AOZ1282DI EZBuck™ 1.2A Simple Buck Regulator General Description Features The AOZ1282DI is a high efficiency, simple to use, 1.2A buck regulator flexible enough to be optimized for a variety of applications. The AOZ1282DI works from a 4.5V to 36V input voltage range, and provides up to 1.2A of continuous output current. The output voltage is adjustable down to 0.8V. The fixed switching frequency of 450kHz PWM operation reduces inductor size. 4.5V to 36V operating input voltage range 240mΩ internal NMOS Up to 95% efficiency Internal compensation 1.2A continuous output current Fixed 450kHz PWM operation Internal soft start Output voltage adjustable down to 0.8V Cycle-by-cycle current limit Short-circuit protection Thermal shutdown Small size DFN2x2-8L Applications Point of load DC/DC conversion Set top boxes and cable modems DVD drives and HDDs LCD Monitors & TVs Telecom/Networking/Datacom equipment Typical Application VIN C3 C1 4.7µF VIN BS EN L1 AOZ1282DI LX VOUT 22µH R1 GND C2 10µF FB R2 Figure 1. 1.2A Buck Regulator Rev. 0.6 September 2012 www.aosmd.com Page 1 of 13 AOZ1282DI Ordering Information Part Number Ambient Temperature Range Package Environmental AOZ1282DI -40 °C to +85 °C DFN2x2-8L Green Product AOS Green Products use reduced levels of Halogens, and are also RoHS compliant. Please visit www.aosmd.com/media/AOSGreenPolicy.pdf for additional information. Pin Configuration LX 1 VIN 2 8 BST 7 GND EPAD VIN 3 6 GND EN 4 5 FB DFN 2x2-8L (Top View) Pin Description Pin Number Pin Name Pin Function 1 LX PWM Output. Connect to inductor. 2, 3 VIN Supply Voltage Input. Range from 4.5V to 36V. When VIN rises above the UVLO threshold the device starts up. 4 EN Enable Pin. The enable pin is active high. Connect EN pin to VIN through current limiting resistor. Do not leave the EN pin floating. 5 FB Feedback Input. It is regulated to 0.8V. The FB pin is used to determine the PWM output voltage via a resistor divider between the output and GND. 6, 7 GND Ground. 8 BST Bootstrap Voltage Input. High side driver supply. Connected to 100nF capacitor between BST and LX. Exposed Pad EPAD Thermal Exposed Pad. Pad can be connected to GND if necessary for improved thermal performance. Rev. 0.6 September 2012 www.aosmd.com Page 2 of 13 AOZ1282DI Absolute Maximum Ratings Recommended Operating Conditions Exceeding the Absolute Maximum Ratings may damage the device. The device is not guaranteed to operate beyond the Recommended Operating Conditions. Parameter Rating Supply Voltage (VIN) Parameter 40V LX to GND -0.7V to VVIN+ 0.3V EN to GND -0.3V to 40V FB to GND -0.3V to 6V VLX + 6V BST to GND Junction Temperature (TJ) +150°C Storage Temperature (TS) -65°C to +150°C ESD Rating (1) 2kV Rating Supply Voltage (VIN) 4.5V to 36V Output Voltage (VOUT) 0.8V to VVIN Ambient Temperature (TA) -40°C to +85°C (2) Package Thermal Resistance (JA) DFN 2x2-8L 55°C/W Note: 2. The value of JA is measured with the device mounted on a 1-in2 FR-4 board with 2 oz. Copper, in a still air environment with TA = 25 °C. The value in any given application depends on the user’s specific board design. Note: 1. Devices are inherently ESD sensitive, handling precautions are required. Human body model rating: 1.5kΩ in series with 100pF. Electrical Characteristics TA = 25 °C, VIN = VEN = 12V, unless otherwise specified. Specifications in BOLD indicate a temperature range of -40°C to +85°C. These specifications are guaranteed by design. Symbol VIN VUVLO Parameter Conditions Supply Voltage Input Under-Voltage Lockout Threshold Min. 4.5 VIN rising VIN falling Max. 36 V V V 260 Supply Current (Quiescent) IOUT = 0, VFB = 1V, VEN > 1.2V IOFF Shutdown Supply Current VEN = 0V VFB Feedback Voltage TA = 25ºC 1 784 800 Units 2.9 2.3 UVLO Hysteresis IIN Typ. mV 1.5 mA 8 A 816 mV VFB_LOAD Load Regulation 120mA < Load < 1.08A 0.5 % VFB_LINE Line Regulation Load = 600mA 0.03 %/V Feedback Voltage Input Current VFB = 800mV 500 nA VEN_OFF VEN_ON EN Input Threshold Off threshold On threshold VEN_HYS EN Input Hysteresis IEN Enable Input Current IFB ENABLE 0.4 1.2 200 V V mV 3 A 540 kHz MODULATOR fO DMAX TON_MIN ILIM Frequency 450 Maximum Duty Cycle 87 % Minimum On Time 150 ns 1.9 A 150 110 °C °C 1.5 ms Current Limit Over-Temperature Shutdown Limit TSS 360 1.5 TJ rising TJ falling Soft Start Interval POWER STATE OUTPUT ILEAKAGE NMOS Leakage VEN = 0V, VLX = 0V RDS(ON) NMOS On-Resistance VIN = 12V Rev. 0.6 September 2012 