Datasheet

LT6108-1/LT6108-2
15
610812fa
APPLICATIONS INFORMATION
Output Current Limitations Due to Power Dissipation
The LT6108 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 LT6108 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
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 LT6108 due to the comparator is often insignificant
and can be calculated as follows:
P
OUTC
= (V
OUTC
– V
–
) • I
OUTC
The total power dissipated is the sum of these
dissipations:
P
TOTAL
= P
OUTA
+ P
OUTC
+ P
S
At maximum supply and maximum output currents, the
total power dissipation can exceed 150mW. This will cause
significant heating of the LT6108 die. In order to prevent
damage to the LT6108, the maximum expected dissipa-
tion in each application should be calculated. This number
can be multiplied by the θ
JA
value, 163°C/W for the MS8
package or 64°C/W for the DFN, 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
be reduced if an external resistor, R
IN
+
, is connected as
shown in Figure 5, the error is then reduced to:
V
OUT(IBIAS)
= ±R
OUT
• I
OS
; I
OS
= I
B
+
– I
B
–
Minimizing low current errors will maximize the dynamic
range of the circuit.
Figure 6. Gain Error vs Resistor Tolerance
Figure 5. R
IN
+
Reduces Error Due to I
B
SENSEHI
LT6108
I
SENSE
R
SENSE
V
+
7
V
–
4
V
+
R
IN
V
BATT
SENSELO
8
1
OUTA 6
610812 F05
R
OUT
V
OUT
R
IN
+
–
+
RESISTOR TOLERANCE (%)
0.01
0.01
RESULTING GAIN ERROR (%)
0.1
1
10
0.1 1 10
610812 F06
R
IN
= 100Ω
R
IN
= 1k
Output Voltage Error, ∆V
OUT(GAIN ERROR)
, Due to
External Resistors
The LT6108 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.