AME, Inc.
3-1/2 Digit A/D Converter
High Accuracy, Low Power
AME7106/AME7106A/AME7106R
AME7107/AME7107A/AME7107R
output is at 3.2V. Maximum power dissipation is 8.1 mA
X 3.2 V X 24 segments = 622mW. However, once the
AME7107 output voltage is above two volts, the LED cur-
rent is essentially constant as output voltage increases.
Reducing the output voltage by 0.7V, results in 7.7mA of
LED current, only a 5 percent reduction. Maximum power
dissipation is only 7.7mA X 2.5V X 24 = 462 mW, a re-
duction of 26%. An output voltage reduction of 1 volt
reduces LED current by 10% (7.3mA) power dissipation
by 38%. (7.3mA X 2.2V X 24 = 385mW).
diode, on the other hand, will result in a relatively steady
output voltage.
In addition to limiting maximum power dissipation, the
resistor reduces the change in power dissipation as the
display changes. As fewer segments are “ON,” each
“ON” output drops more voltage and current. For the
best case of six segments (a “111” display) to worst
case (a “1888” display) the resistor will change about
230 mW. While a circuit without the resistor will change
about 470 mW. Therefore, the resistor will reduce the
variation of power dissipation by about 50%.
There are two ways to reduce the power dissipation:
either a 5.1 ohm resistor or a 1 Amp diode placed in
series with the display (but not in series with the
AME7107). The resistor will reduce the AME7107 out-
put voltage, when all 24 segments are “ON”. When
segments turn off, the output voltage will increase. The
The change in LED brightness caused by the resistor is
almost unnoticeable as more segments turn off. If steady
display brightness is very important, a diode is recom-
mended.
n Typical Applications
14
AME [ ANALOG MICROELECTRONICS ]
