Rainbow Electronics MAX1499 - Manual (Page 28)

MAX1497/MAX1499
3.5- and 4.5-Digit, Single-Chip ADCs with LED
Drivers and µC Interface
28 ______________________________________________________________________________________
Choosing Supply Voltage to Minimize
Power Dissipation
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-
formance is not degraded, a diode can be inserted
between the power supply and VLED.
Computing Power Dissipation
The following can be used to compute power dissipa-
tion:
PD = (VLED x I VLED ) + (VLED - V DIODE )
(DUTY x I SEG x N) + V SUPPLY x I SUPPLY
VLED = LED driver supply voltage
I VLED = VLED bias current
V DIODE = LED forward voltage
DUTY = segment ON time during each digit ON time
I SEG = segment current set by R ISET
N = number of segments driven (worst case is eight)
V SUPPLY = supply voltage of the part
I SUPPLY = supply current from V DD for the MAX1497 or
AV DD + DV DD for the MAX1499
Dissipation Example
For I SEG = 25.5mA, N = 8, DUTY = 127 / 128, V DIODE =
1.5V at 25.5mA, VLED = V SUPPLY = 5.25V:
PD = (5.25 x 2mA) + (5.25V - 1.5) [(127 / 128)
x 25.5mA x 8)] + 5.25 x 1.080mA
PD = 0.7751W
28-Pin SSOP Package Example
For the 28-pin SSOP package (TJA = 1 / 0.009496 =
+105.3°C/W), the maximum allowed ambient tempera-
ture T
A is given by:
TJ (max) = T A + (PD x TJA) =
+125°C = T A + (0.7751W x +105.3°C/W)
T A = +43°C
Thus, the device cannot operate safely at a maximum
package temperature of +85°C. The power dissipates
in the part need to be lowered.
(PD x TJA) max = (+125°C) - (+85°C) = +40°C
PD (max) = +40°C /+105.3°C/W = 380mW
(VLED - V DIODE ) = [380mW - (5.25V x 2mA) - 5.25V x
1.080mA] / [(127 / 128) x 25.5mA x 8]
VLED - V DIODE = 1.854V
VLED - V DIODE should have the following condition to
ensure it operates safely:
0.8V < VLED - V DIODE < 2.08V
28-Pin PDIP Package Example
PD x TJA (max) = (+125°C) - (+85°C) = +40°C
PD (max) = +40°C /+70°C/W = 571mW
VLED - V DIODE = [571mW - (5.25V x 2mA) - 5.25V x
1.080mA] / [(127 / 128) x 25.5mA x 8]
VLED - V DIODE = 2.80V
For a 28-pin PDIP package, VLED - V DIODE should
have the following condition to ensure it operates safe-
ly:
0.8V < VLED - V DIODE < 3.18V
32-Pin TQFP Package
The MAX1499 TQFP package can operate safely for all
supply voltages provided V
DIODE > 1.5V.