www.aosmd.com 10 420 A mΩ Page 3 of 13 AOZ1282DI Block Diagram VIN Regulator EN Enable Detect Current Sense SoftStart Ramp Generator OSC BST LDO OC CLK FB Driver PWM Logic 0.8V Error Amplifier BST LX PWM Comparator GND Rev. 0.6 September 2012 www.aosmd.com Page 4 of 13 AOZ1282DI Typical Performance Characteristics Circuit of Figure 1. TA = 25°C, VIN = VEN = 12V, VOUT = 3.3 V, unless otherwise specified. Full Load Operation Light Load Operation IN Voltage (500mV/div) IN Voltage (1V/div) OUT Voltage (100mV/div) OUT Voltage (100mV/div) LX Voltage (10V/div) LX Voltage (10V/div) LOAD Current (1A/div) LOAD Current (1A/div) 2µs/div 2µs/div Start Up to Full Load Load Transient IN Voltage (5V/div) OUT Voltage (100mV/div) OUT Current (1A/div) OUT Voltage (2V/div) OUT Current (1A/div) 5ms/div 200µs/div Short Circuit Protection Short Circuit Recovery LX Voltage (10V/div) LX Voltage (10V/div) OUT Voltage (2V/div) OUT Voltage (2V/div) LOAD Current (1A/div) 2ms/div 2ms/div Rev. 0.6 September 2012 LOAD Current (1A/div) www.aosmd.com Page 5 of 13 AOZ1282DI Typical Performance Characteristics (continued) Efficiency (Vo=5V) vs. Load Current Efficiency (Vo=3.3V) vs. Load Current 100 100 95 5V–3.3V 90 85 Efficiency (%) Efficiency (%) 90 24V–5V 80 75 70 65 85 80 70 55 55 0.2 0.4 0.6 0.8 Load Current (A) 1.0 1.2 18V–3.3V 65 60 0 24V–3.3V 75 60 50 12V–3.3V 95 12V–5V 18V–5V 50 0 0.2 0.4 0.6 0.8 Load Current (A) 1.0 1.2 Current Limit vs. Input Voltage (Vo=3.3V) 2.0 Current Limit (A) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 5 9 13 Rev. 0.6 September 2012 17 21 25 29 Input Voltage (V) 33 37 www.aosmd.com Page 6 of 13 AOZ1282DI Detailed Description The AOZ1282DI is a current-mode step down regulator with integrated high side NMOS switch. It operates from a 4.5V to 36V input voltage range and supplies up to 1.2A of load current. Features include enable control, under voltage lock-out, internal soft-start, output overvoltage protection, over-current protection and thermal shut down. The AOZ1282DI is available in DFN2x2-8L package. Enable and Soft Start The AOZ1282DI has internal soft start feature to limit inrush current and ensure the output voltage ramps up smoothly to regulation voltage. A soft start process begins when the input voltage rises to the voltage higher than UVLO and voltage on EN pin is HIGH. In soft start process, the output voltage is ramped to regulation voltage in typically 400µs. The 400µs soft start time is set internally. The EN pin of the AOZ1282DI is active high. Connect the EN pin to VIN if enable function is not used. Pull it to ground will disable the AOZ1282DI. Do not leave it open. The voltage on EN pin must be above 1.2 V to enable the AOZ1282DI. When voltage on EN pin falls below 0.4V, the AOZ1282DI is disabled. Switching Frequency The AOZ1282DI switching frequency is fixed and set by an internal oscillator. The switching frequency is set internally 450kHz. Output Voltage Programming Output voltage can be set by feeding back the output to the FB pin with a resistor divider network. In the application circuit shown in Figure 1. The resistor divider network includes R1 and R2. Usually, a design is started by picking a fixed R2 value and calculating the required R1 with equation below. R 1 V O = 0.8 1 + ------- R 2 Some standard values of R1 and R2 for the most commonly used output voltage values are listed in Table 1. Vo (V) R1 (kΩ) R2 (kΩ) 1.8 80.6 64.2 2.5 49.9 23.4 3.3 49.9 15.8 5.0 49.9 9.53 Steady-State Operation Under steady-state conditions, the converter operates in fixed frequency and Continuous-Conduction Mode (CCM). The AOZ1282DI integrates an internal NMOS as the high-side switch. Inductor current is sensed by amplifying the voltage drop across the drain to source of the high side power MOSFET. Output voltage is divided down by the external voltage divider at the FB pin. The difference of the FB pin voltage and reference is amplified by the internal transconductance error amplifier. The error voltage is compared against the current signal, which is sum of inductor current signal and ramp compensation signal, at PWM comparator input. If the current signal is less than the error voltage, the internal high-side switch is on. The inductor current flows from the input through the inductor to the output. When the current signal exceeds the error voltage, the high-side switch is off. The inductor current is freewheeling through the