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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
VS
IR Receiver for Data Communication U2538B
U2538B
Input Amplifier and filter Detector mC
AGC/ATC
Modulated IR signal carrier frequency 20 to 60 kHz minimum 6 pulses/burst
Rev. 4717A-IRRC-05/03
1
Block Diagram
VS RF0 RF VS
BIAS IN BPF
+ -
Comp 1
100 kW
OUT & dt
Vth + -
Comp 2 INT ST
TIA
CGA
AGC
ATC
DEM
AGND
CAGC
CAGC
DGND
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
4717A-IRRC-05/03
U2538B
Pin Configuration
Figure 2. Pinning SO8
NC 8 RF 7 AGND 6 IN 5
U2538B
1 VS
2 CAGC
3 OUT
4 DGND
Pin Description
Pin 1 2 3 4 5 6 7 8 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
4717A-IRRC-05/03
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 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 kW), 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 (Zi < 10 kW). If the input resistance is too high, the useful signal will be lost due to the junction capacitance of the photodiode. 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 capacitive coupling of the individual stages. The cut-off frequency is approximately 20 kHz. 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 calculating the resistor, Rf0:
8855 R fO (kW ) = ------------------ - 13 f0 (kHz)
Controlled Gain Amplifier (CGA)
Bandpass Filter (BPF)
where: 20 kHz < f0 < 60 kHz Figure 3. Characteristic of the Bandpass Filter
120
100
80
Grel
60
40
20
0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
f/f0
4
U2538B
4717A-IRRC-05/03
U2538B
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 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 VTh (Comp 1). That is the case when the input signal strength is more than twice as much as the minimum detectable signal intensity. 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 gradually 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 capacitor, CAGC , maintains the sensitivity for a certain time period. A higher capacitance enables a longer transmission time. A capacitance of C1 = 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
Automatic Gain Control (AGC)
Figure 4. Duty Cycle
Transmitted Burst (N cycles)
t pl =
N f
DC =
t pl T
T
N 6; f = 20 kHz to 60 kHz
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 output 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.
5
4717A-IRRC-05/03
Absolute Maximum Ratings
Parameters Symbol Value Unit
Supply voltage Input voltages Input current Power dissipation Tamb = 105C Junction temperature Ambient temperature Storage temperature
Pin 1 Pin 2, 3, and 5 Pin 7 Pin 7
VS VIN IIN Ptot Tj Tamb Tstg
-0.3 to +6.0 -0.3 to VS -0.3 to +1.5 0 to 0.1 110 125 -40 to +105 -40 to +125
V V mA mW
C C C
Thermal Resistance
Parameters Symbol Value Unit
Junction ambient
RthJA
180
K/W
Electrical Characteristics
Tamb = 25C, VS = 5 V
Parameters Test Conditions Symbol Min. Typ. Max. Unit
Supply voltage Supply current Maximum input current VIN = 0 Output voltage low: IOL = 2 mA Internal pull-up resistor Center frequency of bandpass RF = 240 k Q factor Freqency range AGC current source sink AGC slope Number of pulses required Sensitivity Switch-on delay, iIN = 0.7 nA (rms) Switch-off delay, iIN = 0.7 nA (rms) Pulse width, iIN = 0.7 nA (rms), 6 pulse bursts
Pin 1 Pin 1 Pin 5 Pin 3 Pin 3
VS IS IIN VOL RL f0 Q f
4.5 0.35 0.6
5.5 0.65
V mA mA
0.2 75 33.3 100 35 7 20 90 70 6 120 100 20 0.7 60 155 140 125 36.7
V kW kHz
kHz nA nA dB/V nA(rms) Period Period Period
Pin 2 Pin 2 Pin 5 Pin 3, see Figure 5 Pin 3, see Figure 5 Pin 3, see Figure 5 tdon tdoff tpo
3 5 4.5
7.5 10 10
8855 RfO (kW ) = ------------------ - 13 kW f0 (kHz)
6
U2538B
4717A-IRRC-05/03
U2538B
Figure 5. Switch On/Off Delay
VIN Burst, X pulses Repetition rate = 10 ms
VOUT tpo tdon tdoff
Figure 6. Application Circuit
C1 R1 220 3 VBatt R2 > 10 k 2 OUT 470 p(1) 1 GND feedback reduction C4
(1) (1)
C2 100 n C3 10 n 2 CAGC 1 VS
RfO
10 m 16 V
NC
8
RF
7
U2538B
3 OUT AGND 6 D1 (1) 5 D2 (1) D3 (1)
4
DGND
IN
(1)
optional
7
4717A-IRRC-05/03
Ordering Information
Extended Type Number Package Remarks
U2538B-MFP U2538B-MFPG3
SO8 SO8
Tube Taped and reeled
Package Information
Package SO8
Dimensions in mm
5.00 4.85 1.4 0.4 1.27 3.81 8 5 0.25 0.10 0.2 3.8 6.15 5.85 5.2 4.8 3.7
technical drawings according to DIN specifications
1
4
8
U2538B
4717A-IRRC-05/03
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4717A-IRRC-05/03 xM

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