AN2264 Motorola Semiconductor Products, AN2264 Datasheet - Page 3

• Manufacturer Part Number: AN2264
• Description: Freescale Semiconductor, Inc.
• Manufacturer: Motorola Semiconductor Products

MOTOROLA
As the LEDs are driven directly by the MCU, care had to be taken to avoid
drawing excessive currents from port pins or exceeding the specified current or
dissipation capability of the 5 volt regulator. The digit current for an acceptable
brightness is too high for a port pin so FET buffers are incorporated. The
segment resistors of 220R were chosen to give a segment current of 10mA so
the digit current can be up to 80mA. As the digits are driven with a 25% duty
cycle this corresponds to an average current of 2.5mA per segment and 20mA
for a digit with all 8 segments lit. The highest total current is drawn with a display
of 88.0. This involves 21 active segments which, with a duty cycle of 25%,
requires 52.5mA (2.5mA x 21).
With an MCU I
14volts), the maximum dissipation of the LT1121 can be calculated:
With an assumed ambient temperature of 26°C and a junction to ambient
thermal resistance of 100C°/W, the maximum junction temperature of the
LT1121 is:
This is well within the specified maximum of 125°C. If a significantly higher
segment current were required, then segment drivers and a higher dissipation
regulator should be employed.
Current versions of the MC68HC908AZ60A data sheet specify a maximum
total source current of only 10mA. This was inserted as an interim figure while
characterisation was done to check that the Voh and Vol level specifications
were met under all possible conditions. In this application these levels are not
critical so the only concern is that the total current is not high enough to cause
long term damage to the device. There is, however no risk of this as long as the
absolute maximum figure of 100mA per pin is observed. In this case the
maximum Idd is significantly less than 100mA. Future versions of the data
sheet will remove this unnecessary limitation.
A simple sleep mode was incorporated in the module when no bus activity
occurs for 2 seconds. When this condition is detected, the MCU lowers the
enable line to the MC33399 which in turn lowers the inhibit signal to the LT1121
regulator. The regulator goes into its standby mode and powers down the MCU.
A resumption of bus activity wakes the MC33399 and the regulator switches the
MCU on. In order to facilitate low sleep mode current consumption, an
Average
Freescale Semiconductor, Inc.
Peak
For More Information On This Product,
Dissipation = 9volts x (52.5mA+15mA) ~ 610mW.
T
J
= T
LIN Node Temperature Display
Go to: www.freescale.com
Amb
1 segment
dd
2.5mA
10mA
of 15mA and 9 volts across the regulator (assuming a V
+ 610mW x 100C°/W = 26 + 61 = 87°C
1 digit (all segments)
80mA
20mA
Display: 88.0
52.5mA
80mA
AN2264/D
Hardware
Bat
of
3