New Tri-Color LED Driver Minimizes Control Lines on MCU

AND8091/D
New Tri-Color LED Driver
Minimizes Control Lines
on MCU
Prepared by: Fred Zlotnick
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
the diagram shows 15 total segments, a larger number of
drivers may be used for more LEDs. In all cases just one data
line and one clock line are needed, and one or more Enable
lines depending upon the circuit requirements. The
NLSF595 is an 8 bit device. It is capable of sinking 12 mA
or more. Resistor values should be calculated assuming the
part will have about a 0.5 V drop under normal usage. The
microcontroller (any with an SPI output) is assumed to be in
the MISO mode, Master IN Slave OUT. The 595 is a slave
peripheral, or 3 I/O lines of the MCU may be used.
Figure 1 shows two NLSF595s in cascade. The LEDs are
connected common anode, to a 5.0 V supply. The diagram
shows 220 resistors series limiting resistor which will
allow 12 mA of current. The two devices are cascaded,
with the serial out pin feeding the serial in pin of the second
device. Connections to the MCU are simply Data, Clock,
Enable (low). If there are more SPI devices, the only pin
dedicated to the function, is the EN pin, which is a negative
going latch enable.
When it comes to driving LEDs, a serial bus connection
eliminates the need for many connections to the MCU. The
SPI–Serial Peripheral Interface was introduced in the ’80s
by Motorola and been around for many years. It is still very
popular, and the purpose of this note is to show how to use
a two standard new open drain shift register to interface an
MCU. ON Semiconductor is introducing a new shift register
especially aimed at driving the newly available tri–color
LEDs (red, green, blue). The NLSF595 may be used as an
SPI peripheral, or with standard I/O.
Circuit
The design calls for driving 5 tri–color displays from
either an SPI (data, clock, enable) bus or simply from 3 I/O
pins. Only 1 pin needs to be dedicated to the display, the
Enable (bar) pin. Operating voltage is assumed to be +2.7 V,
with the LEDs operating from +5.0 V. The MCU is
providing the data to the 5 tri–color LEDs via this new
device. The MCU writes the 16 data bits as two 8 bit words
(one extra bit), in serial form, and latches the data. The
display is driven and can be ignored until it needs to be
changed. The NLSF595 includes a shift register, a latch and
8–12 mA drivers in one inexpensive 16–pin package. The
outputs are open drain, so they may be operated at a different
voltage from the main supply. Two or more devices may be
cascaded together to drive many multiple LEDs. Although
 Semiconductor Components Industries, LLC, 2002
August, 2002 – Rev. 2
Conclusion
The NLSF595 makes interfacing multiple tri–color LEDs
easy and trouble–free. Two devices can drive 5 independent
tri–color displays. If needed, any unused outputs may be
paralleled for increased drive output.
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Publication Order Number:
AND8091/D
AND8091/D
2.7 V
5.0 V
220 SCLR
Red
OE
2.7 V
Green
NLSF595
Blue
I/O or SPI
(MISO)
Data
SI
Clock
SCK
5 Additional Outputs
EN
RCK
SQH
Serial
Data Out
MCU
NLSF595
8 Additional Outputs
Total of 5
3–Color Displays
16 VCC
QC 2
15 QA
NLSF595DTR2
QB 1
QD 3
QE 4
QF 5
QG 6
QH 7
14 SI
13 OE
12 RCK
11 SCK
10 SCLR
GND 8
9 SQH
Figure 1. NLSF595 Shown Driving 5 3–Color LEDs
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Notes
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