LTC6104 LINER [Linear Technology], LTC6104 Datasheet - Page 12

• Manufacturer Part Number: LTC6104
• Description: High Voltage, High Side, Bi-Directional Current Sense Amplifi er
• Manufacturer: LINER [Linear Technology]

LTC6104
APPLICATIONS INFORMATION
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 fl ows through R
enters the current sense amp via the –IN pin. The power
dissipated in the LTC6104 due to the output signal is:
There is also power dissipated due to the quiescent sup-
ply current:
The total power dissipated is the output dissipation plus
the quiescent dissipation:
At maximum supply and maximum output current, the
total power dissipation can exceed 100mW. This will cause
signifi cant 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 θ
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:
12
P
P
P
P
P
θ
T
= 39.6°C
T
P
119.6°C
T
JA
RISE
MAX
MAX
OUT
Q
TOTAL
Q(MAX)
OUT(MAX)
TOTAL(MAX)
= I
= 300C˚/W
≈ V
= θ
= T
must be <150°C
S
= P
• V
= I
JA
AMBIENT
S
S
S(MAX)
• |I
OUT
= I
• P
≈ 132mW and the max die temp will be
OUT
OUT
TOTAL(MAX)
+ P
|
• V
+ T
• V
Q
+
RISE
+
(MAX) = 1.2mA • 60V = 72mW
(MAX) = 1mA • 60V = 60mW
JA
= 300C°/W • (72mW + 60mW)
= 80˚C + 39.6˚C = 119.6˚C
value to fi nd the maximum
IN
and
If this same circuit must run at 125°C, the maximum die
temperature will exceed 150°C. (Note that supply current,
and therefore P
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, V
makes fi ltering straightforward. Any circuit may be used
which generates the required Z
response. For example, a capacitor in parallel with R
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:
Useful Equations
Input Voltage: V
Current Gain:
f
Voltage
Transconductance:
Transimpedance:
–
3
dB
=
2
G G ain:
• •
π
Q
, is proportional to temperature. Refer to
I I
R
V
SENSE
I
SENSE
OUT
V
OUT
SENSE
1
OUT
I
SENSE
V
OUT
•
OUT
V
C
=
I
SEN
= I
OUT
, is simply I
=
OUT
R
SENSE
R
S S E
SENSE
R
=
R
OUT
OUT
IN
IN
R
=
SENSE
R
to get the desired fi lter
•
1
IN
R
SENSE
OUT
•
R
R
O O UT
IN
• Z
OUT
. This
OUT
6104f