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EL6207
Data Sheet October 10, 2002 FN7221
Dual Laser Driver Oscillator
The EL6207 is a dual push-pull oscillator used to reduce laser noise. It is optimized for 350MHz operation, allowing reduced concern for harmonic EMI. It uses the standard interface to existing ROM controllers. The frequency and amplitude are each set with a separate resistor connected to ground, for each output. The tiny package and harmonic reduction allow the part to be placed close to a laser with low RF emissions. If the voltage at both IOUT pins is less than 1.0V, the chip will be powered down and not oscillate. If the voltage at either IOUT pin is above 1.4V, the chip will be powered up and oscillating. If both IOUT pins are above 1.4V, the chip will also be powered down, and not oscillate. The current drawn by the oscillator consists of a small utility current, plus the peak oscillator amplitude in the positive cycle, which is routed to the enabled IOUT pin. In the negative cycle the oscillator subtracts peak oscillator amplitude from the laser APC current. The part operates from a single 5V supply, and is specified for operation from 0C to +85C.
Features
* Low power dissipation * User-selectable frequency from 100MHz to 600MHz controlled with a single resistor for each laser * User-specified amplitude from 10mAPK-PK to 100mAPK controlled with a single resistor for each output * Auto turn-off threshold * Soft edges for reduced EMI * Small 8-pin LPP package
Applications
* Combi drive using dual laser
Ordering Information
PART NUMBER EL6207CL PACKAGE 8-Pin LPP TAPE & REEL PKG. NO. MDP0047
Pinout
EL6207 (8-PIN LPP) TOP VIEW
VDD 1 IOUT1 2 GND 3 IOUT2 4
8 RAMP1 7 RAMP2 6 RFREQ1 5 RFREQ
1
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EL6207
Absolute Maximum Ratings (TA = 25C)
Voltages Applied to: VDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +6.0V IOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +6.0V RFREQ, RAMP . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +6.0V Operating Ambient Temperature Range . . . . . . . . . . . 0C to +85C Maximum Die Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . +150C Storage Temperature Range . . . . . . . . . . . . . . . . . .-65C to +150C IOUT Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100mAPK-PK Power Dissipation (maximum) . . . . . . . . . . . . . . . . . . . . See Curves
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed. Typical values are for information purposes only. Unless otherwise noted, all tests are at the specified temperature and are pulsed tests, therefore: TJ = TC = TA
Supply & Reference Voltage Characteristics VDD = +5V, TA = 25C, RL = 10, RFREQ = 5210 (FREQ = 360MHz), RAMP =
2540 (Amp = 50mAP-P measured at 95MHz), VOUT = 2.2V (One channel active) PARAMETER PSOR ISO ISTYP ISLO ISHI VCUTOFF VFREQ VRAMP DESCRIPTION Power Supply Operating Range Supply Current Disabled Supply Current Typical Conditions Supply Current Low Conditions Supply Current High Conditions Output Cutoff Voltage Voltage at RFREQ Pin Voltage on RAMP Pin VOUT1 and VOUT2 < 1.3V RFREQ = 5.21k, RAMP = 2.54k, VOUT1 or VOUT2 >1.4V RFREQ = 18.2k, RAMP = 13k, VOUT1 or VOUT2 >1.4V RFREQ = 3.05k, RAMP = 1.3k, VOUT1 or VOUT2 >1.4 Average voltage at cutoff 1.1 1.27 1.27 CONDITIONS MIN 4.5 550 17 6 32 1.4 TYP MAX 5.5 750 22 UNIT V A mA mA mA V V V
Oscillator Characteristics
PARAMETER FOSC FHIGH FLOW TCOSC PSRROSC
VDD = +5V, TA = 25C, RL = 10, RFREQ = 5210 (FREQ = 360MHz), RAMP = 2540 (Amp = 50mAP-P measured at 100MHz), VOUT = 2.2V (One channel active) CONDITIONS Unit-unit frequency variation RFREQ = 3.0k RFREQ = 18.2k MIN 310 TYP 360 600 100 50 1 MAX 410 UNIT MHz MHz MHz ppm/C %
DESCRIPTION Frequency Tolerance Frequency Range High Frequency Range Low
Frequency Temperature Sensitivity -40C to +85C ambient Frequency Change F/F VDD from 4.5V to 5.5V
Driver Characteristics
PARAMETER AMPHIGH AMPLOW IOFFNOM IOFFHIGH IOFFLOW IOUTP-P Duty Cycle PSRRAMP
VDD = +5V, TA = 25C, RL = 10, RFREQ = 18.2k (FREQ = 100MHz), RAMP = 2540 (Amp = 50mAP-P measured at 100MHz), VOUT = 2.2V (One channel active) DESCRIPTION CONDITIONS RAMP = 1.27k RAMP = 12.7k RFREQ = 5210, 10 load, VOUT = 2.2V RFREQ = 5210, 10 load, VOUT = 3.0V RFREQ = 5210, 10 load, VOUT = 1.8V Defined as on standard deviation RFREQ = 5210 VDD from 4.5V to 5.5V MIN TYP 100 10 -4 -4.8 -3.5 2 43 -54 MAX UNIT mAP-P mAP-P mA mA mA % % dB
Amplitude Range High Amplitude Range Low Average Output Current @ 2.2V Average Output Current @ 2.8V Average Output Current @ 1.8V Output Current Tolerance Output Push Time/Cycle Time Amplitude Change of Output I/I
2
EL6207
Driver Characteristics
PARAMETER TON TOFF INOUT VDD = +5V, TA = 25C, RL = 10, RFREQ = 18.2k (FREQ = 100MHz), RAMP = 2540 (Amp = 50mAP-P measured at 100MHz), VOUT = 2.2V (One channel active) (Continued) DESCRIPTION Auto Turn-on Time Auto Turn-off Time CONDITIONS Output voltage step from 0V to 2.2V Output voltage step from 2.2V to 0V MIN TYP 15 0.5 2.5 MAX UNIT s s nA/Hz
IOUT Current Output Noise Density RFREQ = 5210, FMEASURE = 10MHz
Pin Descriptions
