ChipFind - документация

Электронный компонент: U2538B-MFP

Скачать:  PDF   ZIP
1
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
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. Block Diagram with Typical Circuit
Amplifier
and filter
Detector
Input
AGC/ATC
V
S
U2538B
m
C
Modulated IR signal
carrier frequency 20 to 60 kHz
minimum 6 pulses/burst
IR Receiver for
Data
Communication
U2538B
Rev. 4717AIRRC05/03
2
U2538B
4717AIRRC05/03
Block Diagram
+
-
&
Vth
Comp 1
Comp 2
ATC
DEM
+
-
BPF
AGC
BIAS
IN
A
GND
D
GND
TIA
CGA
INT
ST
OUT
V
S
100 k
W
V
S
R
F0
R
F
C
AGC
dt
C
AGC
TIA
Transimpedance amplifier
ATC
Automatic threshold control
CGA
Controlled gain amplifier
DEM
Demodulator
BPF
Bandpass filter
INT
Integrator
AGC
Automatic gain control
ST
Schmitt trigger
3
U2538B
4717AIRRC05/03
Pin Configuration
Figure 2. Pinning SO8
8
7
6
5
1
2
3
4
NC
RF
AGND
IN
VS
CAGC
OUT
DGND
U2538B
Pin Description
Pin
Symbol
Function
1
VS
Supply voltage
2
CAGC
AGC capacitor
3
OUT
Data output
4
DGND
GND - DEM/INT/ST
5
IN
Input pin diode
6
AGND
GND amplifier
7
RF
Frequency determination
8
NC
Not connected
4
U2538B
4717AIRRC05/03
Functional Description
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 sep-
arate 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 k
W
),
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 k
W
). If the input resistance is too high, the useful
signal will be lost due to the junction capacitance of the photodiode.
Controlled Gain
Amplifier (CGA)
The controlled gain amplifier accounts for the greatest part of the voltage gain and can
be controlled via the voltage at CAGC (Pin 2). Gain control is needed to support the
interference suppression of the detector. High-pass behaviour results from the capaci-
tive coupling of the individual stages. The cut-off frequency is approximately 20 kHz.
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). The following formula can be used for cal-
culating the resistor, R
f0
:
where: 20 kHz < f
0
< 60 kHz
Figure 3. Characteristic of the Bandpass Filter
R
fO
(k
W)
8855
f
0
(kHz)
-------------------
13
=
0
20
40
60
80
100
120
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
f/f
0
G
re
l
5
U2538B
4717AIRRC05/03
Automatic Threshold
Control (ATC)
During the reception of an incoming telegram, the ATC reduces the sensitivity of the
demodulator to establish the highest possible signal-to-noise ratio according to the sig-
nal 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.
Automatic Gain Control
(AGC)
The automatic gaincontrol 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 grad-
ually reduces the sensitivity, but only if the duty cycle exceeds a specific value
(see Figure 4). When using telegrams with higher duty cycles than this value, the capac-
itor, 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:
Figure 4. Duty Cycle
Detector
The output signal of the bandpass filter is compared to a fixed reference (Comp 1) and
to a reference 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 out-
put free of short-time interference.
The integrator drives the output stage after being processed through a Schmitt trigger.
The internal pull-up resistor can replace an external resistor in some applications.
DC
max
N
14.2
1.1
N
+
-------------------------------------
=
T
Transmitted Burst (N cycles)
N
6; f = 20 kHz to 60 kHz
f
N
t
pl
=
T
t
DC
pl
=