MAX1497 Maxim, MAX1497 Datasheet - Page 28

• Manufacturer Part Number: MAX1497
• Description: 3.5- and 4.5-Digit / Single-Chip ADCs with LED Drivers and C Interface
• Manufacturer: Maxim

possible with double-sided boards. With this technique,
the component side of the board is dedicated to ground
planes while signals are placed on the solder side.
Good decoupling is important when using high-resolution
ADCs. Decouple the supplies with 0.1µF ceramic capaci-
tors 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
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 MAX1497/MAX1499 drive a peak current of
25.5mA into LEDs with a 2.2V forward-voltage drop
when operated from a supply voltage of at least 3.0V.
Therefore, the minimum voltage drop across the inter-
nal LED drivers is (3.0V - 2.2V) = 0.8V. The MAX1497/
MAX1499 sink (8 x 25.5mA = 204mA) when the outputs
are operating and LED segment drivers are at full cur-
rent. For a 3.3V supply, the MAX1497/MAX1499 dissi-
pate (3.3V - 2.2V) x 204 = 224.4mW. If a higher supply
voltage is used, the driver absorbs a higher voltage,
and the driver’s power dissipation increases according-
ly. However, if the LEDs used have a higher forward
voltage drop than 2.2V, the supply voltage must be
raised accordingly to ensure that the driver always has
at least 0.8V headroom.
For a VLED supply voltage of 2.7V, the maximum LED
forward voltage is 1.9V to ensure 0.8V driver headroom.
The voltage drop across the drivers with a nominal +5V
supply (5.0V - 2.2V = 2.8V) is almost three times the
drop across the drivers with a nominal 3.3V supply
(3.3V - 2.2V = 1.1V). Therefore, the driver’s power dissi-
pation increases three times. The power dissipation in
the part causes the junction temperature to rise accord-
ingly. In the high ambient temperature case, the total
junction temperature may be very high (>+125°C). At
higher junction temperatures, the ADC performance
degrades. To ensure the dissipation limit for the
MAX1497/MAX1499 is not exceeded and the ADC per-
3.5- and 4.5-Digit, Single-Chip ADCs with LED
Drivers and µC Interface
28
ISET
______________________________________________________________________________________
values below 25kΩ increase the I
Choosing Supply Voltage to Minimize
I
SEG
SEG
Table 6
Segment-Current Selection
values, the proper operation of the
=



1 20
R
.
ISET
for more detail. Use the fol-
V

 ×

Power Dissipation
450
SEG
SEG
to less than
. However,
formance is not degraded, a diode can be inserted
between the power supply and VLED.
The following can be used to compute power dissipa-
tion:
VLED = LED driver supply voltage
I
V
DUTY = segment ON time during each digit ON time
I
N = number of segments driven (worst case is eight)
V
I
AV
For I
1.5V at 25.5mA, VLED = V
For the 28-pin SSOP package (TJA = 1 / 0.009496 =
+105.3°C/W), the maximum allowed ambient tempera-
ture T
Thus, the device cannot operate safely at a maximum
package temperature of +85°C. The power dissipates
in the part need to be lowered.
VLED - V
ensure it operates safely:
VLED
SEG
SUPPLY
DIODE
SUPPLY
(VLED - V
DD
SEG
(PD x TJA) max = (+125°C) - (+85°C) = +40°C
PD x TJA (max) = (+125°C) - (+85°C) = +40°C
= segment current set by R
PD = (5.25 x 2mA) + (5.25V - 1.5) [(127 / 128)
A
= VLED bias current
+ DV
PD (max) = +40°C /+105.3°C/W = 380mW
is given by:
+125°C = T
PD = (VLED x I
= LED forward voltage
PD (max) = +40°C /+70°C/W = 571mW
= supply current from V
= 25.5mA, N = 8, DUTY = 127 / 128, V
= supply voltage of the part
DIODE
DIODE
DD
x 25.5mA x 8)] + 5.25 x 1.080mA
0.8V < VLED - V
TJ (max) = T
(DUTY x I
for the MAX1499
VLED - V
should have the following condition to
) = [380mW - (5.25V x 2mA) - 5.25V x
Computing Power Dissipation
1.080mA] / [(127 / 128) x 25.5mA x 8]
A
PD = 0.7751W
+ (0.7751W x +105.3°C/W)
T
28-Pin SSOP Package Example
VLED
SEG
A
28-Pin PDIP Package Example
DIODE
= +43°C
A
SUPPLY
x N) + V
+ (PD x TJA) =
) + (VLED - V
DIODE
= 1.854V
ISET
DD
Dissipation Example
= 5.25V:
< 2.08V
SUPPLY
for the MAX1497 or
DIODE
x I
SUPPLY
DIODE
)
=