PIN NAME 1 2 3 4 5 6 7 8 PIN TYPE VDD IOUT1 GND IOUT1 RFREQ2 RFREQ1 RAMP2 RAMP1 PIN DESCRIPTION Positive power for laser driver (4.5V - 5.5V) Current output to laser anode Chip ground pin (0V) Current output to laser anode Set pin for oscillator frequency Set pin for oscillator frequency Amplitude control input pin Amplitude control input pin
Recommended Operating Conditions
VDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5V 10% VOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2V - 3V RFREQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3k (min) RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3k (min) FOSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80-600MHz AOSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-100mAPK-PK
IOUT Control
VOUT1 Less than VCUTOFF Less than VCUTOFF More than VCUTOFF More than VCUTOFF VOUT2 Less than VCUTOFF More than VCUTOFF Less than VCUTOFF More than VCUTOFF IOUT OFF OFF Normal Operation OFF IOUT OFF Normal Operation OFF OFF
3
EL6207 Block Diagram
Typical ROM Laser Driver Gain Setting Resistor EMI Reduction Supply Filter
+5V
BEAD
4.7F 0.1uF 0.1uF
PNP
Amplitude Setting Resistors
1 EMI Reduction Filters Controller PNP 0.1uF 3 BEAD 4 0.1uF Laser BEAD 2
VDD
RAMP1
8
IOUT1
RAMP2
7
GND
RFREQ1
6
IOUT2
RFREQ2
5
GND Rear Photodiode Calibration Pots Frequency Setting Resistor
Main Board
Flex
On Pickup
4
EL6207 Block Diagram
(Continued)
VDD
1
BAND GAP REFERENCE
IOUT 1
3
DRIVER
AMPLIFIER
8
RAMP 1
AMPLIFIER
7
RAMP 2
GND
2 OSCILLATOR SELECT 6 RFREQ 1
ENABLE LOGIC
5
RFREQ 2
IOUT 2
4
DRIVER
5
EL6207
~10mW
Laser Output Power Laser Output Power
Threshold Current
0mW 0mA ~60mA Laser Current
Oscillator Current
External Read Current
FIGURE 1.
Amplitude vs RAMP 100 Amplitude = 127x1k / RAMP mAPK-PK 80 Amplitude (mA pk-pk) Frequency MHz 500 600
Frequency vs RFREQ Frequency = 1840 x1k/ RFREQ MHz
400
60
300
40
200
20
100
0 0 4k 8k RAMP Value 12k 16k
0 0 4k 8k RFREQ 12k 16k 20k
6
EL6207 Applications Information
Theory of Operation
A typical semiconductor laser will emit a small amount of incoherent light at low values of forward laser current. But after the threshold current is reached, the laser will emit coherent light. Further increases in forward current will cause rapid increases in laser output power. A typical threshold current is 30mA and a typical slope efficiency is 0.7mW/mA. When the laser is lasing, it will often change its mode of operation slightly, due to changes in current, temperature, or optical feedback into the laser. In a DVD-ROM, the optical feedback from the moving disk forms a significant noise factor due to feedback induced mode hopping. In addition to the mode hopping noise, a diode laser will typically have a noise level above threshold that is almost constant regardless of the power level. The signal-to-noise ratio of the output power can be defined by the DC power level divided by the laser noise power. Because of the almost constant noise power above threshold, higher level of output power have higher SNR (signal-to-noise ratio). The RF oscillator is designed to produce a low noise oscillating current that is added to the external DC current. The effect of the AC current is to cause the laser power to change at twice the oscillator frequency. This changing power level causes the laser to go through rapid mode hopping. The low frequency component of laser power noise due to mode hoping is translated up to sidebands around the oscillator frequency by this action. Since the oscillator frequency can be filtered out of the low frequency read and servo channels, the net result is that the laser noise seems to be reduced. The second source of laser noise reduction is caused by the increase in the laser power above the average laser power during the on cycle. The SNR is better at higher laser powers. In addition, when the laser is off, the noise is also very low. Generally it is desirable to make the oscillator current as large as possible to obtain the greatest reduction in laser noise. But it is not a trivial matter to determine this critical value. The amplitude depends on the waveshape of the oscillator current reaching the laser junction. If the output current is sinusoidal, and the components in the output circuit are fixed and linear, then the shape of the current will be sinusoidal. But the amount of current reaching the laser junction is a function of the circuit parasitics. Also, the amount of junction current causing laser emission is variable with frequency due to the junction capacitance. But even this easy case is not available because the output impedance of the oscillator changes somewhat with output voltage. In conclusion, the size of the RAMP resistor must be determined experimentally. But, a good starting point is to use a peak amplitude that is less than the minimum laser threshold current.
Setting the RAMP Resistor
The laser should always have a forward current during operation. This will prevent the laser voltage from collapsing, and ensure that the high frequency components reach the junction without having to charge the junction capacitance. By looking at Figure 1, it can be seen that the applied DC current should be larger than the peak oscillator current.
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