Datasheet

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SBVS070B − JUNE 2006 − REVISED MAY 2009
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16
COMPENSATION DRIVER
The compensation coil driver provides the driving current
for the compensation coil. A fully differential driver stage
offers high signal voltages to overcome the wire resistance
of the coil with only +5V supply. The compensation coil is
connected between I
COMP1
and I
COMP2
, both generating an
analog voltage across the coil (see Figure 3) that turns into
current from the wire resistance (and eventually from the
inductance). The compensation current represents the
primary current transformed by the turns ratio. A shunt
resistor is connected in this loop and the high-precision
difference amplifier translates the voltage from this shunt
to an output voltage.
Both compensation driver outputs provide low impedance
over a wide frequency range to insure smooth transitions
between the closed-loop compensation frequency range
and the high-frequency range, where the primary winding
directly couples the primary current into the compensation
coil at a rate set by the winding ratio.
The two compensation driver outputs are designed with
protection circuitry to handle inductive energy. However,
additional external protection diodes might be necessary
for high current sensors.
For reliable operation, a wire break in the compensation
circuit can be detected. If the feedback loop is broken, the
integrating filter drives the outputs I
COMP1
and I
COMP2
to the
opposite rails. With one of these pins coming within 300mV
to ground, a comparator tests for a minimum current flowing
between I
COMP1
and I
COMP2
. If this current stays below the
threshold current level for at least 100µs, the ERROR pin is
asserted active (low). The threshold current level for this test
is less than 57mA at 25°C and 65mA at −40°C, if the I
COMP
pins are fully railed (see the Typical Characteristics).
For sensors with high winding resistance (compensation coil
resistance + R
SHUNT
) or connected to an external
compensation driver, this function should be disabled by
pulling the CCdiag pin low.
R
MAX
+
V
OUT
65mA
Where:
V
OUT
equals the peak voltage between I
COMP1
and I
COMP2
at a 65mA drive current.
R
MAX
equals the sum of the coil and the shunt resistance.
EXTERNAL COMPENSATION COIL DRIVER
An external driver for the compensation coil can be
connected to the I
COMP1
and I
COMP2
outputs. To prevent a
wire break indication, CCdiag has to be asserted low.
An external driver can provide both a higher drive voltage
and more drive current. It also moves the power
dissipation to the external transistors, thereby allowing a
higher winding resistance in the compensation coil and
more current. Figure 4 shows a block diagram of an
external compensation coil driver. To drive the buffer,
either one or both I
COMP
outputs can be used. Note,
however, that the additional voltage gain could cause
instability of the loop. Therefore, the internal gain can be
reduced by approximately 8dB by asserting the GAIN pin
high. R
SHUNT
is connected to GND to allow for a
single-ended external compensation driver. The
differential amplifier can continue to sense the voltage,
and used for the gain and over-range comparator or
ERROR flag.
DRV401
External
Buffer
V+
V
−
Compensation
Coil
R
SHUNT
I
COMP1
I
COMP2
Figure 4. DRV401 with External Compensation Coil Driver and R
SHUNT
Connected to GND
(1)