MAX1367 Maxim Integrated Products, MAX1367 Datasheet - Page 19

• Manufacturer Part Number: MAX1367
• Description: (MAX1365 / MAX1367) Digit Panel Meters
• Manufacturer: Maxim Integrated Products

The transfer function for the MAX1367 with AIN+ - AIN-
≥ 0 and RANGE = DV
The transfer function for the MAX1367 with AIN+ - AIN-
< 0 and RANGE = DV
Figure 11 shows the DAC transfer function for the
MAX1365/MAX1367 in unipolar and bipolar modes.
The transfer function for the DAC in the MAX1365/
MAX1367 unipolar mode is:
where N = two’s complement ADC output code.
In unipolar mode, V
complement ADC codes less than zero (see Figure 12).
The transfer function for the DAC in the MAX1365/
MAX1367 in bipolar mode is:
where N = two’s complement ADC output.
Figures 12 and 13 show the MAX1365/MAX1367 trans-
fer function of the output current (4-20OUT) versus the
ADC input code.
The transfer function for the MAX1365/MAX1367 with
the current offset enabled (EN_I is high) is:
The transfer function for the MAX1365/MAX1367 with
the current offset disabled (EN_I is low) is:
Note: The input at V
ance of typically 6kΩ when driving V
( )
( )
7
8
COUNT
COUNT
IOUT
V
DACVOUT
=
=
V
Voltage-to-Current Transfer Function
1 024
1 024
DACVOUT
.
.
IOUT
______________________________________________________________________________________
≅
DACVOUT
Stand-Alone, 4.5-/3.5-Digit Panel Meters
16
1 25
CONV_IN
DD
DD
.






mA
V
V
≅
=
V
V
REF
REF
is:
AIN
is:
AIN
16
1 25
N
=
.
x V
mA
65 536
+19 999
+ −
+ −
+ −
+ −
32 768 1
DAC Transfer Functions
is equal to 0V for all two’s
,
expects a source imped-
,
CONV IN
,
V
V
V
V
N
x V
AIN
REF
AIN
REF
CONV_IN
−
CONV IN
−
−
_
x V
−
−
x V
x
x
REF
2000
2000
+
REF
_
externally.
4
mA






x
x
10
10 1
+
Power up AV
input and external-reference voltage to the device. If
this is not possible, limit the current into these inputs to
50mA. When the analog and digital supplies come from
the same source, isolate the digital supply from the
analog supply with a low-value resistor (10Ω) or ferrite
bead. For best performance, ground the MAX1365/
MAX1367 to the analog ground plane of the circuit
board. Avoid running digital lines under the device as
this can couple noise onto the IC. Run the analog
ground plane under the MAX1365/MAX1367 to mini-
mize coupling of digital noise. Make the power-supply
lines to the MAX1365/MAX1367 as wide as possible to
provide low-impedance paths and reduce the effects of
glitches on the power-supply line. Shield fast-switching
signals, such as clocks, with digital ground to avoid
radiating noise to other sections of the board. Avoid
running clock signals near the analog inputs. Avoid
crossover of digital and analog signals. Running traces
that are on opposite sides of the board at right angles to
each other reduces feedthrough effects. Good decou-
pling is important when using high-resolution ADCs.
Decouple the supplies with 0.1µF ceramic capacitors to
GND. Place these components as close to the device
as possible to achieve the best decoupling.
A resistor from ISET to ground sets the current for each
LED segment. See Table 6 for more detail. Use the fol-
lowing formula to set the segment current:
R
the internal current-limit circuit limits the I
30mA. At higher I
device is not guaranteed. In addition, the power dissipat-
ed may exceed the package power-dissipation limit.
The MAX1365/MAX1367 drive a peak current of 25.5mA
into LEDs with a 2.2V forward voltage drop when operat-
ed from a supply voltage of at least 3.0V. Therefore, the
minimum voltage drop across the internal LED drivers is
0.8V (3.0V - 2.2V = 0.8V). The MAX1365/MAX1367 sink
when the outputs are operating and the LED segment
drivers are at full current (8 x 25.5mA = 204mA). For a
3.3V supply, the MAX1365/MAX1367 dissipate 224.4mW
((3.3V - 2.2V) x 204 = 224.4mW). If a higher supply volt-
age is used, the driver absorbs a higher voltage, and the
driver’s power dissipation increases accordingly.
ISET
with 4–20mA Output
values below 25kΩ increase the I
Choosing Supply Voltage to Minimize
DD
Supplies, Layout, and Bypassing
I
SEG
and DV
SEG
Selecting Segment Current
=



values, proper operation of the
1 20
R
DD
.
ISET
before applying an analog
V



x
Power Dissipation
450
SEG
SEG
to less than
. However,
19