SP8126

PRELIMINARY SP8126B High Speed Differential APC Amplifier FEATURES ■ ■ ■ ■ ■ ■ ■ ■ Dual Wavelength Slew Rate of 300V/µs Fast Settling Time - 7ns Gain Control 2V Output Swing Low Offset Voltage: < 10mV Low Offset Drift: < 20µV/°C 12dB External Gain Adjust Range VCC GAIN RGCOM GND 1 2 3 4 8 7 VOUT + VOUT RGAIN2 RGAIN1 SP8126B 8 Pin COB 6 5 APPLICATIONS ■ DVDRW ■ DVDRAM ■ CDRW DESCRIPTION The SP8126B is a high-speed, differential output APC amplifier that integrates the photodiode and adjustable gain block on one chip. Independent gain control allows individual adjustment for 780mn and 650nm wavelength operation, as found in CD/DVD optical storage drives. This allows the user to control the laser power of the system in high-speed DVDRW, DVDRAM and CDRW systems. The wide 2V output swing also allows better system performance, through improved dynamic range. GAIN VCC + Buffer VOUT+ 1VP-P R1 500 R2 500 (Depend on FPC) 50pF 1k GND TIA + Gain Control R3 500 (Flexible Flat Cable) Buffer VOUT1VP-P 50pF 1k + RGCOM RGAIN1 RGAIN2 RG1 RG2 Figure 1. SP8126B Functional Diagram and Typical Application Rev:B Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 1 ABSOLUTE MAXIMUM RATINGS TJ(MAX) ........................................................................................ 120°C VS(MAX) ............................................................................................... 6V VIN(MAX) .............................................................................................. 6V Power Supply Voltage Continuous Power Dissipation (TAMB = 70°C) COB (derate 13.3mW/°C above +70°C) ......... 90mW These are stress ratings only and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifications below is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. ELECTRICAL SPECIFICATIONS Unless otherwise noted: VCC = 5.0V, CLOAD = 50pF to GND, RLOADP = RLOADN = 1kΩ to GND, RGAIN = 510Ω (Nominal Gain), -20°C ≤ TA ≤ +85°C, Output measured differentially. PARAMETER Supply Voltage Output Sensitivity Output Sensitivity vs. Temp Output Sensitivity vs. Vcc Input Optical Power Required to Produce 2V Output Swing Full Scale Linear Output Voltage Swing Output Common Mode Voltage CONDITIONS MIN 4.5 TYP 5 3000 MAX 5.5 3600 0.1 2 UNITS V V/W %/C %/V µW V Laser Beam Diameter = 0.70mm, uniform density RGAIN=1230Ω RGAIN=1230Ω 2400 533 2 2.3 4.5V < Vcc < 5.5V Vcc/2.2 -10 RGAIN = 1230Ω (High Gain) -20 -25 RGAIN = 1230Ω (High Gain) -50 666 2.8 2.5 Vcc/2 0.5 1.0 -11 -13 1.6 2.4 800 2.7 Vcc/1.8 10 20 25 50 2 5 V V mV mV µV/C µV/C mVrms mVrms MHz MHz dB dB Output Offset Voltage Output Offset Voltage Drift Output Noise BW = 100MHz BW = 100MHz, RGAIN = 1230Ω (High Gain) Bandwidth -3dB -3dB, RGAIN = 1230Ω (High Gain) 70 50 55 50 100 68 76 73 7 12 10 15 PSRR 4.5V < Vcc < 5.5V 4.5V < Vcc < 5.5V, RGAIN = 1230Ω (High Gain) Output Settling Time(650nm) (1% of Final Value) 2VPPStep 2VPP Step, RGAIN = 1230Ω (High Gain) ns ns Rev:A Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 2 ELECTRICAL SPECIFICATIONS: Continued Unless otherwise noted: VCC = 5.0V, CLOAD = 50pF to GND, RLOADP = RLOADN = 1kΩ to GND, RGAIN = 510Ω (Nominal Gain), -20°C ≤ TA ≤ +85°C, Output measured differentially. PARAMETER Output Settling Time (780nm) (1% of Final Value) Output Slew Rate Output Overshoot Power Supply Current CONDITIONS 2VPP Step 2VPP Step, RGAIN = 1230Ω (High Gain) MIN TYP 10 15 MAX 13 18 UNITS ns ns V/µs 200 2VPP Step 300 5 15 % mA -20°C < TA < +85°C Gain Adjust Range Gain Select Input Voltage VIL (TTL Level) Gain Select Input Voltage