external Schottky diode to output. Rev. 0.6 September 2012 Table 1. The combination of R1 and R2 should be large enough to avoid drawing excessive current from the output, which will cause power loss. Protection Features The AOZ1282DI has multiple protection features to prevent system circuit damage under abnormal conditions. Over Current Protection (OCP) The sensed inductor current signal is also used for over current protection. The cycle by cycle current limit threshold is set normal value of 1.9A. When the load current reaches the current limit threshold, the cycle by cycle current limit circuit turns off the high side switch immediately to terminate the current duty cycle. The inductor current stop rising. The cycle by cycle current limit protection directly limits inductor peak current. The average inductor current is also limited due to the limitation on peak inductor current. When cycle by cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreasing. www.aosmd.com Page 7 of 13 AOZ1282DI The AOZ1282DI has internal short circuit protection to protect itself from catastrophic failure under output short circuit conditions. The FB pin voltage is proportional to the output voltage. Whenever FB pin voltage is below 0.2V, the short circuit protection circuit is triggered. As a result, the converter is shut down and hiccups. The converter will start up via a soft start once the short circuit condition disappears. In short circuit protection mode, the inductor average current is greatly reduced. Under Voltage Lock Out (UVLO) An UVLO circuit monitors the input voltage. When the input voltage exceeds 2.9V, the converter starts operation. When input voltage falls below 2.3V, the converter will stop switching. The relationship between the input capacitor RMS current and voltage conversion ratio is calculated and shown in Figure 2. It can be seen that when VO is half of VIN, CIN is under the worst current stress. The worst current stress on CIN is 0.5 x IO. 0.5 0.4 ICIN_RMS(m) 0.3 IO 0.2 0.1 Thermal Protection 0 An internal temperature sensor monitors the junction temperature. It shuts down the internal control circuit and high side NMOS if the junction temperature exceeds 150ºC. The regulator will restart automatically under the control of soft-start circuit when the junction temperature decreases to 110°C. Application Information The basic AOZ1282DI application circuit is shown in Figure 1. Component selection is explained below. Input Capacitor The input capacitor must be connected to the VIN pin and PGND pin of the AOZ1282DI to maintain steady input voltage and filter out the pulsing input current. The voltage rating of input capacitor must be greater than maximum input voltage plus ripple voltage. Since the input current is discontinuous in a buck converter, the current stress on the input capacitor is another concern when selecting the capacitor. For a buck circuit, the RMS value of input capacitor current can be calculated by: VO -------- = m V IN Rev. 0.6 September 2012 1 Figure 2. ICIN vs. Voltage Conversion Ratio For reliable operation and best performance, the input capacitors must have current rating higher than ICIN-RMS at worst operating conditions. Ceramic capacitors are preferred for input capacitors because of their low ESR and high ripple current rating. Depending on the application circuits, other low ESR tantalum capacitor or aluminum electrolytic capacitor may also be used. When selecting ceramic capacitors, X5R or X7R type dielectric ceramic capacitors are preferred for their better temperature and voltage characteristics. Note that the ripple current rating from capacitor manufactures is based on certain amount of life time. Further de-rating may be necessary for practical design requirement. The inductor is used to supply constant current to output when it is driven by a switching voltage. For given input and output voltage, inductance and switching frequency together decide the inductor ripple current, which is: VO VO IO V IN = ----------------- 1 – --------- --------f C IN V IN V IN if we let m equal the conversion ratio: 0.5 m Inductor The input ripple voltage can be approximated by equation below: VO VO - 1 – -------- I CIN_RMS = I O -------V IN V IN 0 VO VO - I L = ----------- 1 – -------fL V IN The peak inductor current is: I L I Lpeak = I O + -------2 High inductance gives low inductor ripple current but requires larger size