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LT1319

Multiple Modulation Standard Infrared Receiver

Linear Technology 

LT1319

APPLICATIONS INFORMATION

current through RC2 which is [VCC – (VBIAS + 0.365V)]/

15kΩ = 182μA where VBIAS = 1.9V. At this new level

(0.335V/15kΩ = 22.3μA), 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 RC3, RC4 and RC6 which forces an offset at the

inputs to the gain stages and preamp. Referring to the

Block Diagram, pulling FILT2L or FILT2 positive a voltage

ΔV provides a voltage of ΔV/11 at the inverting input of the

first gain stage. This offset effectively cuts off a portion of

the tail at high input levels. The magnitude of ΔV is set by

the value of RC3, the current sinking capability of the

transconductance stages (100μA), the value of CF4,CF5 and

the duty cycle of the data pulses. Likewise an offset of

ΔV/10kΩ is created at the preamp input to reduce tail

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 1μs, 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 16Ω which

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 10μF low ESR capacitor should be used as well as a 0.1μF

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.

( ) ILED =

VCC − VLED − VSW

RSERIES

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 1Ω 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.

TYPICAL APPLICATIONS

Optional Clamp Circuit

VCC

2N3904 FOR <1MHz

MMBR941LT1 FOR >1MHz

PIN 2

PIN 4

IN PREOUT

1319 TA03

10

LED Drive Circuit

for IRDA-SIR and Sharp/Newton

VCC

HSDL4220

TSH5400

DN304

RF4

470Ω

VIN

RF3

16Ω

Q1

2N3904

1319 TA04

2 LED Drive Circuit

for IRDA-FIR

VCC

HSDL4220

TSH5400

DN304

RD1

100Ω

VIN

RD2

3.9Ω

Q1

NDS351N

TN0201T

2N7002

1319 TA05

1319fb

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