Datasheet

LT2940
13
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 specifi ed accuracy is V
V
• V
I
= ±0.4V
2
, and it
produces nominally full-scale output currents of I
PMON
=
±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 specifi ed 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 +0.4V
2
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 specifi ed. 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 I
PMON
= 200µA load
line (A) on Figure 5. Spans of voltage or current up to 4:1
naturally fi t 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 I
PMON
= 50µA in Figure 5 covers a span of 16:1 (V
V
= 8V
to 0.5V and V
I
= 200mV to 12.5mV). Note that operating
along line (C), I
PMON
= 25µA allows a span of 32:1, but the
channel offsets reduce the value of doing so. Operating
V
V
(V)
–12
V
I
(mV)
–100
150
200
250
300
–8
–4
0
–200
50
–150
100
–250
–300
0
–50
–10
–6
4
12
–2
2
8106
2940 F04
CURRENT SENSE CLIPPED
LIMITED
BY PMON
COMPLIANCE
II I
III IV
GUARANTEED
ACCURACY
CURRENT SENSE CLIPPED
VOLTAGE SENSE CLIPPED
VOLTAGE SENSE CLIPPED
LIMITED
BY PMON
COMPLIANCE
Figure 5. Various Constant-Power Curves in Quadrant I
2940 F05
V
V
(V)
V
I
(mV)
100
400
1
4
25
50
12.5
200
0.5
2
8
16
CURRENT SENSE CLIPPING
I
PMON
= 200µA
GUARANTEED
ACCURACY
(A)
25µA
50µA
100µA
VOLTAGE SENSE CLIPPING
I
(D)
(B)(C)
Figure 4. Multiplier Operating Regions vs Sense Input
Voltages. Accuracy Is Derataed in Shaded Areas