AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Description Pin Assignments The AP6503 is a 340kHz switching frequency external compensated synchronous DC/DC buck converter. It has integrated low RDSON high and low side MOSFETs. ( Top View ) The AP6503 enables continues load current of up to 3A with efficiency as high as 95%. The AP6503 features current mode control operation, which enables fast transient response times and easy loop stabilization. The AP6503 simplifies board layout and reduces space requirements with its high level of integration and minimal need for external components, making it ideal for distributed power architectures. • • • • • • • • 8 SS IN 2 7 EN SW 3 6 COMP GND 4 5 FB Figure 1. Package Pin Out Applications • • • • • • • • • VIN 4.75V to 23V 3A continuous Output Current, 4A Peak VOUT adjustable to 0.925 to 20V 340kHz switching frequency Programmable Soft-Start Enable pin Protection • OCP • Thermal Shutdown Lead Free Finish/ RoHS Compliant (Note 1) Note: 1 SO-8EP The AP6503 is available in a standard Green SO-8EP package with exposed PAD for improved thermal performance and is RoHS compliant. Features BS Gaming Consoles Flat Screen TV sets and Monitors Set Top Boxes Distributed power systems Home Audio Consumer electronics Network Systems FPGA, DSP and ASIC Supplies Green Electronics 1. EU Directive 2002/95/EC (RoHS). All applicable RoHS exemptions applied. Please visit our website at http://www.diodes.com/products/lead_free.html. Typical Application Circuit 100 EFFICIENCY (%) 90 VIN = 5V 80 VIN = 12V 70 60 50 40 0 VOUT = 3.3V L = 10µH 1 2 LOAD CURRENT (A) Efficiency vs. Load Current AP6503 Document number: DS35077 Rev. 1 - 2 3 Figure 2. Typical Application Circuit 1 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Pin Descriptions Pin # Name Description 1 BS High-Side Gate Drive Boost Input. BS supplies the drive for the high-side N-Channel MOSFET switch. Connect a 0.01µF or greater capacitor from SW to BS to power the high side switch. 2 IN Power Input. IN supplies the power to the IC, as well as the step-down converter switches. Drive IN with a 4.75V to 24V power source. Bypass IN to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor. 3 SW Power Switching Output. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BS to power the high-side switch. 4 GND Ground (Connect the exposed pad to Pin 4). 5 FB Feedback Input. FB senses the output voltage and regulates it. Drive FB with a resistive voltage divider connected to it from the output voltage. The feedback threshold is 0.925V. See Setting the Output Voltage. 6 COMP Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND. In some cases, an additional capacitor from COMP to GND is required. See Compensation Components. 7 EN Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator; low to turn it off. Attach to IN with a 100kΩ pull up resistor for automatic startup. 8 SS Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.1µF capacitor sets the soft-start period to 15ms. To disable the soft-start feature, leave SS floating. Functional Block Diagram Figure 3. Functional Block Diagram AP6503 Document number: DS35077 Rev. 1 - 2 2 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Absolute Maximum Ratings (Note 2) Symbol VIN VSW VBS VFB VEN VCOMP TST TJ TL Parameter Supply Voltage Switch Node Voltage Bootstrap Voltage Feedback Voltage Enable/UVLO Voltage Comp Voltage Storage Temperature Junction Temperature Lead Temperature Rating Unit -0.3 to 26 -1.0 to VIN+0.3 VSW-0.3 to VSW + 6 –0.3V to +6 –0.3V to +6 –0.3V to +6 -65 to +150 +150 +260 V V V V V V °C °C °C 3 250 kV V ESD Susceptibility (Note 3) HBM MM Notes: Human Body Model Machine Model 2. Stresses greater than the 'Absolute Maximum Ratings' specified above, may cause permanent damage to the device. These are stress ratings only; functional operation of the device at these or any other conditions exceeding those indicated in this specification is not implied. Device reliability may be affected by exposure to absolute maximum rating conditions for extended periods of time. 3. Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when handling and transporting these devices. Thermal Resistance (Note 4) Symbol θJA θJC Note: Parameter Junction to Ambient Junction to Case Rating Unit 74 16 °C/W °C/W 4. Test condition for SO-8EP: Measured on approximately 1” square of 1 oz copper Recommended Operating Conditions (Note 5) Parameter Symbol Note: Min Max Unit VIN Supply Voltage 4.75 23 V TA Operating Ambient Temperature Range -40 +85 °C 5. The device function is not guaranteed outside of the recommended operating conditions. AP6503 Document number: DS35077 Rev. 1 - 2 3 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Electrical Characteristics (VIN = 12V, TA = +25°C, unless otherwise noted) Symbol Parameter Test Conditions Min Max Unit IIN Shutdown Supply Current VEN = 0V 0.3 3.0 µA IIN VEN = 2.0V, VFB = 1.0V 0.6 1.5 mA ILimit Supply Current (Quiescent) High-Side Switch On-Resistance (Note 6) Low-Side Switch On-Resistance (Note 6) HS Current Limit ILimit LS Current Limit RDS(ON)1 RDS(ON)2 High-Side Switch Leakage Current AVEA Error Amplifier Voltage Gain (Note 5) GEA Error Amplifier Transconductance 100 mΩ 100 mΩ Minimum duty cycle 5.5 A From Drain to Source VEN = 0V, VSW = 0V, Vsw=12V 0.9 A FSW VFB = 0.75V FFB Fold-back Frequency VFB = 0V DMAX Maximum Duty Cycle VFB = 800mV TON Minimum On Time VFB Feedback Voltage Feedback Overvoltage Threshold VEN_Rising 0 ΔIC = ±10μA COMP to Current Sense Transconductance Oscillator Frequency GCS TA = -40°C to +85°C 300 INUVVth 900 0.7 EN Lockout Threshold Voltage 2.2 INUVHYS TSD Soft-Start Period V/V 1000 uA/V 2.8 A/V 340 380 kHz fSW 90 % 200 ns 925 950 0.8 0.9 2.5 2.7 4.05 mV V 220 3.80 μA 800 1.1 EN Rising Threshold VIN Under Voltage Threshold Rising VIN Under Voltage Threshold Hysteresis Soft-Start Current 10 0.30 EN Lockout Hysteresis Note: Typ. V V mV 4.40 V 250 mV VSS = 0V 6 μA CSS = 0.1µF 15 ms 150 °C Thermal Shutdown 6. Guaranteed by design AP6503 Document number: DS35077 Rev. 1 - 2 4 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typical Performance Graphs (VIN = 12V, VOUT=3.3V ,TA = +25°C, unless otherwise noted) 0.56 0.54 0.52 0.5 6 0.054 5.8 0.044 0.034 0.024 0.014 5 10 15 20 25 INPUT VOLTAGE (V) Quiescent Supply Current vs. Input Voltage 0.92 0.918 3.328 0.916 FEEDBACK VOLTAGE (V) 3.33 3.329 3.327 3.326 3.325 3.324 3.323 14.75 19.75 INPUT VOLTAGE (V) Line Regulation 0.906 0.9 -60 24.75 90 90 85 85 80 80 EFFICIENCY (%) VIN = 5V 70 65 VIN = 12V 60 -40 -20 0 20 40 60 TEMPERATURE (C) Current Limit vs. Temperature 80 100 -20 0 20 40 60 80 TEMPERATURE (°C) Feedback Voltage vs. Temperature -20 0 20 40 60 80 TEMPERATURE (°C) Oscillator Frequency vs. Temperature 100 370 365 360 355 350 -60 100 -40 100 90 VIN = 5V 70 VIN = 12V 65 60 80 70 60 55 55 50 50 VOUT = 1.2V L = 3.3µH 45 40 -40 75 75 5.2 375 0.91 0.902 9.75 10 15 20 25 INPUT VOLTAGE (V) Shutdown Supply Current vs. Input Voltage 0.908 3.321 5.4 4.8 -60 5 0.912 0.904 3.32 4.75 0 0.914 3.322 5.6 5 OSCILLATOR FREQUENCY (Khz) 0 OUTPUT VOLTAGE (V) 0.064 0.004 0.48 EFFICIENCY (%) 6.2 CURRENT LIMIT (A) 0.58 0.074 EFFICIENCY (%) SHUTDOWN SUPPLY CURRENT (µA) QUIESCENT SUPPLY CURRENT (mA) 0.6 0 45 1 2 LOAD CURRENT (A) Efficiency vs. Load Current AP6503 Document number: DS35077 Rev. 1 - 2 3 40 0 50 VIN = 12V VOUT = 5V L = 10µH VOUT = 1.8V L = 3.3µH 1 2 LOAD CURRENT (A) Efficiency vs. Load Current 5 of 13 www.diodes.com 3 40 0 1 2 LOAD CURRENT (A) Efficiency vs. Load Current September 2011 © Diodes Incorporated 3 AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typical Performance Characteristics (VIN = 12V, VOUT=3.3V ,L=3.3µH, C1=22uF, C2=47uF, TA = +25°C, unless otherwise noted) Steady State Test no load Steady State Test 3A Startup Through Enable_no load Time-2us/div Time-2us/div Time-10ms/div Startup Through Enable 3A Shutdown Through Enable_no load Time-10ms/div Time-10ms/div Load Transient Test 1.5 to 3A Short Circuit Test Short Circuit Recovery Time-100us/div Time-20us/div Time-20us/div AP6503 Document number: DS35077 Rev. 1 - 2 6 of 13 www.diodes.com Shutdown Through Enable 3A Time-5ms/div September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Applications Information Theory of Operation The AP6503 is a 3A current mode control, synchronous buck regulator with built in power MOSFETs. Current mode control assures excellent line and load regulation and a wide loop bandwidth for fast response to load transients. Figure 3 depicts the functional block diagram of AP6503. The operation of one switching