Diodes ZXCT1011 - Manual (Page 6)

ZXCT1011
Issue 1- July 2005 6 www.zetex.com
© Zetex Semiconductors plc 2005
A 1 ampere load current is to be measured into a battery. The output signal required to an over
current controller needs to be 3.3V. What is a suitable value of R
out
and what will the error due
to the supply current be?
V
in
= 5.0V - 9.0V
I
load
= 1.0A
Output signal required: 3.3V
Figure 1 ZXCT1011 to measure a 1.0 A load current
To obtain the highest accuracy, choose the of R
sense
to give 100mV:
R
sense
= 0.1/1.0 = 0.1
⍀
The transconductance using a 120 ⍀ resistor will be 10mA/V.
Choose R
out
to give V
out
= 3.3V, when V
sense
= 100mV
Rearranging equation 1 without the offset error gives:
R
out
= (V
out
x R
shunt
) / (1.2 x I
Load
x R
sense
)
R
out
= 3.3k
⍀
Now consider the offset error for this resistor:
Offset due to supply current = R
out
x 4x10
-6
µA = 13.2mV = 0.4%
V
sense+
ZXCT
1011
sense-
V
V
in R
sense
I load = 1A
I
out
2
5 4
R
R
shunt
3
out
R load
R shunt
120R
3.3V
= 5.0V - 9.0V