Diodes ZXCT1107/1109/1110 - Manual (Page 12)

ZXCT1107/1109/1110
LOW POWER HIGH-SIDE CURRENT MONITORS
ZXCT1107/1109/1110
Document number: DS35033 Rev. 3 - 2
12 of 16
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JULY 2011
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Application Information (cont.)
Note that in order to avoid additional error, V
SUPPLY
must be at least 1.5V greater than V
OUT
. This margin depends on the
value of V
SENSE
. For V
SENSE
= 100mV, this margin can be reduced to 1.0V.
R
SENSE
is chosen based on the threshold V
SENSET
of sense voltage selected as above, for the chosen threshold load current,
I
LOADT
.
LOADT
SENSET
SENSE
I
V
R
=
Equation 4
R
GAIN
is chosen to give the required threshold output voltage, V
OUTT
. From Equation 1, the threshold output current is
I
OUTT
= 0.004 * V
SENSET
Equation 1a
OUTT
OUTT
GAIN
I
V
R
=
or
SENSET
OUTT
GAIN
V * 004 . 0
V
R
=
Equation 5
Design Example
A current monitor is required to provide an output voltage of 2V ±6% when the load current of a 12V supply is 2A. Resistors
are available with 1% tolerance.
The sense and output resistors will contribute a total possible error of 2%, so the ZXCT11xx error must not exceed 4%. From
Table 1, the error of ZXCT1107/1109 will be 3.4% if the sense voltage is 100mV. This satisfies the total error requirement of
6%.
Equation 4 gives
R
SENSE
= 100mV/2A = 50m Ω
Equation 5 gives
1 . 0 * 004 . 0
2
R
GAIN
=
= 5k Ω
A suitable preferred value is 5.1k Ω. This gives a systematic error of +2%, which can be compensated elsewhere in the
system if desired. Figure 2 shows the resulting schematic.
S+ ZXCT1107
or
ZXCT1109
S-
50m Ω
V
SUPPLY
OUT
LOAD
GND
5k1
OUTPUT
V
OUT
I
LOAD
Figure 2. Example Circuit using ZXCT1107/ZXCT1109