VIH (TTL Level) 2.0 225Ω < RGAIN < 1230Ω -6 20 6 0.8 mA dB V V PIN DESCRIPTION PIN NUMBER 1 2 3 4 5 6 7 8 NAME VCC GAIN RGCOM GND RGAIN1 RGAIN2 VOUTVOUT+ FUNCTION Supply Voltage Gain Select Common connection point for RGAIN1 and RGAIN2 Power Ground Gain Adjust 1 (Gain Select = LOW) or OPEN Gain Adjust 2 (Gain Select = HIGH) Output Voltage Output Voltage + THEORY OF OPERATION Internal Operation The SP8126B APC circuit has an integrated photo detector and is designed with nominal sensitivities of 3mV/µW for both 650nm and 780nm wavelength laser light. The part's sensitivity can also be adjusted continuously and independently for two different gain modes via two external resistors over a range of ±6dB. The two gain modes are controlled by a TTL compatible logic input, called GAIN SELECT. This logic pin selects between the two external gain setting resistors to allow independent control and settings for the two gain functions. The system is a two stage design, consisting of a Trans-Impedance Amplifier (TIA) and an output buffer stage. In dark condition the outputs VOUT- and VOUT+ are set to a reference voltage Rev:B Date: 3/18/04 that is defined internally as Vcc/2. With 650nm or 780nm wavelength laser light falling on the photodetector, the output will swing differentially around Vcc/2 proportional with the light power according to the chosen gain. TIA and Gain control The first stage is a differential TIA used to convert the photodetector current to a balanced differential voltage. The traditional fixed feedback resistors have been replaced with an active resistor circuit that sets the trans-impedance value. A Resistor Control Block that senses the value of the external gain setting resistor controls the value of the equivalent feedback resistor. © Copyright 2004 Sipex Corporation SP8126B High Speed Differential APC Amplifier 3 THEORY OF OPERATION: Continued The ±6dB Gain adjustment is therefore done directly in the TIA, by adjusting the active feedback blocks proportional with the gain setting resistor. The external resistor is not directly in the signal path, and therefore any parasitic from the off-chip connections does not affect the signal quality. The value of the active feedback is controlled tightly over supply and temperature changes through a Control Block with active feedback circuitry. Gain control is proportional with the external resistor, so the lowest value of RGAIN will produce the -6dB gain adjustment and the highest value will produce the +6dB gain adjustment. Please consult the specification table for the required RGAIN values. Buffer This stage buffers the differential signal from the TIA to the VOUT pins and refers the signal to the internal reference voltage. A balanced current feedback amplifier is used for this purpose to achieve high slew rate and fast settling. The buffer is designed to drive high capacitive loads. The maximum load is 50pF bulk. The actual load is typically a flexible printed circuit (FPC) that acts like a transmission line. This presents a distributed capacitive load plus inductance and resistance. In this case care should be taken to match the characteristic impedance of the line at the far end to avoid standing waves and ringing. The buffer is designed to drive 1kΩ to ground. However, this resistor can be adjusted in value to accommodate the characteristic impedance of the signal trace. The output buffer amplifier is designed to be stable without load and with loads up to 50pF lumped capacitance. 