LT6105
15
6105fa
Error Sources
The current sense system uses an amplifi er, current mirrors
and external resistors to apply gain and level shifting. The
output is then dependent on the matching characteristics
of the current mirrors, characteristics of the amplifi er such
as gain and input offset, as well as matching of external
resistors. Ideally, the circuit output is:
VV
R
R
VIR
OUT SE NS E
OUT
IN
SENSE SENSE SENSE
==•; •
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. Mismatch in the
internal current mirror adds to gain error but is trimmed
to less than 0.3%. Offset voltage and sense input current
are the main cause of any additional error.
Error Due to Input Offset Voltage
Dynamic range is inversely proportional to the input offset
voltage. Dynamic range can be thought of as the maximum
V
SENSE
divided by V
OS
. The offset voltage of the LT6105
is typically only ±100μV.
Error Due to Sense Input Offset Current
Input offset current or mismatches in input bias current will
introduce an additional input offset voltage term. Typical
input offset current is 0.05μA. Lower values of R
IN
will
keep this error to a minimum. For example, if R
IN
= 100Ω,
then the additional offset is 5μV.
Output Current Limitations Due to Power Dissipation
The LT6105 can deliver up to 1mA continuous current to
the output pin. This output current, I
OUT
, is the mirrored
current which fl ows through R
IN2
and enters the current
sense amp via the +IN pin for V
–IN
> 1.6V, and exits out of
–IN through R
IN1
for V
–IN
< 1.6V. The total power dissipa-
tion due to input currents, P
IN
, and the dissipation due to
internal mirrored currents, P
Q
:
P
TOTAL
= P
IN
+ P
Q
P
IN
= (V
+IN
) • I
RIN2
; V
–IN
> 1.6V
or
P
IN
= (V
+
– (V
–IN
)) • I
RIN1
; V
–IN
< 1.6V
Since the current exiting –IN is coming from V
+
, the voltage
is V
+
– V
–IN
. Taking the worst case V
–IN
= 0V, the above
equation becomes:
P
IN
≅ V
+
• I
RIN1
, for V
–IN
< 1.6V.
The power dissipated due to internal mirrored currents:
P
Q
= 2 • I
OUT
• V
+
The factor of 2 is the result of internal current shifting and
1:1 mirroring.
At maximum supply and maximum output current, the
total power dissipation can exceed 100mW. This will
cause signifi cant heating of the LT6105 die. In order to
prevent damage to the LT6105, the maximum expected
dissipation in each application should be calculated. This
number can be multiplied by the θ
JA
value listed in the Pin
Confi guration section 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
LT6105 in the MSOP package is to be run at V
S
+
= 44V and
V
+
= 36V with 1mA output current at 80°C ambient:
P
Q(MAX)
= 2 • I
OUT(MAX)
• V
+
= P
Q(MAX)
= 72mW
P
IN(MAX)
= I
RIN2(MAX)
• V
+IN(MAX)
= 44mW
T
RISE
= θ
JA
• P
TOTAL(MAX)
T
MAX
= T
AMBIENT
+ T
RISE
T
MAX
must be < 150°C
P
TOTAL(MAX)
= 116mW and the maximum die temperature
will be 109°C. If this same circuit must run at 125°C ambi-
ent, the maximum die temperature will increase to 150°C.
Note that supply current, and therefore P
Q
, is proportional
to temperature. Refer to the Typical Performance Charac-
teristics section. In this condition, the maximum output
current should be reduced to avoid device damage. The
DCB package, on the other hand, has a lower θ
JA
and
subsequently, a lower die temperature increase than the
MSOP. With the same condition as above, the DCB will
rise only 7.5°C to 87.5°C and 132.5°C, respectively.
It is important to note that the LT6105 has been designed
to provide at least 1mA to the output when required, and
can deliver more under large V
SENSE
conditions. Care must
be taken to limit the maximum output current by proper
choice of sense resistor and input resistors.
APPLICATIONS INFORMATION
