Datasheet

LT6109-1/LT6109-2
12
610912fa
APPLICATIONS INFORMATION
Selection of External Input Gain Resistor, R
IN
R
IN
should be chosen to allow the required speed and
resolution while limiting the output current to 1mA. The
maximum value for R
IN
is 1k to maintain good loop sta-
bility. For a given V
SENSE
, larger values of R
IN
will lower
power dissipation in the LT6109 due to the reduction
in I
OUT
while smaller values of R
IN
will result in faster
response time due to the increase in I
OUT
. If low sense
currents must be resolved accurately in a system that has
a very wide dynamic range, a smaller R
IN
may be used
if the maximum I
OUTA
current is limited in another way,
such as with a Schottky diode across R
SENSE
(Figure 4).
This will reduce the high current measurement accuracy
by limiting the result, while increasing the low current
measurement resolution.
This approach can be helpful in cases where occasional
bursts of high currents can be ignored.
Care should be taken when designing the board layout for
R
IN
, especially for small R
IN
values. All trace and inter-
connect resistances will increase the effective R
IN
value,
causing a gain error.
The power dissipated in the sense resistor can create a
thermal gradient across a printed circuit board and con-
sequently a gain error if R
IN
and R
OUT
are placed such
that they operate at different temperatures. If significant
power is being dissipated in the sense resistor then care
Figure 3. LT6109-1 Typical Connection
OUTA
I
OUTA
–
+
–
+
V
+
C1
SENSEHI
INC2
INC1
5
4
3
2
1
R1*
610912 F03
V
–
V
+
V
+
V
–
LT6109-1
SENSELO
EN/RST
OUTC2
V
RESET
R
C
V
PULLUP
LOAD
V
SUPPLY
V
SENSE
R
SENSE
UNDERCURRENT
FLAG
OVERCURRENT
FLAG
R
IN
+
–
OUTC1
*R
OUT
= R1 + R2 + R3
–
+
V
–
V
–
V
+
I
SENSE
=
V
SENSE
R
SENSE
R
C
R2*
6
7
8
9
10
R3* C
L
V
OUT
400mV
REFERENCE
C
LC
C
LC
D
SENSE
R
SENSE
V
+
LOAD
610912 F04
Figure 4. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging