LT2940_15 Datasheet, PDF (10/24 Pages) Linear Technology – Power and Current Monitor

LT2940
APPLICATIONS INFORMATION
Introduction
Multiplier Operation
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 flexible voltage scaling, simple filtering
and, into a reference, bipolar signals. Its onboard
comparator is the final piece required for integrated
power monitoring. In addition, the LT2940 provides a
proportional-to-current output that allows for equally
straightforward scaling, filtering and monitoring of the
sensed current.
Please note: although standard convention defines 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 defined 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.
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 defined as:
V_V = V_V+ – V_V–
(1a)
V _I = V _I+ – V _I–
(1b)
The full scale output of the multiplier core is ±0.4V², 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_PMON
=
500
µA
V²
(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)
µA
G_IMON = 1000 _V
(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
V_I = V_I+ – V_I–
±200mV (MAX)
V_V = V_V+ – V_V–
±8V (MAX)
V⁺
8
V^–
7
11 10
I⁺ I^–
μA
+ ^{GIMON = 1000 V}
LT2940
IMON
5
–
V_V • V_I = ±0.4V² FULL-SCALE
+
K_PMON
=
500
μA
V²
PMON
4
–
±200μA
FULL-SCALE
±200μA
FULL-SCALE
10
2940 F01
Figure 1. LT2940 Signal Path Diagram
2940f