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LT2940

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 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.

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 deﬁned as:

VV = VV+ – VV–

(1a)

VI = VI+ – VI–

(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 KPMON.

IPMON = KPMON • VV • VI

(2)

µA

PMON = 500

(3)

The voltage across the current sense input pins is converted

to a current by the IMON output driver through the scale

factor of GIMON.

IIMON = GIMON • VI

(4)

µA

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, VCC. Note also that

the inputs accept signals of either polarity, and that the

Figure 1. LT2940 Signal Path Diagram

LT2940

VI = VI+ – VI–

±200mV (MAX)

2940 F01

±200μA

FULL-SCALE

11 10

PMON

V +

V –

I +

I –

VV • VI = ±0.4V

2 FULL-SCALE

KPMON = 500

μA

±200μA

FULL-SCALE

IMON

GIMON = 1000

μA

VV = VV+ – VV–

±8V (MAX)

–

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