LT1319 Linear Technology, LT1319 Datasheet - Page 10

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

APPLICATIONS
LT1319
current through R
15k
(0.335V/15k = 22.3 A), Q1 turns on which clamps the
preamp output. The collector current of Q1 provides base
drive for Q2 which saturates and pulls its collector close to
5V. The FILT1, FILT2L and FILT2 inputs are now pulled
positive by R
inputs to the gain stages and preamp. Referring to the
Block Diagram, pulling FILT2L or FILT2 positive a voltage
first gain stage. This offset effectively cuts off a portion of
the tail at high input levels. The magnitude of V is set by
the value of R
transconductance stages (100 A), the value of C
the duty cycle of the data pulses. Likewise an offset of
current contributions.
LED Drive Circuits
There are several simple circuits for driving LEDs. For low
speed modulation methods such as IRDA-SIR and Sharp/
Newton with pulses over 1 s, a 2N3904 in a SOT-23
package can be used as a switch with a series resistor in the
collector to limit the current drive. This circuit is shown
below with a suggested limiting resistor of 16
typically sets the current at 200mA. The supply voltage
must be well bypassed at the connection to the LED in order
for the supply not to sag when hit with a fast current pulse.
A 10 F low ESR capacitor should be used as well as a 0.1 F
10
TYPICAL APPLICATIONS
V provides a voltage of V/11 at the inverting input of the
V/10k
MMBR941LT1 FOR >1MHz
= 182 A where V
2N3904 FOR <1MHz
Optional Clamp Circuit
is created at the preamp input to reduce tail
C3
, R
C3
, the current sinking capability of the
PIN 2
C4
C2
U
and R
V
IN
which is [V
CC
PREOUT
INFORMATION
BIAS
U
C6
PIN 4
which forces an offset at the
= 1.9V. At this new level
CC
1319 TA03
U
W
– (V
BIAS
+ 0.365V)]/
U
F4
for IRDA-SIR and Sharp/Newton
,C
F5
which
V
and
IN
HSDL4220
LED Drive Circuit
TSH5400
470
R
DN304
F4
RF quality capacitor to reduce the high frequency spikes.
The current must be selected to achieve the minimum
output light intensity at a given angle and must be lower
than the manufacturer’s maximum current rating at the
maximum duty cycle of the modulation method. The opti-
mum current is a function of the LED output, the LED
forward voltage, the drop across the transistor and the
minimum supply voltage.
The minimum light output then can be obtained from the
LED data sheet. For IRDA-SIR the minimum intensity at 15
off axis is 40mW/sr. For IRDA-FIR the spec rises to
100mW/sr. To increase light output and distance of the
link, a second LED can be inserted in series with the first to
obtain twice the light output without consuming additional
supply current. The current variation will now be greater
because two LED forward drops must be accounted for and
the drop across the series resistor is greatly reduced.
For pulse widths less than 500ns the NPN should be
replaced by an N-channel MOSFET with on-resistance of
less than 1 with 5V on the gate. The FET can turn off
much more quickly than the saturated NPN and provides
a lower effective on-resistance. A suggested circuit is
shown below and includes three devices available in the
SOT-23 package.
V
CC
I
Q1
2N3904
R
16
LED
F3
1319 TA04
V
CC
R
V
SERIES
LED
V
SW
V
IN
2 LED Drive Circuit
HSDL4220
100
R
TSH5400
for IRDA-FIR
D1
DN304
V
CC
Q1
NDS351N
TN0201T
2N7002
R
3.9
D2
1319 TA05