MAX1447 Datasheet, PDF (18/24 Pages) Maxim Integrated Products – 3.5- and 4.5-Digit, Single-Chip ADCs with LED Drivers

3.5- and 4.5-Digit, Single-Chip ADCs
with LED Drivers
Selecting Segment Current
A resistor from ISET to ground sets the current for each
LED segment. See Table 5 for more detail. Use the fol-
lowing formula to set the segment current:
I_SEG
=
 1.20V 
 R_ISET 
×
450
R_ISET values below 25kΩ increase the I_SEG. However,
the internal current-limit circuit limits the I_SEG to less than
30mA. At higher I_SEG values, proper operation of the
device is not guaranteed. In addition, the power dissipat-
ed may exceed the package power-dissipation limit.
Choosing Supply Voltage to Minimize
Power Dissipation
The MAX1447/MAX1496/MAX1498 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 MAX1447/
MAX1496/MAX1498 sink (8 x 25.5mA = 204mA) when
the outputs are operating and the LED segment drivers
are at full current. For a 3.3V supply, the MAX1447/
MAX1496/MAX1498 dissipate (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 dissi-
pation increases accordingly. 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
MAX1447/MAX1496/MAX1498 is not exceeded and the
ADC performance is not degraded, a diode can be
inserted between the power supply and VLED.
Table 5. Segment-Current Selection
R_ISET (kΩ)
25
50
100
500
>2500
I_SEG (mA)
21.6
10.8
5.4
1.1
LED driver disabled
Computing Power Dissipation
The following can be used to compute power dissipation:
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 MAX1496 or
AV_DD + DV_DD for the MAX1447/MAX1498.
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 (T_JA = 1 / 0.009496 =
+105.3°C/W), the maximum allowed ambient tempera-
ture T_A is given by:
T_J (max) = T_A + (PD x T_JA) =
+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.
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