Datasheet

LT6100
10
6100fc
The LT6100 high side current sense amplifier (Figure 1)
provides accurate unidirectional monitoring of current
through a user-selected sense resistor. The LT6100 fea-
tures a fully specified 4.1V to 48V input common mode
range. A high PSRR V
CC
supply (2.7V to 36V) powers the
current sense amplifier and the internal op amp circuitry.
The input sense voltage is level shifted from the positive
sense power supply to the ground reference and amplified
by a user-selected gain to the output. The buffered output
voltage is directly proportional to the current flowing
through the sense resistor.
Theory of Operation (Refer to Figure 1)
Current from the source at V
S
+
flows through R
SENSE
to
the load at V
S
–
, creating a sense voltage, V
SENSE
. Inputs
V
S
+
and V
S
–
apply the sense voltage to 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 current
through R
G2
is forced to flow through transistor Q1 and is
sourced to node V
O1
. The current from R
G2
flowing through
resistor R
O
gives a voltage gain of ten, V
O1
/V
SENSE
= R
O
/
R
G2
= 10V/V. The sense amplifier output at V
O1
is ampli-
fied again by amplifier A2. The inputs of amplifier A2 can
operate to ground which ensures that small sense voltage
signals are detected. Amplifier A2 can be programmed to
different gains via Pin 6 and Pin 7. Thus, the total gain of
the system becomes A
V
= 10 • A2 and V
OUT
= V
SENSE
• A
V
.
Gain Setting
The LT6100 gain is set by strapping (or floating) the two
gain pins (see Table 1). This feature allows the user to
“zoom in” by increasing the gain for accurate measure-
ment of low currents.
A
V
= 10V/V • G2, G2 is the gain of op amp A2.
Table 1. Gain Set with Pin 6 and Pin 7
A2 (PIN 6) A4 (PIN 7) G2 A
V
Open Open 1 10
V
EE
Out 1.25 12.5
V
EE
Open 2 20
Out V
EE
2.5 25
Open V
EE
4 40
V
EE
V
EE
5 50
Selection of External Current Sense Resistor
External R
SENSE
resistor selection is a delicate trade-off
between power dissipation in the resistor and current mea-
surement accuracy. The maximum sense voltage may be
as large as ±300mV to get maximum dynamic range. For
high current applications, the user may want to minimize
the sense voltage to minimize the power dissipation in
the sense resistor. The LT6100’s low input offset voltage
of 80µV allows for high resolution of low sense voltages.
This allows limiting the maximum sense voltage while still
providing high resolution current monitoring.
Kelvin connection of the LT6100’s V
S
+
and V
S
–
inputs to
the sense resistor should be used to provide the highest
accuracy in high current applications. Solder connections
and PC board interconnect resistance (approximately
0.5mΩ per square) can be a large error in high current
systems. A 5A application might choose a 20mΩ sense
resistor to give a 100mV full-scale input to the LT6100.
Input offset voltage will limit resolution to 4mA.
Neglect-
ing
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.
applicaTions inForMaTion