inductor to avoid saturation. Low ripple current reduces inductor core losses. It also reduces RMS current through inductor and switches, which results in less conduction loss. www.aosmd.com Page 8 of 13 AOZ1282DI When selecting the inductor, make sure it is able to handle the peak current without saturation even at the highest operating temperature. The inductor takes the highest current in a buck circuit. The conduction loss on inductor needs to be checked for thermal and efficiency requirements. Surface mount inductors in different shape and styles are available from Coilcraft, Elytone and Murata. Shielded inductors are small and radiate less EMI noise. But they cost more than unshielded inductors. The choice depends on EMI requirement, price and size. Output Capacitor The output capacitor is selected based on the DC output voltage rating, output ripple voltage specification and ripple current rating. The selected output capacitor must have a higher rated voltage specification than the maximum desired output voltage including ripple. De-rating needs to be considered for long term reliability. Output ripple voltage specification is another important factor for selecting the output capacitor. In a buck converter circuit, output ripple voltage is determined by inductor value, switching frequency, output capacitor value and ESR. It can be calculated by the equation below: 1 V O = I L ESR CO + ------------------------- 8fC In a buck converter, output capacitor current is continuous. The RMS current of output capacitor is decided by the peak to peak inductor ripple current. It can be calculated by: I L I CO_RMS = ---------12 Usually, the ripple current rating of the output capacitor is a smaller issue because of the low current stress. When the buck inductor is selected to be very small and inductor ripple current is high, output capacitor could be overstressed. Schottky Diode Selection The external freewheeling diode supplies the current to the inductor when the high side NMOS switch is off. To reduce the losses due to the forward voltage drop and recovery of diode, Schottky diode is recommended to use. The maximum reverse voltage rating of the chosen Schottky diode should be greater than the maximum input voltage, and the current rating should be greater than the maximum load current. Thermal Management and Layout Consideration O where, CO is output capacitor value, and ESRCO is the equivalent series resistance of the output capacitor. When low ESR ceramic capacitor is used as output capacitor, the impedance of the capacitor at the switching frequency dominates. Output ripple is mainly caused by capacitor value and inductor ripple current. The output ripple voltage calculation can be simplified to: 1 V O = I L ------------------------- 8 f C O If the impedance of ESR at switching frequency dominates, the output ripple voltage is mainly decided by capacitor ESR and inductor ripple current. The output ripple voltage calculation can be further simplified to: V O = I L ESR CO For lower output ripple voltage across the entire operating temperature range, X5R or X7R dielectric type of ceramic, or other low ESR tantalum capacitor or aluminum electrolytic capacitor may also be used as output capacitors. In the AOZ1282DI buck regulator circuit, high pulsing current flows through two circuit loops. The first loop starts from the input capacitors, to the VIN pin, to the LX pins, to the filter inductor, to the output capacitor and load, and then return to the input capacitor through ground. Current flows in the first loop when the high side switch is on. The second loop starts from inductor, to the output capacitors and load, to the anode of Schottky diode, to the cathode of Schottky diode. Current flows in the second loop when the low side diode is on. In PCB layout, minimizing the two loops area reduces the noise of this circuit and improves efficiency. A ground plane is strongly recommended to connect input capacitor, output capacitor, and PGND pin of the AOZ1282DI. In the AOZ1282DI buck regulator circuit, the major power dissipating components are the AOZ1282DI, the Schottky diode and output inductor. The total power dissipation of converter circuit can be measured by input power minus output power. P total_loss = V IN I IN – V O V IN Rev. 0.6 September 2012 www.aosmd.com Page 9 of 13 AOZ1282DI The power dissipation in Schottky can be approximated as: