參數(shù)資料
型號(hào)： AD7877ACPZ-500RL7
廠商： ANALOG DEVICES INC
元件分類： 消費(fèi)家電
英文描述： Touch Screen Controller
中文描述： SPECIALTY CONSUMER CIRCUIT, QCC32
封裝： 5 X 5 MM, ROHS COMPLIANT, MO-220VHHD-2, LFCSP-32
文件頁(yè)數(shù)： 27/44頁(yè)
文件大小： 1002K
代理商： AD7877ACPZ-500RL7
AD7877
V
OUT
falls. If the DAC output exceeds V
REF
, it starts to source
current, and V
OUT
has to further decrease to compensate. When
the DAC output is at full scale, V
OUT
is at its minimum.
Rev. A | Page 27 of 44
Note that the effect of the DAC on V
OUT
is opposite in voltage
mode to that in current mode. In current mode, increasing DAC
code increases the sink current, so V
OUT
increases with
increasing DAC code. In voltage mode, increasing DAC code
increases the DAC output voltage, reducing the sink current.
Calculate the resistor values as follows:
1.
Decide on the feedback current as before.
2.
Calculate the parallel combination of R1 and R3 when the
DAC output is zero:
R
P
=
V
REF
/
I
FB
3.
Calculate R2 as before, but use R
P
and V
OUTMAX
:
R2
=
R
P
(
V
OUT(MAX)
V
REF
)/
V
REF
4.
Calculate the change in feedback current between
minimum and maximum output voltages as before using
I
=
V
R2(MAX)
/
R2
V
R2(MIN)
/
R2
This is equal to the change in current through R1 between
zero output and full scale, which is also given by
I
=
current at zero
current at full scale
=
V
/
R1
(
V
REF
V
)/
R1
=
V
/
R1
5.
R1
=
V
FS
/
.
6.
Calculate R3 from R1 and R using
R3
= (
R1
×
R
P
)/(
R1
R
P
)
Example:
1.
V
CC
= 5 V and
V
FS
=
V
CC
.
V
OUT(MIN)
is 20 V and
V
OUT(MAX)
is
25 V.
V
REF
is 1.25 V. Allow 100 μA around the feedback
loop.
2.
R
P
= 1.25 V/100 μA = 12.5 k.
3.
R2
= 12.5 k × (25 1.25 )/1.25 = 237 k.
Use nearest preferred value of 240 k.
4.
I
= 25 V/240 k 20 V/240 k = 21 μA.
5.
R1
= 5 V/21 μA = 238 k.
Use nearest preferred value of 250 k.
6.
R3
= (180 k × 12.5 k)/(180 k 12.5 k) =13.4 k.
Use nearest preferred value of 13 k.
The actual adjustment range using these values is 21 V to 26 V.
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