cycle can be explained as follows. At the beginning of each cycle, HS (high-side) MOSFET is off. The EA output voltage is higher than the current sense amplifier output, and the current comparator’s output is low. The rising edge of the 340kHz oscillator clock signal sets the RS Flip-Flop. Its output turns on HS MOSFET. The current sense amplifier is reset for every switching cycle. When the HS MOSFET is on, inductor current starts to increase. The Current Sense Amplifier senses and amplifies the inductor current. Since the current mode control is subject to sub-harmonic oscillations that peak at half the switching frequency, Ramp slope compensation is utilized. This will help to stabilize the power supply. This Ramp compensation is summed to the Current Sense Amplifier output and compared to the Error Amplifier output by the PWM Comparator. When the sum of the Current Sense Amplifier output and the Slope Compensation signal exceeds the EA output voltage, the RS Flip-Flop is reset and HS MOSFET is turned off. For one whole cycle, if the sum of the Current Sense Amplifier output and the Slope Compensation signal does not exceed the EA output, then the falling edge of the oscillator clock resets the Flip-Flop. The output of the Error Amplifier increases when feedback voltage (VFB) is lower than the reference voltage of 0.925V. This also increases the inductor current as it is proportional to the EA voltage. If in one cycle the current in the power MOSFET does not reach the COMP set current value, the power MOSFET will be forced to turn off. When the HS MOSFET turns off, the synchronous LS MOSFET turns on until the next clock cycle begins. There is a “dead time” between the HS turn off and LS turn on that prevents the switches from “shooting through” from the input supply to ground. The voltage loop is compensated through an internal transconductance amplifier and can be adjusted through the external compensation components. Enable The enable (EN) input allows the user to control turning on or off the regulator. To enable the regulator EN must be pulled above the ‘EN Rising Threshold’ and to disable the regulator EN must be pulled below ‘EN falling Threshold’ (EN rising threshold – En threshold Hysteresis). AP6503 Document number: DS35077 Rev. 1 - 2 External Soft Start Soft start is traditionally implemented to prevent the excess inrush current. This in turn prevents the converter output voltage from overshooting when it reaches regulation. The AP6503 has an internal current source with a soft start capacitor to ramp the reference voltage from 0V to 0.925V. The soft start current is 6uA. The soft start sequence is reset when there is a Thermal Shutdown, Under Voltage Lockout (UVLO) or when the part is disabled using the EN pin. External Soft Start can be calculated from the formula below: ISS = C * DV DT Where; Iss = Soft Start Current C = External Capacitor DV=change in feedback voltage from 0V to maximum voltage DT = Soft Start Time Current Limit Protection In order to reduce the total power dissipation and to protect the application, AP6503 has cycle-by-cycle current limiting implementation. The voltage drop across the internal high-side MOSFET is sensed and compared with the internally set current limit threshold. This voltage drop is sensed at about 30ns after the HS turns on. When the peak inductor current exceeds the set current limit threshold, current limit protection is activated. During this time the feedback voltage (VFB) drops down. When the voltage at the FB pin reaches 0.3V, the internal oscillator shifts the frequency from the normal operating frequency of 340Khz to a fold-back frequency of 102Khz. The current limit is reduced to 70% of nominal current limit when the part is operating at 102Khz. This low Fold-back frequency prevents runaway current. Under Voltage Lockout (UVLO) Under Voltage Lockout is implemented to prevent the IC from insufficient input voltages. The AP6503 has a UVLO comparator that monitors the input voltage and the internal bandgap reference. If the input voltage falls below 4.0V, the AP6503 will latch an under voltage fault. In this event the output will be pulled low and power has to be re-cycled to reset the UVLO fault. Over Voltage Protection When the AP6503 FB pin exceeds 20% of the