7.0k 6.0k 5.0k 4.0k 3.0k 2.0k 1.0k 0.0k 0.0 250 500 750 1.0k 1.25K 1.5k RGAIN Figure 2. Sensitivity versus RGAIN 180M -9.5dB 160M 140M 120M -6.0dB 0.0dB (Hz) 100M 80M 6 .0dB 60M 40M 0.0 250 500 750 1.0k 1.25K 1.5k RGAIN Ω Figure 3. Bandwidth versus RGAIN TABLE 1: APC SYSTEM TARGET GAIN SET POINTS. VOUTP-VOUTN = 2VP-P, VCC = 5V, T = 27°C Gain (dB) Sensitivity (V/W) RGAIN (Ω) Ts 1% (nS) BW (MHz) Pin (µW) +6 0 -6 6000 3000 1500 1230 510 225 12 7 5.5 68 119 160 335 666 1331 Rev:A Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 4 LAYOUT AND ROUTING CONSIDERATIONS A major consideration in developing the optical pick-up head system is the extremely small form factor, which is additionally aggravated by the physical locations required by the light path. Special care must be taken when designing the Flex or PCB for this part. The output peak current requirement is in the order of 12.5mA when driving 50pF of capacitive load with a slew rate of 250V/µs Therefore care must be taken to provide low inductance, low resistance paths for power and ground and output traces. Supply coupling is also very important. Good supply decoupling is important to ensure the high frequency performance of the system by eliminating supply lead inductance effects. The decoupling capacitor C1, as shown in Figure 5, should be as close to the part as possible. This capacitor should be 0.1µF ceramic. C2 is optional to improve decoupling and is recommended to be 1µF tantalum. The layout of the PCB is pictured here. Note the wide and short traces on the supply lines. The traces for the gain resistors RGAIN1 and RGAIN2 are kept as short as possible to avoid excessive parasitics. Any parasitics on these nodes will limit the performance of the system. RGAIN1 and RGAIN2 are subminiature potentiometers in the application. This is a single layer board using FR4 material. In order to minimize coupling capacitance into the gain setting resistor nodes, it is also critical that VOUT+ and VOUT- are routed away from the traces associated with the gain-setting resistors. Top Figure 4. Test and Evaluation PCB Layout for COB 8 Lead Package Bottom Rev:B Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 5 +5V + C1 0.1µF VCC GAIN GAIN RGCOM GND 1 2 3 4 8 VOUT+ VOUTRGAIN2 RGAIN1 10 RL2 1k 10 + VOUT+ RL1 1k CL1 50pF SP8126B 7 6 5 + CL2 50pF VOUTRG1 510 RG2 510 Figure 5. Test and Evaluation Schematic Rev:A Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 6 0.090mm 1 2 3 4 8 7 6 5 b 0.600 D e S H TOP VIEW E 5° ( 2X) F L BOTTOM VIEW DIMENSIONS in mm Min/Max A 0.90/1.10 PIN SIDE VIEW 8–PIN COB B A C B 0.127/0.33 b 0.30/0.50 C 0.50 nom D 2.90/3.10 E 3.00/3.20 e 0.75 nom H 3.40/3.60 SIDE VIEW DETECTOR SIZE 0.35mm 0.70mm L 0.40/0.60 FF S 0.28/0.48 0.28/0.48 0.075/0.275 Detector Area 8 PIN COB (3.0mm X 3.5mm) Rev:B Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 7 ORDERING INFORMATION Part Number Temperature Range Package Type SP8126CB-3 ........................................... -20°C to +85°C .............. 8 lead COB (3.0mm x 3.5mm) Corporation ANALOG EXCELLENCE Sipex Corporation Headquarters and Sales Office 233 South Hillview Drive Milpitas, CA 95035 TEL: (408) 934-7500 FAX: (408) 935-7600 Sales Office 22 Linnell Circle Billerica, MA 01821 TEL: (978) 667-8700 FAX: (978) 670-9001 e-mail: sales@sipex.com Sipex Corporation reserves the right to make changes to any products described herein. Sipex does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights nor the rights of others. Rev:A Date: 3/18/04 SP8126B High Speed Differential APC Amplifier © Copyright 2004 Sipex Corporation 8