lt2940 Linear Technology Corporation, lt2940 Datasheet - Page 12

• Manufacturer Part Number: lt2940
• Description: Power And Current Monitor
• Manufacturer: Linear Technology Corporation

LT2940
APPLICATIONS INFORMATION
the full-scale output current of ±200μA, the achievable
full-scale power is:
In some applications the PMON output is converted to a
voltage by a load resistor:
The complete end-to-end scaling is then given by:
The current monitor output current at IMON is found by
combining Equations 4 and 8a:
The output current may be positive (sourcing) or negative
(sinking) depending on the signs of I
that the magnitude of V
full-scale output current of ±200μA the achievable full-
scale input current is:
If IMON current is converted to a voltage by a load resis-
tor, then:
and the fi nal end-to-end scaling is given by:
12
V
V
I
V
V
P
I
IMON
IN FS
IN FS
PMON
PMON
IMON
IMON
( )
( )
= I
= I
= I
=
= I
= P
=
IN
0 2
IMON
IN
k
PMON
0 4
V
P
IN
.
k
• G
IN
V
IN
.
• G
I
V
= V
• K
•
V
IMON
• R
k
IMON
IN
2
• R
PMON
R2
R1
I
• I
IMON
IN
PMON
• k
• k
Figure 3. Basic Power Sensing Application Showing Derivation of k
8
7
I
• k
I
V
V
does not exceed 200mV, at the
I
+
–
• R
V
• k
R
11 10
IMON
SENSE
I
I
IN
+
I
• R
I
–
PMON
IN
and k
+
–
+
–
I
. Provided
PMON
LT2940
(10)
(11)
(12)
(13)
(14)
(15)
(16)
IMON
2940 F03
LOAD
5
4
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
typically ±2%, and is defi ned as:
Contributors to the power output accuracy such as the scal-
ing (K
and current sense input offsets (V
separately specifi ed at key conditions and may be totaled
using the root sum-of-squares (RSS) method.
The total error of the I
±1.5%, and is defi ned as:
Contributors to the current output accuracy such as the
scaling (G
are separately specifi ed at key conditions. Here again, use
the RSS method of totaling errors.
R
R
IMON
PMON
E
E
PMON
IMON
PMON
I
I
IMON
PMON
IMON
=
V
V
=
), the output offset (I
IMON
PMON
I
I
IMON
PMON
) and the current sense input offset (V
V
− 1000
V
200
V
= V
− 500
I
V
= I
IN
and k
IN
200
•
μ
IMON
• R
R1 + R2
A
SENSE
R1
I
µA
V
μ
µA
V
A
2
output, E
• (
PMON(OS)
•
V
V
V(OSP)
I
PMON
k
k
V
I
•
V
= R
=
• )
100
R1 + R2
SENSE
V
I
IMON
R1
output, E
), and the voltage
and V
%
•
, is typically
100
I(OSP)
PMON
%
I(OSI)
), are
(17)
(18)
2940f
, is
)