Features
•
•
•
•
Few External Components
Low Power Consumption
Microcomputer Compatible
Insensitive to Ambient Light and Other Continuous Interferences
Applications
•
Keyless Entry Systems
•
Remote Control
•
Wireless Data Transfer up to 4 kbit/s
IR Receiver for
Data
Communication
U2538B
1. Description
The IC U2538B is a complete IR receiver for data communication. The useful input
signals are separated by a special input circuit and amplified by a gain-controlled
amplifier. The bandpass filter suppresses the off-band signals. The signal detector,
consisting of a demodulator, an integrator and a Schmitt trigger, forms the input signal
to an output pulse that can be interfaced to a microcomputer. The AGC and the ATC
circuit control the receiver's sensitivity, making it insensitive to ambient light sources.
Figure 1-1.
Block Diagram with Typical Circuit
V
S
U2538B
Input
Amplifier
and filter
Detector
µC
AGC/ATC
Modulated IR signal
carrier frequency 20 to 60 kHz
minimum 6 pulses/burst
Rev. 4717B–IRRC–09/05
Figure 1-2.
Block Diagram
V
S
R
F0
R
F
V
S
BIAS
IN
BPF
+
-
Comp 1
100 kΩ
OUT
&
dt
Vth
+
-
Comp 2
INT
ST
TIA
CGA
AGC
ATC
DEM
A
GND
C
AGC
C
AGC
D
GND
TIA
CGA
BPF
AGC
Transimpedance amplifier
Controlled gain amplifier
Bandpass filter
Automatic gain control
ATC
DEM
INT
ST
Automatic threshold control
Demodulator
Integrator
Schmitt trigger
2
U2538B
4717B–IRRC–09/05
U2538B
2. Pin Configuration
Figure 2-1.
Pinning SO8
NC
8
RF
7
AGND
6
IN
5
U2538B
1
VS
2
CAGC
3
OUT
4
DGND
Table 2-1.
Pin
1
2
3
4
5
6
7
8
Pin Description
Symbol
VS
CAGC
OUT
DGND
IN
AGND
RF
NC
Function
Supply voltage
AGC capacitor
Data output
GND - DEM/INT/ST
Input pin diode
GND amplifier
Frequency determination
Not connected
3
4717B–IRRC–09/05
3. Functional Description
3.1
Input Stage (TIA)
The input stage provides the necessary bias voltage for the photo diode and ensures decoupling
of the useful signal. This involves processing the DC and AC portions in separate parts of the
circuit: the bias voltage (BIAS) and the transimpedance amplifier circuit (TIA). The bias voltage
circuit operates like a load resistor with respect to the photo diode, the value of which is low for
DC and low-frequency signals (3 to 100 kΩ), but as high as possible for the operating frequency
(100 kHz to 1 MHz) depending on the input current). The ac portion of the input signal feeds an
inverted amplifier with a sufficiently low input resistance (Z
i
< 10 kΩ). If the input resistance is too
high, the useful signal will be lost due to the junction capacitance of the photodiode.
3.2
Controlled Gain Amplifier (CGA)
The controlled gain amplifier accounts for the greatest part of the voltage gain and can be con-
trolled via the voltage at CAGC (pin 2). Gain control is needed to support the interference
suppression of the detector. High-pass behavior results from the capacitive coupling of the indi-
vidual stages. The cut-off frequency is approximately 20 kHz.
3.3
Bandpass Filter (BPF)
The bandpass filter basically consists of integrated components. An external resistor determines
the mid-frequency. The filter quality is about 7 and is practically independent of the selected mid-
frequency (see
Figure 3-1).
The following formula can be used for calculating the resistor, R
f0
:
8855
-
R
fO
(kΩ)
= ------------------ –
13
f
0
(kHz)
where: 20 kHz < f
0
< 60 kHz
Figure 3-1.
Characteristic of the Bandpass Filter
120
100
80
G
rel
60
40
20
0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
f/f
0
4
U2538B
4717B–IRRC–09/05
U2538B
3.4
Automatic Threshold Control (ATC)
During the reception of an incoming telegram, the ATC reduces the sensitivity of the demodula-
tor to establish the highest possible signal-to-noise ratio according to the signal strength. This
prevents interferences which may arise during transmission from affecting the output. The
advantage of the circuit is achieved if its output voltage exceeds V
Th
(Comp 1). That is the case
when the input signal strength is more than twice as much as the minimum detectable signal
intensity.
3.5
Automatic Gain Control (AGC)
The automatic gain control improves the circuit's resistance to interference by adapting the
amplification of the gain-controlled amplifier to the relevant existing interference level. In order to
prevent the circuit from responding to transmitted data signals, it gradually reduces the sensitiv-
ity, but only if the duty cycle exceeds a specific value (see
Figure 3-2).
When using telegrams
with higher duty cycles than this value, the capacitor, C
AGC
, maintains the sensitivity for a certain
time period. A higher capacitance enables a longer transmission time. A capacitance of
C
1
= 22 nF is adequate for most known telegrams. A typical value for the maximum duty cycle
(DC) can be calculated by the following formula:
N
DC
max
= -------------------------------------
-
14.2
+
1.1
×
N
Figure 3-2.
Duty Cycle
Transmitted Burst (N cycles)
t
pl
=
N
f
DC
=
t
pl
T
T
N
≥
6; f = 20 kHz to 60 kHz
3.6
Detector
The output signal of the bandpass filter is compared to a fixed reference (Comp 1) and to a ref-
erence generated by the ATC circuit (Comp 2). The output of the comparator with the higher
threshold voltage controls the integrator. Using the integrator keeps the output free of short-time
interference.
The integrator drives the output stage after being processed through a Schmitt trigger. The inter-
nal pull-up resistor can replace an external resistor in some applications.
5
4717B–IRRC–09/05
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