AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Description Pin Assignments The AP6508 is a 500kHz switching frequency internal compensated synchronous DCDC buck converter. It has integrated compensation, and low RDSON high and low side MOSFETs. (Top View) The AP6508 features current mode control operation, which enables fast transient response times and easy loop stabilization. The AP6508 has external programmable softstart and a Power Good indicator enabling sequencing and ramp control. 1 14 AGND SW 2 13 GND SW 3 12 GND SW 4 11 VCC SW 5 10 SS BST 6 9 PG EN 7 8 FB The AP6508 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. The AP6508 is available in a standard Green DFN4030-14 package with exposed PAD for improved thermal performance and is RoHS compliant. Applications • VIN 4.5V to 21V • • • • • • VOUT adjustable to 0.8V 500kHz switching frequency Enable pin External Softstart Power Good Protection o OCP o Thermal Shutdown Lead Free Finish/ RoHS Compliant (Note 1) • • • • • • • Note: Exposed Pad DFN4030-14 Features Gaming Consoles TV sets and Monitors Set Top Boxes Distributed power systems Home Audio Consumer 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 VOUT = 2.5V 90 EFFICIENCY (%) NEW PRODUCT The AP6508 enables continues load current of up to 3A with efficiency as high as 93%. IN 80 70 60 50 VIN = 12V VIN = 5V 40 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current AP6508 Document number: DS33437 Rev. 4 - 2 Typical Application Circuit 3 1 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Pin Descriptions NEW PRODUCT Pin # Name Description Supply Voltage. The AP6508 operates from a 4.5V to 21V input rail. C1 is needed to decouple the input rail. Use wide PCB trace to make the connection. 1 IN 2,3,4,5 SW Switch Output. Use wide PCB trace to make the connection. 6 BST Bootstrap. A capacitor connected between SW and BS pins is required to form a floating supply across the high-side switch driver. 7 EN EN=1 to enable the chip. For automatic start-up, connect EN pin to VIN by proper EN resistor divider as Figure 1 shows. 8 FB Feedback. An external resistor divider from the output to GND, tapped to the FB pin, sets the output voltage. To prevent current limit run away during a short circuit fault condition the frequency fold-back comparator lowers the oscillator frequency when the FB voltage is below 500mV. 9 PG Power Good 10 SS External Softstart 11 VCC BIAS Supply. Decouple with 0.μ1F – 0.22μF cap. And the capacitance should be no more than 0.22μF 12, 13 GND System Ground. This pin is the reference ground for the regulated output voltage. For this reason care must be taken in its PCB layout. Suggested to be connected to GND with copper and vias. 14 AGND Exposed PAD Analog Ground No internal connection. It is recommended to connect exposed pad to GND plane for optimal thermal performance Functional Block Diagram AP6508 Document number: DS33437 Rev. 4 - 2 2 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Absolute Maximum Ratings (Note 2) Symbol VIN Supply Voltage VSW Switch Node Voltage Rating Unit 22 V -0.3 to 23 V VBS Bootstrap Voltage VSW + 6 V VFB Feedback Voltage –0.3V to +6 V VEN VCOMP NEW PRODUCT Parameter TST Enable/UVLO Voltage –0.3V to +6 V Comp Voltage –0.3V to +6 V Storage Temperature -65 to +150 °C TJ Junction Temperature +150 °C TL Lead Temperature +260 °C 4 400 1 kV V kV ESD Susceptibility (Note 3) HBM MM CDM Human Body Model Machine Model Charged Device Model Thermal Resistance (Note 4) Symbol Rating Unit θJA Junction to Ambient Parameter 48 °C/W θJC Junction to Case 11 °C/W Recommended Operating Conditions (Note 5) Symbol Notes: Min Max Unit VIN Supply Voltage Parameter 4.5 21 V TA Operating Ambient Temperature Range -20 +85 °C 2. Exceeding these ratings may damage the device. 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 device. 