Datasheet

LTC6101/LTC6101HV
11
6101fh
APPLICATIONS INFORMATION
6101 F03b
–
+
–
+
–
+
R
5
7.5k
V
IN
301301
V
OUT
I
LOAD
LTC6101
R
SENSE LO
100m
M1
Si4465
10k
CMPZ4697
7.5k
V
IN
1.74M
4.7k
Q1
CMPT5551
40.2k
3
4
5
6
12
8
7
619k
HIGH
RANGE
INDICATOR
(I
LOAD
> 1.2A)
V
LOGIC
(3.3V TO 5V)
LOW CURRENT RANGE OUT
2.5V/A
( V
LOGIC
+5V ) ≤ V
IN
≤ 60V
0 ≤ I
LOAD
≤ 10A
HIGH CURRENT RANGE OUT
250mV/A
301 301
LTC6101
R
SENSE HI
10m
V
LOGIC
BAT54C
LTC1540
V
+
V
–
OUT
–IN+IN
V
+
V
–
OUT
–IN+IN
Selection of External Input Resistor, R
IN
The external input resistor, R
IN
, controls the transconduc-
tance of the current sense circuit. Since I
OUT
= V
SENSE
/R
IN
,
transconductance g
m
= 1/R
IN
. For example, if R
IN
= 100,
then I
OUT
= V
SENSE
/100 or I
OUT
= 1mA for V
SENSE
= 100mV.
R
IN
should be chosen to allow the required resolution
while limiting the output current. At low supply voltage,
I
OUT
may be as much as 1mA. By setting R
IN
such that
the largest expected sense voltage gives I
OUT
= 1mA, then
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current and the maximum allowed output voltage, as
well as the minimum practical output signal. If less dynamic
range is required, then R
IN
can be increased accordingly,
reducing the max output current and power dissipation.
If low sense currents must be resolved accurately in a
system that has very wide dynamic range, a smaller R
IN
than the max current spec allows may be used if the max
current is limited in another way, such as with a Schottky
diode across R
SENSE
(Figure 3a). This will reduce the high
current measurement accuracy by limiting the result, while
increasing the low current measurement resolution.
Figure 3b. Dual LTC6101s Allow High-Low Current Ranging
V
+
LOAD
D
SENSE
6101 F03a
R
SENSE
Figure 3a. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
This approach can be helpful in cases where occasional
large burst currents may be ignored. It can also be used
in a multirange confi guration where a low current circuit
is added to a high current circuit (Figure 3b). Note that
a comparator (LTC1540) is used to select the range, and
transistor M1 limits the voltage across R
SENSE LO
.
Care should be taken when designing the board layout
for R
IN,
especially for small R
IN
values. All trace and inter-
connect impedances will increase the effective R
IN
value,
causing a gain error. In addition, internal device resistance
will add approximately 0.2 to R
IN
.
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