Datasheet

LT2940
11
2940f
Table 1. LT2940 Essential Operating Parameters to Achieve Specifi ed Accuracy (V
CC
Operating Range = 6V to 80V)
PARA-
METER
SENSE
INPUT
PINS
PIN VOLTAGE
LIMIT
INPUT
OPERATING
RANGE
SCALING
TO
OUTPUT
MONITOR
OUTPUT
PINS
OUTPUT
OPERATING
RANGE
OUTPUT
VOLTAGE COMPLIANCE
Voltage V
+
, V
–
0V to V
CC
– 3V at V
CC
≤ 12V
0V TO 9V at 12V < V
CC
< 30V
0V to 18V at V
CC
≥ 30V
V
V
= ±8V - - - -
Current I
+
, I
–
4V to 80V* V
I
= ±200mV G
IMON
=
1000µA /V
IMON I
IMON
=
±200µA
Sourcing:
0V to V
CC
– 4.5V at V
CC
≤ 7.5V
0V to 7.5V at 12V < V
CC
< 30V
0V to 12V at V
CC
≥ 30V
Sinking:
As Above, Except Minimum is 0.5V
Power V
+
, V
–
,
I
+
, I
–
See Above Limits V
V
• V
I
= ±0.4V
2
K
PMON
=
500µA/V
2
PMON I
PMON
=
±200µA
* The current sense range is completely independent of the supply voltage.
APPLICATIONS INFORMATION
PMON and IMON outputs are capable of indicating forward
and reverse fl ow of power and current, provided they are
advantageously biased.
The multiplier core full-scale product of ±0.4V
2
may
be reached over a range of voltage and current inputs,
as shown in Figure 2. For example, voltage sense and
current sense combinations of 8V and 50mV, 4V and
100mV, and 2V and 200mV each multiply to 0.4V
2
, and
thus produce 200µA at PMON. This arrangement allows
the core to operate at full-scale, and therefore at best ac-
curacy, over a 4:1 range of current and voltage, a readily
appreciated feature when monitoring power in variable
supply applications.
Essential Design Equations
A few equations are needed to calculate input scaling factors
and achieve a desired output. Consider the basic applica-
tion in Figure 3, where the power P
IN
is to be measured
as the product of voltage V
IN
and current I
IN
:
P
IN
= V
IN
• I
IN
(6)
The actual measured quantities V
IN
and I
IN
are scaled to be
level-compatible with the LT2940. In this basic application,
a simple resistive voltage divider scales V
IN
, and a sense
resistor scales I
IN
.
V
V
= V
IN
• k
V
(7a)
k
R
RR
V
=
+
1
12
(7b)
V
I
= I
IN
• k
I
(8a)
k
I
= R
SENSE
(8b)
The PMON output current is given by:
I
PMON
= K
PMON
• V
IN
• k
V
• I
IN
• k
I
(9a)
or
I
PMON
= P
IN
• K
PMON
• k
V
• k
I
(9b)
The output current may be positive (sourcing) or
negative (sinking) depending on the signs of V
IN
, k
V
,
I
IN
, and k
I
. Provided that the magnitudes of V
V
and V
I
do not exceed 8V and 200mV as shown in Figure 2, at
Figure 2. PMON Output Current as a Function
of Sense Input Voltages
2940 F02
V
V
= V
V
+
– V
V
–
(V)
V
I
= V
I
+ – V
I
–
(mV)
100
1
4
25
50
12.5
200
0.5
2
8
I
PMON
= 200µA
25µA
50µA
100µA
12.5µA