4. Test condition for DFN4030-14: Measured on approximately 1” square of 1 oz copper. 5. The device function is not guaranteed outside of the recommended operating conditions. AP6508 Document number: DS33437 Rev. 4 - 2 3 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Electrical Characteristics (VIN = 12V, TA = +25°C, unless otherwise noted) Symbol Test Conditions IIN Shutdown Supply Current VEN = 0V IIN Supply Current (Quiescent) VEN = 2.0V, VFB = 1.0V Min Typ. Max Unit 0 µA 1.2 mA RDS(ON)1 High-Side Switch On-Resistance (Note 6) 120 mΩ RDS(ON)2 Low-Side Switch On-Resistance (Note 6) 20 mΩ SWLKG NEW PRODUCT Parameter Switch Leakage Current VEN = 0V, VSW = 0V ILimit Current Limit FSW Oscillator Frequency VFB = 0.75V FFB Fold-back Frequency VFB = 300mV DMAX 0 350 500 Maximum Duty Cycle VFB = 700mV 80 85 Feedback Voltage TA = -20°C to +85°C 791 807 IFB Feedback Current VFB = 800mV 1.1 EN Rising Threshold VEN_HYS EN Threshold Hysteresis IEN EN Input Current µA A 650 kHz fSW 0.3 VFB VEN_Rising 10 5.8 % 823 mV 10 50 nA 1.3 1.5 V 0.4 VEN = 2V 2 VEN = 0V 0 V μA ENTD-Off EN Turn Off Delay (Note 6) PGVth-Hi Power Good Rising Threshold 0.9 VFB PGVth-Lo PGTD VPG IPG_LEAK ISS Power Good Falling Threshold 0.7 VFB Power Good Delay 20 μs Power Good Sink Current Capability 0.4 Power Good Leakage Current VIN Under Voltage Threshold Rising INUVHYS VIN Under Voltage Threshold Hysteresis TSD Note: 4.2 200 Icc=5mA Thermal Shutdown nA μA 10.5 4.0 VCC Regulator VCC Load Regulation V 10 Soft-Start Current INUVVth VCC μs 5 4.4 V mV 5 V 5 % 140 °C 6. Guaranteed by design AP6508 Document number: DS33437 Rev. 4 - 2 4 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typical Performance Characteristics 16 SHUTDOWN SUPPLY CURRENT (µA) QUIESCENT SUPPLY CURRENT (mA) 1.25 1.2 1.15 1.1 1.05 1 0 14 12 10 8 6 4 2 0 5 10 15 20 25 INPUT VOLTAGE Quiescent Supply Current vs. Input Voltage 5.05 0 5 10 15 20 INPUT VOLTAGE (V) Shutdown Supply vs. Input Voltage 5.045 VIN = 12V VOUT = 1.2V CURRENT LIMIT (A) 6.6 5.04 VCC (V) 25 7 6.8 5.035 5.03 6.4 6.2 6 5.8 5.6 5.4 5.025 5.2 5.02 0 5 VCC 10 15 20 INPUT VOLTAGE (V) Regulator Line Regulation 5 -20 -10 0 25 10 20 30 40 50 60 70 80 90 TEMPERATURE (°C) Current Limit vs. Temperature 1.225 1.21 1.2245 1.208 1.224 1.206 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) NEW PRODUCT 1.3 1.2235 1.223 1.2225 1.222 1.2215 1.221 1.2205 VIN = 12V 1.204 1.202 VIN = 5V 1.2 1.198 1.196 1.194 1.22 0 5 10 15 20 INPUT VOLTAGE (V) Line Regulation vs. Output Current AP6508 Document number: DS33437 Rev. 4 - 2 25 5 of 14 www.diodes.com 1.192 0 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) Load Regulation vs. Output Current 3 July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typical Performace Characteristics (cont.) 100 100 VOUT = 1.2V VOUT = 1.8V EFFICIENCY (%) EFFICIENCY (%) 90 80 70 60 50 80 70 60 50 VIN = 12V VIN = 5V 40 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current VIN = 12V VIN = 5V 40 3 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current 3 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current 3 100 100 VOUT = 2.5V 90 EFFICIENCY (%) EFFICIENCY (%) 90 80 70 60 80 70 60 50 50 VIN = 12V VIN = 5V 40 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current 3 40 100 VIN = 12V VOUT = 5V 90 EFFICIENCY (%) NEW PRODUCT 90 80 70 60 50 40 0 2 1 LOAD CURRENT (A) Efficiency vs. Load Current AP6508 Document number: DS33437 Rev. 4 - 2 3 6 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typcal Performance Characteristics NEW PRODUCT VIN = 12V, VOUT = 1.2V, L = 3.3µH, C1 = 22µF, C2 = 47µF, TA = +25°C, unless otherwise noted. Time- 2µs/div Steady State Test IOUT = 3A Time- 200µs/div Load Transient Test IOUT = 1.5A to 3A. Step at 0.8A/µs Time- 500µs/div Start-up Through Enable (No Load) Time- 2ms/div Start-up through VIN (No load) Time- 50µs/div Shutdown Through Enable (No Load) Time- 50µs/div