●
Auxiliary comparator with complementary
outputs
As configured, DC-1495A features:
●
Single quadrant power and current meas-
urement with unidirectional outputs
●
8V to 40V, 0A to 3A measurement range
●
30W full scale power monitor output, 10W/V
●
3A full scale current monitor output, 1A/V
●
75V maximum input, limited by clamp
●
V
CC
connected to INPUT turret
●
LEDs indicate load power above (red) or be-
low (green) 12.5W
DC-1495A includes provision for optional compo-
nents, allowing the board to be reconfigured for
different scaling and a variety of applications.
Components
Components are divided into four basic groups:
R12A, R12B, R12C, C1, D1, D2: these compo-
nents are associated with V
CC
(D1 supplies con-
stant current to the LEDs). A zero ohm jumper
installed in any one of the R12 positions selects
the source of power for V
CC
: input, output, or ex-
ternal supply connected to the V
CC
turret. Use a
resistor if filtering is desired. As built R12B is
stuffed with 10
Ω
and V
CC
is bypassed with 100nF,
serving both as a V
CC
filter and also as a snubber
for the input.
An SMAT70A clamp diode, D2, is connected di-
rectly across the input to ground. It has a maxi-
mum dc standoff rating of 75V. Do not exceed
75Vdc or D2 will be permanently damaged. The
LT2940 can tolerate up to 100V input absolute
maximum on the V
CC
, I+, I– and LATCH pins. Nev-
ertheless, when connecting to supplies greater
than 50V it is possible to exceed 100V owing to
ringing. D2 is included to prevent destruction of
the LT2940 while performing bench tests.
R9A, C2, R9B: configuration resistors and noise
filter for the LATCH pin. As stuffed the comparator
operates in “flow-through” mode; install 47k
Ω
at
R9B or tie the LATCH turret high (>2.5V) for latch-
ing behavior. Grounding the LATCH turret clears
the comparator after latching.
RJ-2, R2A, R1A, R1C, R2B, R1B: divider compo-
nents for voltage sense pins V+ and V–. As
stuffed, R2A and R1A form a 5:1 divider that moni-
tors the output voltage, with a 40V full scale value.
By re-arranging the components it is possible to
achieve a variety of configurations for single-ended
and differential voltage measurements.
C3, R3, RJ-4, RJ-5, R4B, R4A, C4, R5B, R5A, C5:
configuration and scaling resistors and integration
or filter capacitors for PMON, IMON and the com-
parator input, CMP+. DC-1495A is stuffed with
15k
Ω
load resistors for PMON and IMON (R4A
and R5A), and RJ-4 connects PMON to CMP+.
Positions are provided for 3 sense resistors, allow-
ing for high current operation or for combining two
or three resistors in parallel to achieve a specific
value. Sufficient copper is present on the circuit
board to handle more than 10A; beyond this point
DC-1495A can be connected to an off-board sense
resistor or shunt using the INPUT and OUTPUT
terminals. In this case remove RS1, RS2, and
RS3. The I+ and I– terminals allow precise Kelvin
examination of the current sense signal, as seen by
the LT2940.
Multiplier Operation
The LT2940 has two differential inputs which we
will call V
V
and V
I
. V
V
is the voltage across the V+
and V– voltage input pins, and V
I
is the voltage
across the I+ and I– current sense input pins. The
voltages at these inputs are multiplied together by
a four-quadrant Gilbert cell, producing a current at
the PMON output proportional to the product of V
V
and V
I
. Specifically,
PMON Output Current = V
V
×
V
I
×
500µA/V
2
Where 500µA/V
2
is the gain or transfer function of
the multiplier, and a V
V
×
V
I
product of 0.4V
2
drives the PMON output to 200µA full scale.
Although the multiplier core can handle a maxi-
mum V
V
×
V
I
product of just ±0.4V
2
, the voltage
