lt2940 Linear Technology Corporation, lt2940 Datasheet - Page 13

• Manufacturer Part Number: lt2940
• Description: Power And Current Monitor
• Manufacturer: Linear Technology Corporation

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