Shutdown Through Enable (Iout =1A) Time- 50µs/div Short Circuit Entry Time- 100µs/div Short Circuit Recovery AP6508 Document number: DS33437 Rev. 4 - 2 7 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Typcal Performance Characteristics NEW PRODUCT VIN = 12V, VOUT = 1.2V, L = 3.3µH, C1 = 22µF, C2 = 47µF, TA = +25°C, unless otherwise noted. Time- 1µs/div Input Voltage Ripple Time- 2µs/div Output Voltage Ripple 90% of VFB 72% of VFB Time- 1µs/div Powergood FallingThreshold Time- 1us/div Powergood Rising Threshold AP6508 Document number: DS33437 Rev. 4 - 2 8 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Application Information Theory of Operation NEW PRODUCT The AP6508 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. 2 depicts the functional block diagram of AP6508. 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 500kHz 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. 5V regulation and hence the output voltage would also drop from regulation. 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). Few conditions on EN function: 1) EN must be pulled low for atleast 5us to disable the regulator. 2) The voltage on EN cannot exceed 5V. 3) AP6508 can be enabled by Vin through a voltage divider as shown in the figure 1 below. 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 0.807V. 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. Figure 1. EN Divider Network VIN −RISE = VEN −RISE Where VEN −RISE = 1.3V(TYP) When the HS MOSFET turns off, the synchronous LS MOSFET turns on until the next clock cycle begins. There is a 520ns “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 internally compensated with the 50pF and 200kΩ RC network. The maximum EAMP voltage output is precisely clamped at 2.1V. Internal Regulator Most of the internal circuitries including the bottom driver are powered from the 5V internal regulator. This regulator uses the Vin input to regulate at 5V. When Vin is less than 5V, this internal regulator cannot maintain the AP6508 Document number: DS33437 Rev. 4 - 2 (R TOP + RBOT || 1MΩ RBOT || 1MΩ VIN−FALL = VEN−FALL (RTOP + RBOT || 1MΩ RBOT || 1MΩ Where VEN −FALL = 0.9V(TYP) Power Good Power Good is an open drain and active high output. This output can be pulled up high to the appropriate level with an external resistor. The PGOOD is flagged low when Vfb=0.7V and is an open drain output when Vfb=0.9V. The PGOOD output can deliver a max of 4 mA sink current at 0.4 V when de- asserted. The PGOOD pin is held low during soft-start. Once output voltage reaches 90% of its final value, PGOOD goes high if there are no faults. 9 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Application Information (cont.) NEW PRODUCT 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 AP6508 has an internal current source with a soft start capacitor to ramp the reference voltage from 0V to 0.807V. The soft start time is internally fixed at 2ms (TYP). The soft start time can be extended > 2ms by adding a soft start capacitor externally. 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: DV ISS = C * DT Where; Iss = Soft Start Current C = External Capacitor DV=change in output voltage from 0V to maximum voltage DT = Soft Start Time 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. Setting the Output Voltage The output voltage can be adjusted from 0.81V to 15V 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 R1 = R 2 ⋅ ⎜⎜ OUT − 1⎟⎟ ⎠ ⎝ 0.81 Current Limit Protection The AP6508 has cycle-by-cycle current limiting implementation. The voltage drop across the internal highside 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 500kHz to a foldback frequency of 150kHz. The current limit is reduced to 70% of nominal current limit when the part is operating at 150kHz. 