LTC6104
12
6104f
the high current path, as this will increase the voltage drop
and escalate this error.
Output Current Limitations Due to Power Dissipation
The LTC6104 can deliver up to ±1mA continuous current
to the output pin. This current ows through RIN and
enters the current sense amp via the –IN pin. The power
dissipated in the LTC6104 due to the output signal is:
POUT ≈ VS |IOUT|
There is also power dissipated due to the quiescent sup-
ply current:
PQ = IS VS
The total power dissipated is the output dissipation plus
the quiescent dissipation:
PTOTAL = POUT + PQ
At maximum supply and maximum output current, the
total power dissipation can exceed 100mW. This will cause
signicant heating of the LTC6104 die. In order to prevent
damage to the LTC6104, the maximum expected dissipa-
tion in each application should be calculated. This number
can be multiplied by the
θJA value to 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 LTC6104 in the MS8 package is to be
run at 55V ±5V supply with 1mA output current at 80°C:
PQ(MAX) = IS(MAX) V
+(MAX) = 1.2mA 60V = 72mW
POUT(MAX) = IOUT V
+(MAX) = 1mA 60V = 60mW
θJA = 300C/W
TRISE = θJA PTOTAL(MAX) = 300C°/W (72mW + 60mW)
= 39.6°C
TMAX = TAMBIENT + TRISE = 80C + 39.6C = 119.6C
PTOTAL(MAX) ≈ 132mW and the max die temp will be
119.6°C
TMAX must be <150°C
If this same circuit must run at 125°C, the maximum die
temperature will exceed 150°C. (Note that supply current,
and therefore PQ, is proportional to temperature. Refer to
Typical Performance Characteristics.) In this condition,
the maximum output current should be reduced to avoid
device damage. It is important to note that the LTC6104
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 sense resistor and, if
input fault conditions exist, external clamps.
Output Filtering
The output voltage, VOUT, is simply IOUT ZOUT. This
makes ltering straightforward. Any circuit may be used
which generates the required ZOUT to get the desired lter
response. For example, a capacitor in parallel with ROUT
will give a lowpass response. This will reduce unwanted
noise from the output, and may also be useful as a charge
reservoir to keep the output steady while driving a switch-
ing circuit such as a MUX or ADC. This output capacitor
in parallel with an output resistor will create a pole in the
output response at:
f
RC
dB
OUT
–
3
1
2
=
π
Useful Equations
Input Voltage: VSENSE = IR
Voltage
SENSE
G
Gain:
V
Current Gain:
I
OUT
V
R
SENSE
OUT
IN
=
II
R
V
SENSE
IN
SEN
=
Transconductance:
IOUT
S
SE
IN
SENSE
R
I
R
=
1
Transimpedance:
VOUT
O
OUT
IN
R
APPLICATIONS INFORMATION