lt2940 Linear Technology Corporation, lt2940 Datasheet - Page 11

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

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
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
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.
Table 1. LT2940 Essential Operating Parameters to Achieve Specifi ed Accuracy (V
PARA-
METER
Voltage
Current
Power
* The current sense range is completely independent of the supply voltage.
Figure 2. PMON Output Current as a Function
of Sense Input Voltages
SENSE
INPUT
V
V
12.5
200
PINS
100
I
I
+
50
25
+
+
+
, V
, V
, I
, I
0.5
–
–
–
–
,
0V TO 9V at 12V < V
0V to V
0V to 18V at V
1
12.5μA
See Above Limits
V
V
PIN VOLTAGE
CC
25μA
4V to 80V*
= V
– 3V at V
V +
LIMIT
2
50μA
– V
V –
CC
100μA
(V)
CC
CC
≥ 30V
≤ 12V
4
< 30V
= 200μA
I
PMON
2940 F02
8
V
V
V
OPERATING
I
• V
V
= ±200mV
RANGE
INPUT
V
I
= ±8V
= ±0.4V
2
2
, and
may
2
1000μA /V
500μA / V
SCALING
OUTPUT
K
G
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
as the product of voltage V
The actual measured quantities V
level-compatible with the LT2940. In this basic application,
a simple resistive voltage divider scales V
resistor scales I
The PMON output current is given by:
or
The output current may be positive (sourcing) or
negative (sinking) depending on the signs of V
I
do not exceed 8V and 200mV as shown in Figure 2, at
PMON
IMON
IN
TO
-
P
V
V
k
I
I
, and k
k
PMON
PMON
I
IN
V
I
=
V
=
= R
= I
2
= V
= V
=
IN
SENSE
R
MONITOR
= K
= P
IN
OUTPUT
IN
I
CC
1
. Provided that the magnitudes of V
PMON
IMON
• k
PINS
R
• k
+
• I
Operating Range = 6V to 80V)
-
PMON
IN
1
I
R
V
IN
• K
2
IN
.
PMON
• V
OPERATING
OUTPUT
±200μA
I
±200μA
RANGE
I
PMON
IMON
IN
-
• k
• k
=
=
V
V
IN
• I
• k
As Above, Except Minimum is 0.5V
and current I
IN
I
0V to V
0V to 7.5V at 12V < V
IN
• k
0V to 12V at V
VOLTAGE COMPLIANCE
and I
I
IN
CC
is to be measured
– 4.5V at V
Sourcing:
IN
Sinking:
OUTPUT
are scaled to be
IN
-
IN
LT2940
, and a sense
:
CC
≥ 30V
CC
CC
V
11
≤ 7.5V
< 30V
and V
IN
(7b)
(8b)
(9b)
, k
(7a)
(8a)
(9a)
2940f
(6)
V
,
I