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 AP6508 has a UVLO comparator that monitors the inout voltage and internal bandgap reference. If the input voltage falls below 3.8V, the AP6508 will latch an under voltage fault. In this event the output will be pulled low and power has to be recycled to reset the UVLO fault. Thermal Shutdown The AP6508 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 140°C, the thermal protection feature gets activated .The internal thermal sense circuitry turns the IC off thus preventing the power switch from damage. AP6508 Document number: DS33437 Rev. 4 - 2 Figure 2. Feedback Divider Network When output voltage is low, T-type network as shown in Figure 2 is recommended. VOUT (V) R1 (kΩ) R2 (kΩ) Rt (kΩ) 1.2 1.8 2.5 3.3 5 4.99 4.99 (1%) 40.2 (1%) 40.2 (1%) 40.2 (1%) 10.2 4.02 (1%) 19.1 (1%) 13 (1%) 7.68 (1%) 24.9 24.9 0 10 35.7 Table 1.Resistor Selection for Common Output Voltages 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; V ⋅ (VIN − VOUT ) L = OUT 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: 10 of 14 www.diodes.com IL(MAX) = ILOAD + ΔIL 2 July 2011 © Diodes Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER Application Information (cont.) Vout capacitor = ΔIinductor * ESR Inductor (cont.) Peak current determines the required saturation current rating, which influences the size of the inductor. An output capacitor with ample capacitance and low ESR is the best option. For most applications, a 22µF ceramic capacitor will be sufficient. NEW PRODUCT 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. 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. 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 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. External feedback resistor dividers must be placed close to the FB pin 34mm 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. Input capacitor C1 must be placed as close as possible to the IC and to L1. 52mm AP6508 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. Maximum capacitance required can be calculated from the following equation: ΔI L(Iout + inductor ) 2 2 Co = (Δ V + Vout ) 2 − Vout 2 Where ΔV is the maximum output voltage overshoot. Document number: DS33437 Rev. 4 - 2 BST 11 of 14 www.diodes.com 6 AP6508 SW ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from the equation below: AP6508 5V BOOST DIODE 10nF 2,3,4,5 Figure 3. External Bootstrap Diode July 2011 © Diodes Incorporated AP P6508 50 00kHz 21V 3A SYNCH HRONOUS S DC/DC BU UCK CONV VERTER Ordering Information n NEW PRODUCT AP650 08 FE - 13 Package Packing FE E : DFN4030 13 : Tape & Ree el 13” Tape T and Reel Pa art Number Suffix Device e Pac ckage Code Pa ackaging (Note 7) Qu uantity AP6508FE E-13 FE F DFN N4030-14 3000/Ta ape & Reel Note: -13 7. Pad layout as shown on Diodes Inc. sugg gested pad layout document d AP02001 1, which can be fou und on our website at http://www.diodes.com m/datasheets/ap020 001.pdf. Marking In nformation n ( Top View w) XX Y WX XX : E8 E : AP6508 8 Y : 0~9 Y : Year W : Week W : A~Z : 1~26 wee ek; a : 27~52 a~z 2 week; z : represen nts 52 and 53 5 A : Green n X : A~Z Part Numb ber P Package Identificatio on Code AP6508F FE DFN4030-14 E8 Package Outline O Dim mensions (All Dimensio ons in mm) AP6508 Document numberr: DS33437 Rev. 4 - 2 12 of 14 ww ww.diodes.com July 2011 © Diodess Incorporated AP6508 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER NEW PRODUCT Tape Orientation (Note 8) Note: 8. The taping orientation of the other package type can be found on our website at http://www.diodes.com/datasheets/ap02007.pdf AP6508 Document number: DS33437 Rev. 4 - 2 13 of 14 www.diodes.com July 2011 © Diodes Incorporated AP6508 500kHz 21V 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). NEW PRODUCT Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes without further notice to this document and any product described herein. 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