DN152 - The LT1328: A Low Cost 4Mbps IrDA Receiver in MS8 and SO-8 Package

A Low Cost 4Mbps IrDA Receiver in MS8 and SO-8 Packages
Design Note 152
Alexander Strong
Introduction
The need for ever increasing data rates required by a vast
array of devices, such as notebook computers, printers,
mobile phones, pagers and modems, has been satisfied
by the technology of infrared data transmission. The
Infrared Data Association (IrDA) standard, which covers
data rates from 2400bps to 4Mbps, is the overwhelming
choice for infrared data transmission. The LT®1328 is
a photodiode receiver that supports IrDA data rates up
to 4Mbps, as well as other modulation methods, such
as Sharp ASK and TV remote control.
The LT1328, in the MS8 and SO-8 packages, contains
all the necessary circuitry to convert current pulses
from an external photodiode to a digital TTL output
while rejecting unwanted lower frequency interference.
The LT1328 plus five external components is all that is
required to make the IrDA-compatible receiver shown
in Figure 1. An IrDA-compatible transmitter can also be
implemented with only six components, as shown in
C6
1000pF
LT1328
LIGHT IN
TEMIC
BPV22NF
1
D1
IN
2
C1
10nF
C4
330pF
FILT
3
FILT LO
4
GND
VBIAS
MODE
VCC
DATA
Figure 2. Power requirements for the LT1328 are minimal: a single 5V supply and 2mA of quiescent current.
LT1328 Functional Description
Figure 3 is a block diagram of the LT1328. Photodiode
current from D1 is transformed into a voltage by feedback resistor RFB. The DC level of the preamp is held
at VBIAS by the servo action of the transconductance
amplifier’s gm. The servo action only suppresses
frequencies below the Rgm/CFILT pole. This highpass
filtering attenuates interfering signals, such as sunlight
or incandescent or fluorescent lamps, and is selectable
at Pin 7 for low or high data rates. For high data rates,
Pin 7 should be held low. The highpass filter breakpoint
is set by the capacitor C4 at f = 25/(2π • Rgm • C4), where
Rgm = 60k. The 330pF capacitor (C4) sets a 200kHz
corner frequency and is used for data rates above
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HIGH: SIR
LOW: FIR AND 4ppm
8
VBIAS
7
C2
4.7μF
5
5V
PREAMP
1
IN
BIAS
D1
RIN
TRANSMIT
INPUT
R2
10k
RGM
gm CELL
MODE
7
VCC
6
DATA
5
–
2
VCC
D2
HSDL-4220
+
DN152 F01
Figure 1. LT1328 IrDA Receiver
R1
100Ω
C6
1000pF
RFB
6
TTL
DATA OUT
C5
4.7μF
8
C1
10nF 3
FILTER
FILTER LO
C4
330pF
R3
3.9Ω
1/2W
Q3
2N7002
4
GND
–
COMP
+
DN152 F02
DN152 F03
Figure 2. IrDA Transmitter
04/97/152_conv
Figure 3. LT1328 Block Diagram
115kbps. For low data rates (115kbps and below), the
capacitance at Pin 2 is increased by taking Pin 7 to a
TTL high. This switches C1 in parallel with C4, lowering
the highpass filter breakpoint. A 10nF capacitor (C1)
produces a 6.6kHz corner. Signals processed by the
preamp/gm amplifier combination cause the comparator
output to swing low.
IrDA SIR
The LT1328 circuit in Figure 1 operates over the full
1cm to 1 meter range of the IrDA standard at the
stipulated light levels. For IrDA data rates of 115kbps
and below, a 1.6μs pulse width is used for a zero and
no pulse for a one. Light levels are 40mW/sr (Watts per
steradian) to 500mW/sr. Figure 4 shows a scope photo
for a transmitter input (bottom trace) and the LT1328
output (top trace). Note that the input to the transmitter
is inverted; that is, transmitted light produces a high at
the input, which results in a zero at the output of the
transmitter. The MODE pin (Pin 7) should be high for
these data rates.
RECEIVER
OUTPUT
TRANSMITTER
INPUT
DN152 F05
Figure 5. IrDA-FIR Modulation
4ppm
The last IrDA encoding method is for 4Mbps and uses
pulse position modulation, thus its name: 4ppm. Two
bits are encoded by the location of a 125ns wide pulse at
one of the four positions within a 500ns interval (2 bits •
1/500ns = 4Mbps). Range and input levels are the
same as for 1.152Mbps. Figure 6 shows the LT1328
reproduction of this modulation.
RECEIVER
OUTPUT
RECEIVER
OUTPUT
TRANSMITTER
INPUT
TRANSMITTER
INPUT
DN152 F04
Figure 4. IrDA-SIR Modulation
IrDA FIR
The second fastest tier of the IrDA standard addresses
576kbps and 1.152Mbps data rates, with pulse widths of
1/4 of the bit interval for zero and no pulse for one.
The 1.152Mbps rate, for example, uses a pulse width
of 217ns; the total bit time is 870ns. Light levels are
100mW/sr to 500mW/sr over the 1cm to 1 meter range.
A photo of a transmitted input and LT1328 output is
shown in Figure 5. The LT1328 output pulse width will
be less than 800ns wide over all of the above conditions
at 1.152Mbps. Pin 7 should be held low for these data
rates and above.
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DN152 F06
Figure 6. IrDA-4ppm Modulation
Conclusion
In summary, the LT1328 can be used to build a low cost
receiver compatible with IrDA standards. Its ease of
use and flexibility also allow it to provide solutions to
numerous other photodiode receiver applications. The
tiny MSOP package saves on PC board area.
For applications help,
call (408) 432-1900
dn152f_conv LT/GP 0497 185K • PRINTED IN THE USA
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