Applications information, Zxct1050, Gain 20 r – Diodes ZXCT1050 User Manual
Page 7
ZXCT1050
© Zetex Semiconductors plc 2007
Applications information
The ZXCT1050 is a current output version of the ZXCT1051 and as such uses a separate power
supply pin. All biasing for the internal amplifiers comes from its separate V
CC
input and is not ‘line
powered’, unlike the ZXCT1021.
This means that at very small sense voltages the ZXCT1050 draws very little current (<1
A) from
the lines being sensed.
The separate V
CC
pin enables the ZXCT1050 to be operated at sense line voltages down to 0V,
where the ZXCT1021 would switch off. This feature enables the ZXCT1050 to be used to sense the
currents flowing through lines that have been shorted to ground.
Basic operation
Load current, I
L
, from V
RAIL
is drawn through R
SENSE
developing a voltage V
SENSE
across the
sense inputs of the ZXCT1050.
The internal amplifier forces V
SENSE
across external resistance R
SH
(internal on the ZXCT1051)
causing a current to flow through transistor Q1 and out of the output pin, OUT. This current is then
converted to a voltage by a resistor, R
G
, between OUT and GND.
The overall gain of the ZXCT1050 is determined by the following expression:
A ratio of 1:2 between R
SH
and R
G
creates the fixed gain of 10 with an output impedance equal to
RG (see electrical characteristics for output current-voltage characteristics).
The ZXCT1050 has both R
G
and R
SH
external. This allows R
G
and R
SH
to be varied so that the
required gain can be achieved at the required output impedance.
For low power applications both R
G
and R
SH
can be increased whereas for driving low impedance
R
G
and R
SH
can be decreased.
The maximum recommended value for R
G
is 40k
⍀ and the maximum recommended value for
RSH is 10k
⍀. Large values of R
SH
start increasing the effective input offset error, while large values
of R
G
can created load errors and reduce bandwidths.
The maximum differential input voltage, V
SENSE
, is 150mV (I
L
* R
SENSE
); however voltages up to
500mV will not damage it. This can be increased further by the inclusion of a resistor, R
LIM
,
between the SENSE+ pin and the rail being sensed, V
RAIL
.
For best performance R
SENSE
should be connected as close to the SENSE+ and SENSE- pins thus
minimizing any series resistance with R
SENSE
.
ZXCT1050
SENSE+ (4)
Out
(3)
(5) SENSE-
V
CC
(1)
V
RAIL
R
SENSE
R
SH
I
L
Load
Q1
R
G
GND (2)
GND
GAIN
20
R
G
R
SH
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