LT2940CMS datasheet(13/24 Pages) LINER

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LT2940

2940f

APPLICATIONS INFORMATION

Multiplier Operating Regions

The operating regions of the four-quadrant multiplier are

illustrated in Figure 4. Note that while Figure 2’s axes em-

ployed logarithmic (octave) scales to allow constant-power

trajectories to be straight lines, Figure 4’s axes are linear

to better accommodate negative inputs. Constant-power

trajectories are thus arcs.

The heavy line circumscribing the guaranteed accuracy

region is limited both by the product of the sense inputs

(the curved edges) and by each sense input’s differential

range (the straight edges). The maximum product that

realizes the speciﬁed accuracy is VV • VI = ±0.4V2, and it

produces nominally full-scale output currents of IPMON =

±200μA. At the same time, the voltage and current sense

inputs must not exceed ±8V and ±200mV, respectively.

In the shaded functional region, multiplying occurs but

the output current accuracy is derated as speciﬁed in the

Electrical Characteristics section.

The shaded functional region offers headroom beyond the

guaranteed range in all quadrants, and excellent sourcing

current operation beyond the standard +0.4V2 sense prod-

uct limit in quadrants I and III. In quadrants II and IV, the

PMON current is limited by compliance range, so accuracy

is not speciﬁed. See the Electrical Characteristics and Typi-

cal Performance Characteristics sections for operation in

these regions. Inputs beyond those ranges, and out to the

absolute maximum ratings, are clipped internally.

Range and Accuracy Considerations

The LT2940’s performance and operating range may best

be exploited by letting the broad application category steer

design direction.

Constant-power applications comprise power level alarm

circuits, whether tripping a circuit breaker, activating aux-

iliary circuits, or simply raising an alarm, and single-level

power servo loops. In such applications, accuracy is best

when the full-scale output current of the LT2940 represents

the power level of interest, i.e., the IPMON = 200μA load

line (A) on Figure 5. Spans of voltage or current up to 4:1

naturally ﬁt into the operating range of the LT2940.

Special constant-power applications are the same types of

circuits (level measuring, servos) with additional restric-

tions. If operating within the guaranteed accuracy region

of Figure 4 is important over voltage or current spans

wider than 4:1, let a PMON current less than full-scale

represent the power level. For example, the load line (B)

of IPMON = 50μA in Figure 5 covers a span of 16:1 (VV = 8V

to 0.5V and VI = 200mV to 12.5mV). Note that operating

along line (C), IPMON = 25μA allows a span of 32:1, but the

channel offsets reduce the value of doing so. Operating

VV (V)

–12

–100

150

200

250

300

–8

–4

–200

–150

100

–250

–300

–50

–10

–6

–2

810

2940 F04

CURRENT SENSE CLIPPED

LIMITED

BY PMON

COMPLIANCE

III

GUARANTEED

ACCURACY

CURRENT SENSE CLIPPED

LIMITED

BY PMON

COMPLIANCE

Figure 5. Various Constant-Power Curves in Quadrant I

2940 F05

VV (V)

100

400

12.5

200

0.5

CURRENT SENSE CLIPPING

IPMON

= 200μA

GUARANTEED

ACCURACY

(A)

25μA

50μA

100μA

(D)

(B)

(C)

Figure 4. Multiplier Operating Regions vs Sense Input

Voltages. Accuracy Is Derataed in Shaded Areas

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