OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT2940IDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT2940
10
2940f
APPLICATIONS INFORMATION
Introduction
The LT2940 power and current monitor brings together
circuits necessary to measure, monitor and control power.
In circuits where voltage is constant, power is directly
proportional to current. The LT2940 enables power moni-
toring and control in applications where both the current
and the voltage may be variable due to supply voltage
uncertainty, component parametric changes, transient
conditions, time-varying signals, and so forth.
The LT2940’s four-quadrant multiplier calculates in-
stantaneous power from its voltage sense and current
sense inputs. Its output driver sources and sinks cur-
rent proportional to power (magnitude and direction),
which affords fl exible voltage scaling, simple fi ltering
and, into a reference, bipolar signals. Its onboard
comparator is the fi nal piece required for integrated
power monitoring. In addition, the LT2940 provides a
proportional-to-current output that allows for equally
straightforward scaling, fi ltering and monitoring of the
sensed current.
Please note: although standard convention defi nes cur-
rents as positive going into a pin (as is generally the case
in the Electrical Characteristics table), the opposite is
true of the PMON and IMON pins. Throughout this data
sheet the power and current monitor output currents
are defi ned positive coming out of PMON and IMON,
respectively. Adopting this convention lets positive voltage
differences at the current and voltage sense pins yield
positive currents sourced from PMON and IMON that
can be scaled to positive ground referenced voltages
with a resistor.
Multiplier Operation
The LT2940 power and current monitor contains a four-
quadrant multiplier designed to measure the voltage and
current of a generator or load, and output signals propor-
tional to power and current. Figure 1 shows a signal path
block diagram. The operating ranges of the voltage sense
and current sense inputs are included. To simplify the
notation, the differential input voltages are defi ned as:
V
V
= V
V
+
– V
V
–
(1a)
V
I
= V
I
+
– V
I
–
(1b)
The full scale output of the multiplier core is ±0.4V
2
, which
the PMON output driver converts to current through a
scale factor of K
PMON
.
I
PMON
= K
PMON
• V
V
• V
I
(2)
K
µA
V
PMON
= 500
2
(3)
The voltage across the current sense input pins is converted
to a current by the IMON output driver through the scale
factor of G
IMON
.
I
IMON
= G
IMON
• V
I
(4)
G
µA
V
IMON
= 1000
(5)
Both the PMON and IMON outputs reach full-scale at
±200µA.
The headroom and compliance limits for the input and
output pins are summarized in Table 1 for easy reference.
It is important to note that the current sense inputs, I
+
and I
–
, operate over a 4V to 80V range completely inde-
pendent of the LT2940’s supply pin, V
CC
. Note also that
the inputs accept signals of either polarity, and that the
Figure 1. LT2940 Signal Path Diagram
LT2940
V
I
= V
I
+ – V
I
–
±200mV (MAX)
2940 F01
±200µA
FULL-SCALE
7
8
11 10
PMON
V
+
V
–
I
+
I
–
V
V
• V
I
= ±0.4V
2
FULL-SCALE
4
K
PMON
= 500
V
2
µA
±200µA
FULL-SCALE
IMON
5
G
IMON
= 1000
V
µA
V
V
= V
V
+ – V
V
–
±8V (MAX)
+
–
+
–
LT2940
11
2940f
Table 1. LT2940 Essential Operating Parameters to Achieve Specifi ed Accuracy (V
CC
Operating Range = 6V to 80V)
PARA-
METER
SENSE
INPUT
PINS
PIN VOLTAGE
LIMIT
INPUT
OPERATING
RANGE
SCALING
TO
OUTPUT
MONITOR
OUTPUT
PINS
OUTPUT
OPERATING
RANGE
OUTPUT
VOLTAGE COMPLIANCE
Voltage V
+
, V
–
0V to V
CC
– 3V at V
CC
≤ 12V
0V TO 9V at 12V < V
CC
< 30V
0V to 18V at V
CC
≥ 30V
V
V
= ±8V - - - -
Current I
+
, I
–
4V to 80V* V
I
= ±200mV G
IMON
=
1000µA /V
IMON I
IMON
=
±200µA
Sourcing:
0V to V
CC
– 4.5V at V
CC
≤ 7.5V
0V to 7.5V at 12V < V
CC
< 30V
0V to 12V at V
CC
≥ 30V
Sinking:
As Above, Except Minimum is 0.5V
Power V
+
, V
–
,
I
+
, I
–
See Above Limits V
V
• V
I
= ±0.4V
2
K
PMON
=
500µA/V
2
PMON I
PMON
=
±200µA
* The current sense range is completely independent of the supply voltage.
APPLICATIONS INFORMATION
PMON and IMON outputs are capable of indicating forward
and reverse fl ow of power and current, provided they are
advantageously biased.
The multiplier core full-scale product of ±0.4V
2
may
be reached over a range of voltage and current inputs,
as shown in Figure 2. For example, voltage sense and
current sense combinations of 8V and 50mV, 4V and
100mV, and 2V and 200mV each multiply to 0.4V
2
, and
thus produce 200µA at PMON. This arrangement allows
the core to operate at full-scale, and therefore at best ac-
curacy, over a 4:1 range of current and voltage, a readily
appreciated feature when monitoring power in variable
supply applications.
Essential Design Equations
A few equations are needed to calculate input scaling factors
and achieve a desired output. Consider the basic applica-
tion in Figure 3, where the power P
IN
is to be measured
as the product of voltage V
IN
and current I
IN
:
P
IN
= V
IN
• I
IN
(6)
The actual measured quantities V
IN
and I
IN
are scaled to be
level-compatible with the LT2940. In this basic application,
a simple resistive voltage divider scales V
IN
, and a sense
resistor scales I
IN
.
V
V
= V
IN
• k
V
(7a)
k
R
RR
V
=
+
1
12
(7b)
V
I
= I
IN
• k
I
(8a)
k
I
= R
SENSE
(8b)
The PMON output current is given by:
I
PMON
= K
PMON
• V
IN
• k
V
• I
IN
• k
I
(9a)
or
I
PMON
= P
IN
• K
PMON
• k
V
• k
I
(9b)
The output current may be positive (sourcing) or
negative (sinking) depending on the signs of V
IN
, k
V
,
I
IN
, and k
I
. Provided that the magnitudes of V
V
and V
I
do not exceed 8V and 200mV as shown in Figure 2, at
Figure 2. PMON Output Current as a Function
of Sense Input Voltages
2940 F02
V
V
= V
V
+
– V
V
–
(V)
V
I
= V
I
+ – V
I
–
(mV)
100
1
4
25
50
12.5
200
0.5
2
8
I
PMON
= 200µA
25µA
50µA
100µA
12.5µA
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
+
–
+
–
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24

LT2940IDD#PBF

Mfr. #:
Buy LT2940IDD#PBF
Manufacturer:
Analog Devices Inc.
Description:
Current & Power Monitors & Regulators Power & Current Monitor
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet