8
LT1787/LT1787HV
APPLICATIONS INFORMATION
WUUU
Kelvin connection of the LT1787’s V
S+
and V
S–
inputs to
the sense resistor should be used in all but the lowest
power applications. Solder connections and PC board
interconnect resistance (approximately 0.5mΩ per square)
can be a large error in high current systems. A 5-Amp
application might choose a 20mΩ sense resistor to give a
100mV full-scale input to the LT1787. Input offset voltage
will limit resolution to 2mA. Neglecting contact resistance
at solder joints, even one square of PC board copper at
each resistor end will cause an error of 5%. This error will
grow proportionately higher as monitored current levels
rise to tens or hundreds of amperes.
Input Noise Filtering
The LT1787 provides input signal filtering pins FIL
+
and
FIL
–
that are internally connected to the midpoint taps of
resistors R
G1
and R
G2
. These pins may be used to filter the
input signal entering the LT1787’s internal amplifier, and
should be used when fast current ripple or transients may
flow through the sense resistor. High frequency signals
above the 300kHz bandwidth of the LT1787’s internal
amplifier will cause errors. A capacitor connected between
FIL
+
and FIL
–
creates a single pole low pass filter with
corner frequency:
f
–3dB
= 1/(2πRC)
where R = 1.25k. A 0.01µF capacitor creates a pole at
12.7kHz, a good choice for many applications.
Common mode filtering from the FIL
+
and FIL
–
pins should
not be attempted, as mismatch in the capacitors from FIL
+
and FIL
–
will create AC common mode errors. Common
mode filtering must be done at the power supply output.
Output Signal Range
The LT1787’s output signal is developed by summing the
net currents through R
G1
and R
G2
into output resistor
R
OUT
. The pins V
OUT
and V
BIAS
may be connected in
numerous configurations to interface with following cir-
cuitry in either single supply or split supply applications.
Care must be used in connecting the output pins to
preserve signal accuracy. Limitations on the signal swing
at V
OUT
are imposed by the negative supply, V
EE
, and the
input voltage V
S+
. In the negative direction, internal circuit
saturation with loss of accuracy occurs for V
OUT
< 70mV
with absolute minimum swing at 30mV above V
EE
. V
OUT
may swing positive to within 0.75V of V
S+
or a maximum
of 35V, a limit set by internal junction breakdown. Within
these contraints, an amplified, level shifted representation
of the R
SENSE
voltage is developed across R
OUT
.
Split Supply Bipolar Output Swing
Figure 2 shows the LT1787 used with split power supplies.
The V
BIAS
pin is connected to ground, and the output
signal appears at the V
OUT
pin. Bidirectional input currents
can be monitored with the output swinging positive for
current flow from V
S+
and V
S–
. Input currents in the
opposite direction cause V
OUT
to swing below ground.
Figure 2 shows an optional output capacitor connected
from V
OUT
to ground. This capacitor may be used to filter
the output signal before it is processed by other
circuitry.Figure 3 shows the voltage transfer function of
the LT1787 used in this configuration.
Single Supply with Shifted V
BIAS
Figure 4 shows the LT1787 used in a single supply mode
with the V
BIAS
pin shifted positive using an external
LT1634 voltage reference. The V
OUT
output signal can
swing above and below V
BIAS
to allow monitoring of
positive or negative currents through the sense resistor,
as shown in Figure 5. The choice of reference voltage is not
critical except for the precaution that adequate headroom
must be provided for V
OUT
to swing without saturating the
internal circuitry. The component values shown in Figure 4
allow operation with V
S
supplies as low as 3.1V.
Operation with A/D Converter
Figure 6 shows the LT1787 operating with the LTC1286
A/D converter. This low cost circuit is capable of 12-bit
resolution of unipolar currents. The –IN pin of the A/D
converter is biased at 1V by the resistor divider R1 and R2.
This voltage increases as sense current increases, with the