10
LT1787/LT1787HV
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The LT1787 high side current sense amplifier (Figure 1)
provides accurate bidirectional monitoring of current
through a user-selected sense resistor. The sense voltage
is amplified by a fixed gain of 8 and level shifted from the
positive power supply to the ground referenced outputs.
The output signal may be used in a variety of ways to
interface with subsequent signal processing circuitry.
Input and output filtering are easily implemented to elimi-
nate aliasing errors.
Theory of Operation
Inputs V
S+
and V
S–
apply the sense voltage to matched
resistors R
G1
and R
G2
. The opposite ends of resistors R
G1
and R
G2
are forced to be at equal potentials by the voltage
gain of amplifier A1. The currents through R
G1
and R
G2
are
forced to flow through transistors Q1 and Q2 and are
summed at node V
OUT
by the 1:1 current mirror. The net
current from R
G1
and R
G2
flowing through resistor R
OUT
gives a voltage gain of eight. Positive sense voltages result
in V
OUT
being positive with respect to pin V
BIAS
.
Pins V
EE
, V
BIAS
and V
OUT
may be connected in a variety of
ways to interface with subsequent circuitry. Split supply
and single supply output configurations are shown in the
following sections.
Supply current for amplifier A1 is drawn from the V
S–
pin.
The user may choose to include this current in the moni-
tored current through R
SENSE
by careful choice of connec-
tion polarity.
Selection of External Current Sense Resistor
External R
SENSE
resistor selection is a delicate trade-off
between power dissipation in the resistor and current
measurement accuracy. The LT1787 makes this decision
less difficult than with competitors’ products. The maxi-
mum sense voltage may be as large as ±500mV to get
maximum resolution, however, high current applications
will not want to suffer this much power dissipation in the
sense resistor. The LT1787’s input offset voltage of 40µV
gives high resolution for low sense voltages. This wide
operating dynamic range gives the user wide latitude in
tailoring the range and resolution of his supply monitoring
function.
APPLICATIONS INFORMATION
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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