDUCTIVE
LOAD
GND V
+IN V
–IN
REF
AD8205
1 +V
GND V
REF
AD8205
NC = NO CONNECT
REF
5V
NC = NO CONNECT
S
1 +V
2 NC
OUT
REF
5V
S
2 NC
OUT
CONTROLLER
AD8205
2.5V
5V