Several layout tips are listed below for the best electric and thermal performance. P diode_loss = I O 1 – D V FW_Schottky 1. The input capacitor should be connected as close as possible to the VIN pin and the GND pin. where, 2. The inductor should be placed as close as possible to the LX pin and the output capacitor. VFW_Schottky is the Schottky diode forward voltage drop. The power dissipation of inductor can be approximately calculated by output current and DCR of inductor. P inductor_loss = IO2 R inductor 1.1 4. Place the feedback resistors and compensation components as close to the chip as possible. The actual junction temperature can be calculated with power dissipation in the AOZ1282DI and thermal impedance from junction to ambient. T 3. Keep the connection of the schottky diode between the LX pin and the GND pin as short and wide as possible. P –P –P total_loss diode_loss inductor_loss = -------------------------------------------------------------------------------------------------------------------------junction +T JA ambient 5. Keep sensitive signal traces away from the LX pin. 6. Pour a maximized copper area to the VIN pin, the LX pin and especially the GND pin to help thermal dissipation. 7. Pour a copper plane on all unused board area and connect the plane to stable DC nodes, like VIN, GND or VOUT. The maximum junction temperature of AOZ1282DI is 150ºC, which limits the maximum load current capability. The thermal performance of the AOZ1282DI is strongly affected by the PCB layout. Extra care should be taken by users during design process to ensure that the IC will operate under the recommended environmental conditions. Rev. 0.6 September 2012 www.aosmd.com Page 10 of 13 AOZ1282DI Package Dimensions, DFN2x2-8L B D C A B bbb A 8 b e 8 R aaa C 2x E Pin#1 Identification Option 1 E1 L 1 D1 1 a a a C 2x BOTTOM VIEW TOP VIEW 8 ccc C A C C A1 ddd C Pin#1 Identification Option 2 seating plan SIDE VIEW Chamfer 0.2x45° 1 BOTTOM VIEW RECOMMENDED LAND PATTERN 0.50 Dimensions in millimeters 0.25 0.25 0.85 0.90 1.70 0.30 1.50 UNIT: mm Symbols A A1 b c D D1 E E1 e Min. 0.70 0.00 0.18 L R aaa bbb ccc ddd 0.20 1.90 1.35 1.90 0.75 Nom. 0.75 0.02 0.25 0.20 REF 2.00 1.50 2.00 0.90 0.50 BSC Max. 0.80 0.05 0.30 0.30 0.20 0.15 0.10 0.10 0.08 0.40 2.10 1.60 2.10 1.00 Dimensions in inches Symbols A A1 b c D D1 E E1 e Min. 0.028 0.000 0.007 Nom. Max. 0.030 0.031 0.001 0.002 0.010 0.012 0.008 REF 0.075 0.079 0.083 0.053 0.059 0.063 0.075 0.079 0.083 0.030 0.035 0.039 0.020 BSC L R aaa bbb ccc ddd 0.008 0.012 0.008 0.006 0.004 0.004 0.003 0.016 Notes: 1. Dimensioning and tolerancing conform to ASME Y14.5M-1994. 2. Controlling dimension is in millimeter, converted inch dimensions are not necessarily exact. 3. Dimension b applies to matellized terminal and is measured between 0.10mm and 0.30mm from the terminal tip. If the terminal has the optional radius on the other end of the terminal, the dimension b should not be measured in that radius area. 4. Coplanarity ddd applies to the terminals and all other bottom surface metallization. Rev. 0.6 September 2012 www.aosmd.com Page 11 of 13 AOZ1282DI Tape and Reel Dimensions, DFN2x2-8L P1 Carrier Tape T P2 D1 E1 E2 E B0 K0 P0 D0 A0 Feeding Direction UNIT: mm Package A0 B0 K0 D0 DFN 2x2 (8mm) 2.30 0.20 2.30 0.20 1.00 ±0.20 1.00 MIN. D1 E E1 1.50 8.00 1.75 +0.10/-0 +0.30/-0.10 ±0.10 Reel E2 P0 P1 P2 T 3.50 ±0.05 4.00 ±0.20 4.00 ±0.20 2.00 ±0.05 0.30 ±0.05 W1 S G N M K V R H W UNIT: mm Tape Size Reel Size 8mm ø178 M ø178.0 ±1.0 N 70.5 ±1.0 W 9.0 ±0.5 W1 11.8 ±1.1 H ø13.0 +0.5/-0.2 K 10.25 ±0.1 S 2.4 ±0.1 G ø9.8 R N/A V N/A Leader / Trailer & Orientation Trailer Tape 300mm Min. OR 75 Empty Pockets Rev. 0.6 September 2012 Components Tape Orientation in Pocket www.aosmd.com Leader Tape 500mm Min. OR 125 Empty Pockets Page 12 of 13 AOZ1282DI Part Marking AOZ1282DI (2x2 DFN) Part Number Code AF1A 9B12 Assembly Location Code Option Code Assembly Lot Code Year Code Week Code This data sheet contains preliminary data; supplementary data may be published at a later date. Alpha & Omega Semiconductor reserves the right to make changes at any time without notice. LIFE SUPPORT POLICY ALPHA & OMEGA SEMICONDUCTOR PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. Rev. 0.6 September 2012 2. A critical component in any component of a life support, device, or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.aosmd.com Page 13 of 13