Datasheet

LT2940
12
2940f
APPLICATIONS INFORMATION
the full-scale output current of ±200µA, the achievable
full-scale power is:
P
V
kk
IN FS
VI
()
.
•
=
04
2
(10)
In some applications the PMON output is converted to a
voltage by a load resistor:
V
PMON
= I
PMON
• R
PMON
(11)
The complete end-to-end scaling is then given by:
V
PMON
= P
IN
• K
PMON
• k
V
• k
I
• R
PMON
(12)
The current monitor output current at IMON is found by
combining Equations 4 and 8a:
I
IMON
= I
IN
• G
IMON
• k
I
(13)
The output current may be positive (sourcing) or negative
(sinking) depending on the signs of I
IN
and k
I
. Provided
that the magnitude of V
I
does not exceed 200mV, at the
full-scale output current of ±200µA the achievable full-
scale input current is:
I
V
k
IN FS
I
()
.
=
02
(14)
If IMON current is converted to a voltage by a load resis-
tor, then:
V
IMON
= I
IMON
• R
IMON
(15)
and the fi nal end-to-end scaling is given by:
V
IMON
= I
IN
• G
IMON
• k
I
• R
IMON
(16)
Accuracy
The principal accuracies of the power and current monitor
outputs are characterized as absolute percentages of full-
scale output currents, using the nominal values of scaling
parameters. The total error of the I
PMON
output, E
PMON
, is
typically ±2%, and is defi ned as:
E
I
µA
V
VV
µA
PMON
PMON V I
=
− 500
200
100
2
•( • )
•%
(17)
Contributors to the power output accuracy such as the scal-
ing (K
PMON
), the output offset (I
PMON(OS)
), and the voltage
and current sense input offsets (V
V(OSP)
and V
I(OSP)
), are
separately specifi ed at key conditions and may be totaled
using the root sum-of-squares (RSS) method.
The total error of the I
IMON
output, E
IMON
, is typically
±1.5%, and is defi ned as:
E
I
A
V
V
A
IMON
IMON I
=
− 1000
200
100
μ
μ
•
•%
(18)
Contributors to the current output accuracy such as the
scaling (G
IMON
) and the current sense input offset (V
I(OSI)
)
are separately specifi ed at key conditions. Here again, use
the RSS method of totaling errors.
Figure 3. Basic Power Sensing Application Showing Derivation of k
V
and k
I
LT2940
P
IN
= V
IN
• I
IN
2940 F03
7
8
11 10
PMON
R1
LOAD
V
+
V
–
I
+
I
–
4
IMON
5
mk
V
=
V
I
= I
IN
• R
SENSE
mk
I
= R
SENSE
V
V
= V
IN
•
R1 + R2
R1
R1 + R2
R1
V
IN
R2
R
SENSE
I
IN
R
IMON
V
IMON
I
IMON
R
PMON
V
PMON
I
PMON
+
–
+
–