Datasheet

LT6100
11
6100fc
applicaTions inForMaTion
Noise Filtering
The LT6100 provides signal filtering via pin FIL that is
internally connected to the resistors R
E
and R
O
. This pin
may be used to filter the input signal entering the LT6100’s
internal op amp, and should be used when fast ripple cur-
rent or transients flow through the sense resistor. High
frequency signals above the 300kHz bandwidth of the
LT6100’s internal amplifier will cause errors. A capacitor
connected between FIL and V
EE
creates a single pole low
pass filter with corner frequency:
f
–3dB
= 1/(2πR
FIL
C)
where R
FIL
= 60k. A 220pF capacitor creates a pole at
12kHz, a good choice for many applications.
Output Signal Range
The LT6100’s output signal is developed by current through
R
G2
into output resistor R
O
. The current is V
SENSE
/R
G2
. The
sense amplifier output, V
O1
, is buffered by the internal op
amp so that connecting the output pins to other systems will
preserve signal accuracy. For zero V
SENSE
, internal circuit
saturation with loss of accuracy occurs at the minimum
V
OUT
swing, 15mV above V
EE
. V
OUT
may swing positive
to within 75mV of V
CC
or a maximum of 36V, a limit set
by internal junction breakdown. Within these constraints,
an amplified, level shifted representation of the R
SENSE
voltage is developed at V
OUT
. The output is well behaved
driving capacitive loads to 1000pF.
Sense Input Signal Range
The LT6100 has high CMRR over the wide input voltage
range of 4.1V to 48V. The minimum operation voltage of
the sense amplifier input is 1.4V above V
CC
. The output
remains accurate even when the sense inputs are driven
to 48V. Figure 2 shows that V
OS
changes very slightly
over a wide input range. Furthermore, the sense inputs
V
S
+
and V
S
–
can collapse to zero volts without incurring
any damage to the device. The LT6100 can handle dif-
ferential sense voltages up to the voltage of the sense
inputs supplies. For example, V
S
+
= 48V and V
S
–
= 0V can
be a valid condition in a current monitoring application
(Figure 3) when an overload protection fuse is blown and
V
S
–
voltage collapses to ground. Under this condition, the
output of the LT6100 goes to the positive rail, V
OH
. There
is no phase inversion to cause an erroneous output signal.
For the opposite case when V
S
+
collapse to ground with
V
S
–
held up at some higher voltage potential, the output
will sit at V
OL
. If both inputs fall below the minimum CM
voltage, V
CC
+ 1.4V, the output is indeterminate but the
LT6100 will not be damaged.
V
S
INPUT VOLTAGE (V)
0
–3.5
INPUT OFFSET VOLTAGE (mV)
–3.0
–2.0
–1.5
–1.0
1.5
0
20
40
50
6100 F02
–2.5
0.5
1.0
–0.5
10 30
V
SENSE
= 100mV
V
CC
= 3V
T
A
= 25°C
OUTPUT
V
EE
OUT
6100 F03
R
SENSE
FUSE
LT6100
81
V
S
–
V
S
+
A4
2
V
CC
A2
3
4
7
C2
0.1µF
C1
0.1µF
DC
SOURCE
5V
6
5
FIL
TO LOAD
–
+
+
Figure 2. V
OS
vs V
S
Input Voltage Figure 3. Current Monitoring of a Fuse Protected Circuit