MM5450 Micrel, MM5450 Datasheet - Page 5

• Manufacturer Part Number: MM5450
• Description: LED Display Driver
• Manufacturer: Micrel

MM5450/5451
Functional Description
The MM5450 and MM5451 were designed to drive either 4- or
5-digit alphanumeric LED displays with the added benefit of
requiring minimal interface with the display or data source.
Data is transferred serially via 2 signals; clock and serial data.
Data transfer without the added inconvenience of an external
load signal is accomplished by using a format of a leading “1”
followed by the allowed 35 data bits. These 35 data bits are
latched after the 36th has been transferred. This scheme
provides non multiplexed, direct drive to the LED display.
Characters currently displayed (thus, data output) changes
only if the serial data bits differ from those previously transferred.
Control of the output current for LED displays provides for the
display brightness. To prevent oscillations, a 1nF capacitor
should be connected to pin 19, brightness control.
The block diagram is shown in Figure 1. For the MIC5450, the
DATA ENABLE is a metal option and is used instead of the
35th output. The output current is typically 20-times greater
that the current into pin 19, which is set by an external variable
resistor.
There is an external reset connection shown which is available
on unpackaged (die) only. Figure 2 illustrates the die pad
locations for bonding in “chip on board” applications.
Figure 5 shows the input data format. A leading “1” is followed
by 35 bits of data. After the 36th had been transferred, a LOAD
signal is generated synchronously with the clock high state.
This loads the 35 bits of data into the latches. The low side of
the clock is used to generate a RESET signal which clears all
shift registers for the next set of data. All shift registers are
static master-slave, with no clear for the master portion of the
first register, allowing continuous operation.
May 2000
(INTERNAL)
(INTERNAL)
CLOCK
RESET
LOAD
DATA
DATA ENABLE
1
START BIT 1
CLOCK
DATA
Figure 6. Input Data Format
Figure 5.
5
There must be a complete set of 36 clocks or the shift registers
will not clear.
When the chip first powers ON, an internal power ON reset
signal is generated which resets all registers and all latches.
The START bit and the first clock return the chip to its normal
operation.
Figure 3 and 4 show the pin-out of the MIC5450 and MIC5451.
Bit 1 is the first bit following the start bit and it will appear on
pin 18. A logical “1” at the input will turn on the appropriate
LED.
Figure 5 shows the timing relationships between data, clock
and DATA ENABLE. A maximum clock frequency of 0.5MHz
is assumed.
For applications where a lesser number of outputs are used,
it is possible to either increase the current per output, or
operate the part at higher than 1V V
equation can be used for calculations.
T
where:
T
V
I
124 C/W = thermal resistance of the package
T
The above equation was used to plot Figures 7–9.
LED
J
J
OUT
A
= (V
= junction temperature + 150 C max
= ambient temperature
= the LED current
= the voltage at the LED driver outputs
OUT
100 nS MIN
) (I
300 nS MIN
LED
) (No. of segments) (124 C/W) + T
BIT 35
BIT 36
36
37
OUT
. The following
MM5450/5451
A
Micrel