10

LT1319

15k

(0.335V/15k

Ω = 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

inputs to the gain stages and preamp. Referring to the

Block Diagram, pulling FILT2L or FILT2 positive a voltage

∆V provides a voltage of ∆V/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

∆V is set by

transconductance stages (100

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

∆V/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 1

µs, 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

Ω 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 10

µFlowESRcapacitorshouldbeusedaswellasa0.1µF

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.

I

VV

V

R

LED

CC

LED

SW

SERIES

=

−−

()

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

Ω 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.

LED Drive Circuit

for IRDA-SIR and Sharp/Newton

3.9

Ω

100

Ω

HSDL4220

TSH5400

DN304

Q1

NDS351N

TN0201T

2N7002

1319 TA05

16

Ω

470

Ω

HSDL4220

TSH5400

DN304

Q1

2N3904

1319 TA04

IN

PREOUT

PIN 2

PIN 4

2N3904 FOR <1MHz

MMBR941LT1 FOR >1MHz

1319 TA03

Optional Clamp Circuit

2 LED Drive Circuit

for IRDA-FIR