Datasheet

LT6109-1/LT6109-2
14
610912fa
APPLICATIONS INFORMATION
There is also power dissipated due to the quiescent power
supply current:
P
S
= I
S
• V
+
The comparator output current flows into the comparator
output pin and out of the V
–
pin. The power dissipated in
the LT6109 due to each comparator is often insignificant
and can be calculated as follows:
P
OUTC1,C2
= (V
OUTC1,C2
– V
–
) • I
OUTC1,C2
The total power dissipated is the sum of these
dissipations:
P
TOTAL
= P
OUTA
+ P
OUTC1
+ P
OUTC2
+ P
S
At maximum supply and maximum output currents, the
total power dissipation can exceed 100mW. This will
cause significant heating of the LT6109 die. In order to
prevent damage to the LT6109, the maximum expected
dissipation in each application should be calculated. This
number can be multiplied by the θ
JA
value, 160°C/W, to
find the maximum expected die temperature. Proper heat
sinking and thermal relief should be used to ensure that
the die temperature does not exceed the maximum rating.
Output Filtering
The AC output voltage, V
OUT
, is simply I
OUTA
• Z
OUT
. This
makes filtering straightforward. Any circuit may be used
which generates the required Z
OUT
to get the desired filter
response. For example, a capacitor in parallel with R
OUT
will give a lowpass response. This will reduce noise at the
output, and may also be useful as a charge reservoir to
keep the output steady while driving a switching circuit
such as a MUX or ADC. This output capacitor in parallel
with R
OUT
will create an output pole at:
f
–3dB
=
1
2 • π •R
OUT
• C
L
SENSELO, SENSEHI Range
The difference between V
BATT
(see Figure 7) and V
+
, as
well as the maximum value of V
SENSE
, must be considered
to ensure that the SENSELO pin doesn’t exceed the range
listed in the Electrical Characteristics table. The SENSELO
and SENSEHI pins of the LT6109 can function from 0.2V
Figure 6. Gain Error vs Resistor Tolerance
SENSEHI
LT6109
I
SENSE
R
SENSE
V
+
9
V
–
5
V
+
R
IN
V
BATT
SENSELO
10
1
OUTA 8
610912 F05
R
OUT
V
OUT
R
IN
+
–
+
Figure 5. R
IN
+
Reduces Error Due to I
B
Output Voltage Error, ∆V
OUT(GAIN ERROR)
, Due to
External Resistors
The LT6109 exhibits a very low gain error. As a result,
the gain error is only significant when low tolerance
resistors are used to set the gain. Note the gain error is
systematically negative. For instance, if 0.1% resistors
are used for R
IN
and R
OUT
then the resulting worst-case
gain error is –0.4% with R
IN
= 100Ω. Figure 6 is a graph
of the maximum gain error which can be expected versus
the external resistor tolerance.
Output Current Limitations Due to Power Dissipation
The LT6109 can deliver a continuous current of 1mA to the
OUTA pin. This current flows through R
IN
and enters the
current sense amplifier via the SENSEHI pin. The power
dissipated in the LT6109 due to the output signal is:
P
OUT
= (V
SENSEHI
– V
OUTA
) • I
OUTA
Since V
SENSEHI
≈ V
+
, P
OUTA
≈ (V
+
– V
OUTA
) • I
OUTA
RESISTOR TOLERANCE (%)
0.01
0.01
RESULTING GAIN ERROR (%)
0.1
1
10
0.1 1 10
610912 F06
R
IN
= 100Ω
R
IN
= 1k