nominal regulation voltage of 0.925V, the over voltage comparator is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off. 7 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Applications Information (cont.) Thermal Shutdown Compensation Components The AP6503 has on-chip thermal protection that prevents damage to the IC when the die temperature exceeds safe margins. It implements a thermal sensing to monitor the operating junction temperature of the IC. Once the die temperature rises to approximately 150°C, the thermal protection feature gets activated .The internal thermal sense circuitry turns the IC off thus preventing the power switch from damage. A hysteresis in the thermal sense circuit allows the device to cool down to approximately 120°C before the IC is enabled again through soft start. This thermal hysteresis feature prevents undesirable oscillations of the thermal protection circuit. The AP6503 has an external COMP pin through which system stability and transient response can be controlled. COMP pin is the output of the internal trans-conductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by: Setting the Output Voltage The output voltage can be adjusted from 0.925V to 18V using an external resistor divider. Table 1 shows a list of resistor selection for common output voltages. Resistor R1 is selected based on a design tradeoff between efficiency and output voltage accuracy. For high values of R1 there is less current consumption in the feedback network. However the trade off is output voltage accuracy due to the bias current in the error amplifier. R2 can be determined by the following equation: ⎛V ⎞ R 1 = R 2 ⋅ ⎜ OUT − 1⎟ ⎝ 0.925 ⎠ A VDC = RLOAD × GCS × A VEA × Where VFB is the feedback voltage (0.925V), RLOAD is the load resistor value, GCS is the current sense transconductance and AVEA is the error amplifier voltage gain. The control loop transfer function incorporates two poles one is due to the compensation capacitor (C3) and the output resistor of error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at: fP1 = fP2 = GEA 2π × C3 × A VEA 1 2π × C2 × RLOAD Where GEA is the error amplifier trans-conductance. One zero is present due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: fZ1 = Figure 4. Feedback Divider Network When output voltage is low, network as shown in Figure 4 is recommended. Vout(V) 5 3.3 2.5 1.8 1.2 R1(KΩ) 45.3 26.1 16.9 9.53 3 R2(KΩ) 10 10 10 10 10 Document number: DS35077 Rev. 1 - 2 1 2π × C3 × R3 The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is crucial. A rule of thumb is to set the crossover frequency to below one-tenth of the switching frequency. Use the following procedure to optimize the compensation components: 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: V 2π × C2 × fc 2π × C2 × 0.1 × fs VOUT × OUT < × GEA × G CS VFB G ×G VFB CS EA Where fC is the crossover frequency, which is typically less than one tenth of the switching frequency. R3 = Table 1—Resistor Selection for Common Output Voltages AP6503 VFB VOUT 9 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Applications Information (cont.) Compensation Components (cont.) 2. Choose the compensation capacitor (C3) to achieve the desired phase margin set the compensation zero, fZ1, to below one forth of the crossover frequency to provide sufficient phase margin. Determine the C3 value by the following equation: A 1µH to 10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current is recommended for most applications. For highest efficiency, the inductor’s DC resistance should be less than 200mΩ. Use a larger inductance for improved efficiency under light load conditions. Input Capacitor 2 C3 > π × R3 × fc The input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. The input capacitor has to sustain the ripple current produced during the on time on the upper MOSFET. It must hence have a low ESR to minimize the losses. Where R3 is the compensation resistor value. VOUT (V) Cin/C1 (µF) Cout/C2 (µF) Rc/R3 (kΩ) Cc/C3 (nF) L1 (µH) 1.2 1.8 2.5 3.3 5 12 22 22 22 22 22 22 47 47 47 47 47 47 3.24 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 3.3 3.3 10 10 10 15 Table 2—Recommended Component Selection Inductor Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate the inductor value; L= VOUT ⋅ (VIN − VOUT ) VIN ⋅ ΔIL ⋅ fSW Where ΔI L is the inductor ripple current. And f SW is the buck converter switching frequency. Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from: The RMS current rating of the input capacitor is a critical parameter that must be higher than the RMS input current. As a rule of thumb, select an input capacitor which has an RMs rating that is greater than half of the maximum load current. Due to large dI/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum must be used, it must be surge protected. Otherwise, capacitor failure could occur. For most applications, a 4.7µF ceramic capacitor is sufficient. Output Capacitor The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the output voltage. The output capacitor is a basic component for the fast response of the power supply. In fact, during load transient, for the first few microseconds it supplies the current to the load. The converter recognizes the load transient and sets the duty cycle to maximum, but the current slope is limited by the inductor value. Maximum capacitance required can be calculated from the following equation: ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from the equation below: Vout capacitor = ΔIinductor * ESR ΔI IL(MAX) = ILOAD + L 2 Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the inductor decreases the converter efficiency while increasing the temperatures of the inductor and the internal MOSFETs. Hence choosing an inductor with appropriate saturation current rating is important. An output capacitor with ample capacitance and low ESR is the best option. For most applications, a 22µF ceramic capacitor will be sufficient. ΔIinductor 2 ) 2 Co = 2 (Δ V + Vout ) − Vout 2 L(Iout + Where ΔV is the maximum output voltage overshoot. AP6503 Document number: DS35077 Rev. 1 - 2 9 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Applications Information (cont.) PC Board Layout This is a high switching frequency converter. Hence attention must be paid to the switching currents interference in the layout. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. These interconnecting impedances should be minimized by using wide, short printed circuit traces. External feedback resistor dividers must be placed close to the FB pin. AP6503 is exposed at the bottom of the package and must be soldered directly to a well designed thermal pad on the PCB. This will help to increase the power dissipation. External Bootstrap Diode It is recommended that an external bootstrap diode be added when the input voltage is no greater than 5V or the 5V rail is available in the system. This helps to improve the efficiency of the regulator. This solution is also applicable for D > 65%. The bootstrap diode can be a low cost one such as BAT54 or a schottky that has a low Vf. 5V 34mm Input capacitor C1 must be placed as close as possible to the IC and to L1. BST 4 AP6503 SW BOOST DIODE 10nF 3 52mm Figure 7—External Bootstrap Compensation Components Recommended Diodes: Part Number B130 SK13 AP6503 Document number: DS35077 Rev. 1 - 2 10 of 13 www.diodes.com Voltage/Current Rating 30V, 1A 30V, 1A Vendor Diodes Inc Diodes Inc September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Ordering Information AP6503 SP - 13 Device AP6503SP-13 Note: Package Code SP Package Packing SP : SO-8EP 13 : Tape & Reel Packaging (Note 7) SO-8EP 13” Tape and Reel Quantity Part Number Suffix 2500/Tape & Reel -13 7. Pad layout as shown on Diodes Inc. suggested pad layout document AP02001, which can be found on our website at http://www.diodes.com/datasheets/ap02001.pdf. Marking Information AP6503 Document number: DS35077 Rev. 1 - 2 11 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Package Outline Dimensions (All Dimensions in mm) Detail "A" Exposed pad 2.4Ref. 3.70/4.10 45° 0.35max. 3.85/3.95 5.90/6.10 7°~9° 7°~9° 1 1 0.15/0.25 Bottom View 1.75max. 1.30/1.50 3.3Ref. 0/0.13 0.254 0.3/0.5 1.27typ 4.85/4.95 1 Gauge Plane Seating Plane 0.62/0.82 Detail "A" 8x-0.60 5.4 Exposed pad 8x-1.55 6x-1.27 Land Pattem Recommendation (Unit:mm) AP6503 Document number: DS35077 Rev. 1 - 2 12 of 13 www.diodes.com September 2011 © Diodes Incorporated AP6503 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). 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