LT1319 Linear Technology, LT1319 Datasheet - Page 9

• Manufacturer Part Number: LT1319
• Description: Multiple Modulation Standard Infrared Receiver
• Manufacturer: Linear Technology

APPLICATIONS
first trace in the photograph shows the current with the
photodiode 10cm from a source with 100mW/sr intensity.
At 200mV/div, there is about 40 A of peak current and the
decay is consistent with the 210ns time constant. The
lower trace shows the current with the photodiode 2cm
from the LEDs where the photodiode current is theoreti-
cally 25 times greater than at 10cm. The voltage is clamped
by the photodiode to nearly 0.4V, but what is now notice-
able is that there is a tail with a time constant a bit greater
than 1 s. If the signal is AC coupled and has a low duty
cycle, the waveform will be centered at the very bottom
which can result in very wide output pulses. This issue will
be discussed later in more detail and a method to circum-
vent it will be shown.
Threshold Adjustment
The comparator thresholds are set by the current into Pin
11. The simplest method of setting this current is by a
resistor, R
should be bypassed. The current is given by:
The threshold referred to the input of the filter buffer is:
I
V
TH
TH
10cm
2cm
V
R
T1
I
TH
CC
T
1
tied between Pin 11 and Pin 15 (V
400
• •
4 500
0 9
2
k
V V
Photocurrent Waveforms
/
U
V
INFORMATION
U
W
1319 AI01
CC
U
). Pin 11
or nominally 0.68mV for R
value of R
transients at the comparator outputs parasitically coupling
to the FILTIN or FILTINL inputs and is layout dependent.
Extending Short Range Performance
The short range performance of the LT1319 is normally
limited by the photocurrent tail, but in some instances the
peak current level cannot be supported by the output of the
preamplifier and the input will sag at Pin 2. Typically the
maximum input current is 6mA. To increase this current to
20mA or more, place an NPN transistor with its emitter tied
to Pin 2, the base to Pin 4 and collector to the 5V supply.
The choice of transistor is dependent on the bandwidth
required for the preamp. The base-emitter capacitance of
the transistor (C
resistor of the preamplifier and performs a lowpass filter-
ing function. For modest data rates such as IRDA-SIR and
Sharp/Newton a 2N3904 limits the bandwidth to 2MHz
which is ample. For the highest data rates, a transistor with
f
Another issue with large input signals is the photocurrent
tail. When this tail is AC coupled and the data has a low duty
cycle, the output pulse width can become so wide that it
extends into the next bit interval. A highpass filter can reject
this tail, but for the case of IRDA-SIR, rejecting the 1 s time
constant can cause rejection of the 1.6 s pulse which leads
to a loss of sensitivity and reduced maximum link distance.
The circuit on the front page of the data sheet uses a 500kHz
highpass that trades off some sensitivity for rejection of
this tail. Unfortunately both maximum and minimum dis-
tance are compromised. An alternative is shown in the
IRDA-SIR/FIR application. In this instance the final highpass
filter for SIR is moved into 450kHz, but a clamp/squelch
circuit consisting of Q1, Q2, D3 and R
is used as described above to clamp the input, but the input
current level at which the clamp engages has been modified
by R
Without the resistors, Q1 would turn on when the voltage
across the 15k resistor in the preamp reaches about 0.7V
(a current of 0.7V/15k
reduces this voltage by about 365mV. The drop is set by the
T
greater than 1GHz is needed such as MMBR941LT1.
C1
and R
T1
is 39k. The limitation tends to be switching
C2
.
JE
), is in parallel with the 15k feedback
= 47 A). The drop across R
T1
= 30k. The largest practical
C1
to R
C6
is added. Q1
LT1319
9
C1