LTC6103
11
6103f
APPLICATIONS INFORMATION
If the power dissipation of the sense resistor is chosen to
be less than 0.5W then:
R
mW
I
m
SENSE
SENSE MAX
≤=
500
500
2
()
Ω
V
SENSE(MAX)
= I
SENSE(MAX)
• R
SENSE
= 500mV
Gain
R
R
V
V
V
mV
OUT
IN
OUT MAX
SENSE MAX
== = =
()
()
3
500
6
If the maximum output current, I
OUT
, is limited to 1mA:
R
V
mA
k value and
R
k
OUT
IN
=≈
=≈
3
1
301 1
3
6
499
.(% )
Ω (% )1 value
The output error due to DC offset is ±510µV (typ) and the
error due to offset current:
I
OS
is 3kΩ × 100nA = ±300µV (typical)
The maximum output error can therefore reach ±810µV
or 0.027% (–71dB) of the output full scale. Considering
the system input 60dB dynamic range (I
SENSE
= 1mA to
1A), the 71dB performance of the LTC6103 makes this
application feasible.
In many applications the power dissipation of the sense
resistor is of greater importance than the precision of the
measurement. Designing for a V
SENSE(MAX)
of as low as
100mV is recommended in such cases.
Output Current Limitations Due to Power Dissipation
The LTC6103 can deliver up to 1mA continuous current to
the output pin. This current fl ows through R
IN
and enters
the current sense amp via the –IN pin. The power dissipated
in the LTC6103 due to the output signal is:
P
OUT
= (V
IN
–
– V
OUT
) • I
OUT
Since V
IN
–
≈ V
S
, P
OUT
≈ (V
S
– V
OUT
) • I
OUT
There is also power dissipated due to the quiescent sup-
ply current:
P
Q
= I
S
• V
S
The total power dissipated is the output dissipation plus
the quiescent dissipation:
P
TOTAL
= P
OUTA
+ P
OUTB
+ P
QA
+ P
QB
At maximum supply and maximum output current, the
total power dissipation can exceed 100mW. This will
cause signifi cant heating of the LTC6103 die. In order to
prevent damage to the LTC6103, the maximum expected
dissipation in each application should be calculated. This
number can be multiplied by the θ
JA
value listed in the
Package/Order Information to fi nd the maximum expected
die temperature. This must not be allowed to exceed 150°C
or performance may be degraded.
As an example, if an LTC6103 in the MS8 package is to
be run at 55V ±5V supply with 0.5mA output current in
both amplifi ers at 80°C:
P
Q(MAX)
= I
S(MAX)
• V
+
(MAX) • 2 = 82.8mW
P
OUT(MAX)
= I
OUT
• V
+
(MAX) • 2 = 60mW
T
RISE
= θ
JA
• P
TOTAL(MAX)
= 300°C/W • (82.8mW +
60mW) ≈ 43°C
T
MAX
= T
AMBIENT
+ T
RISE
= 80°C + 43°C = 123°C
T
MAX
must be <150°C
P
TOTAL(MAX)
≈ 143mW and the maximum die tempera-
ture will be 123°C
If this same circuit must run at 125°C, the maximum die
temperature will exceed 150°C. (Note that supply current,
and therefore P
Q
, is proportional to temperature. Refer to
the Typical Performance Characteristics.) In this condition,
the maximum output current should be reduced to avoid
device damage. It is important to note that the LTC6103
has been designed to provide at least 1mA to the output
when required, and can deliver more depending on the
conditions. Care must be taken to limit the maximum
output current by proper choice of resistors and, if input
fault conditions exist, external clamps.
Output Filtering
The output voltage, V
OUT
, is simply I
OUT
• Z
OUT
. This
makes fi ltering straightforward. Any circuit may be used
which generates the required Z
OUT
to get the desired fi lter
response. For example, a capacitor in parallel with R
OUT
