RT8260A 1.8A, 24V, 1.4MHz Step-Down Converter General Description Features The RT8260A is a high voltage buck converter that can support the input voltage range from 4.5V to 24V and the output current can be up to 1.8A. Current Mode operation provides fast transient response and eases loop stabilization. z Wide Operating Input Voltage Range : 4.5V to 24V z Adjustable Output Voltage Range : 0.8V to 15V 1.8A Output Current 0.3Ω Ω Internal Power MOSFET Switch High Efficiency up to 92% 1.4MHz Fixed Switching Frequency Stable with Low ESR Output Ceramic Capacitors Thermal Shutdown Cycle-By-Cycle Over Current Protection RoHS Compliant and Halogen Free z z z z The chip also provides protection functions such as cycleby-cycle current limiting and thermal shutdown protection. The RT8260A is available in a WDFN-8L 2x2 package. z z z z Ordering Information RT8260A Applications Package Type QW : WDFN-8L 2x2 (W-Type) z z Lead Plating System G : Green (Halogen Free and Pb Free) z z Note : Richtek products are : Pin Configurations RoHS compliant and compatible with the current require- (TOP VIEW) ments of IPC/JEDEC J-STD-020. ` Suitable for use in SnPb or Pb-free soldering processes. NC SW VIN EN Marking Information 1 2 3 4 GND ` Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators WLED Drivers 9 8 7 6 5 NC BOOT GND FB JG : Product Code JGW WDFN-8L 2x2 W : Date Code Typical Application Circuit VIN 4.5V to 24V CIN 10µF Chip Enable Open = Automatic Startup DS8260A-03 March 2011 3 VIN BOOT RT8260A SW 2 4 EN VOUT 3.3V 7 CBOOT L1 10nF 4.7µH D1 B230A FB 5 GND 6, 9 (Exposed Pad) R1 62k COUT 22µF R2 19.6k www.richtek.com 1 RT8260A Table 1. Recommended Component Selection VOUT (V) 1.2 1.8 2.5 3.3 5 8 10 15 L1 (μH) 2 2 3.6 4.7 6.8 10 10 15 R2 (kΩ) 124 49.9 29.4 19.6 13 8.2 6.49 4.2 R1 (kΩ) 62 62 62 62 68 75 75 75 COUT (μF) 22 22 22 22 22 22 22 22 Functional Pin Description Pin No. Pin Name Pin Function 1, 8 NC No Internal Connection. 2 SW Switch Output. 3 VIN 4 EN 5 FB Supply Voltage. Bypass VIN to GND with a suitable large capacitor to prevent large voltage spikes from appearing at the input. Chip Enable (Active High). If the EN pin is open, it will be pulled to high by internal circuit. Feedback. An external resistor divider from the output to GND tapped to the FB pin sets the output voltage. The value of the divider resistors also set loop bandwidth. Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 6, GND 9 (Exposed Pad) 7 BOOT Bootstrap. A capacitor is connected between SW and BOOT pins to form a floating supply across the power switch driver. This capacitor is needed to drive the power switch‘s gate above the supply voltage. Function Block Diagram VIN - X20 1µA Current Sense Amp EN 3V FB 1.1V 25mΩ Ω Ramp Generator Regulator 10k + BOOT - Oscillator 1.4MHz + Shutdown Reference Comparator S Q + EA - 400k 30pF + - Driver R SW PWM Comparator Bootstrap Control OC Limit Clamp GND 1pF www.richtek.com 2 DS8260A-03 March 2011 RT8260A Absolute Maximum Ratings z z z z z z z z z z (Note 1) Supply Voltage, VIN ----------------------------------------------------------------------------------------- 26V SW Voltage --------------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) BOOT Voltage ------------------------------------------------------------------------------------------------ (VSW − 0.3V) to (VSW + 6V) All Other Pins ------------------------------------------------------------------------------------------------- 0.3V to 6V Power Dissipation, PD @ TA = 25°C WDFN-8L 2x2 ------------------------------------------------------------------------------------------------- 0.833W Package Thermal Resistance (Note 2) WDFN-8L 2x2, θJA ------------------------------------------------------------------------------------------- 120°C/W WDFN-8L 2x2, θJC ------------------------------------------------------------------------------------------- 8.2°C/W Junction Temperature --------------------------------------------------------------------------------------- 150°C Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------- 260°C Storage Temperature Range ------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Mode) --------------------------------------------------------------------------------- 2kV MM (Machine Mode) ---------------------------------------------------------------------------------------- 200V Recommended Operating Conditions z z z z z (Note 4) Supply Voltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 24V Output Voltage, VOUT --------------------------------------------------------------------------------------- 0.8V to 15V EN Voltage, VEN ---------------------------------------------------------------------------------------------- 0V to 5.5V Junction Temperature Range ------------------------------------------------------------------------------ −40°C to 125°C Ambient Temperature Range ------------------------------------------------------------------------------ −40°C to 85°C Electrical Characteristics (VIN = 12V, TA = 25° C unless otherwise specified) Parameter Symbol VFB 4.5V ≤ VIN ≤ 24V I FB VFB = 0.8V RDS(ON) VEN = 0V, VSW = 0V I LIM VBOOT − VSW = 4.8V f SW Feedback Reference Voltage Feedback Current Switch On Resistance Switch Leakage Current Limit Oscillator Frequency Maximum Duty Cycle Minimum On-Time Under Voltage Lockout Threshold Under Voltage Lockout Threshold Hysteresis EN Input Low Voltage EN Input High Voltage EN Pull Up Current Shutdown Current Quiescent Current I SHDN IQ Thermal Shutdown T SD DS8260A-03 March 2011 Test Conditions t ON Rising VEN = 0V VEN = 0V VEN = 2V, VFB = 1V (Not Switching) Min Typ Max Unit 0.79 ---2.2 1.2 --- 0.8 0.1 0.3 -2.9 1.4 75 100 0.81 0.3 -10 -1.6 --- V μA Ω μA A MHz % ns 3.9 4.2 4.5 V -- 200 -- mV -1.4 ---- --1 25 0.55 0.4 ---1 V V μA μA mA -- 150 -- °C www.richtek.com 3 RT8260A Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. Note 2. θJA is measured in natural convection at TA = 25°C on a high effective thermal conductivity four-layer test board of JEDEC 51-7 thermal measurement standard. The measurement case position of θJC is on the exposed pad of the package. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. www.richtek.com 4 DS8260A-03 March 2011 RT8260A Typical Operating Characteristics Efficiency vs. Load Current 100 Efficiency vs. Load Current 100 VIN = 12V 90 80 VIN = 24V 80 70 Efficiency (%) Efficiency (%) VIN = 12V 90 60 50 40 30 20 VIN = 24V 70 60 50 40 30 20 10 10 VOUT = 5V 0 VOUT = 3.3V 0 0 0.3 0.6 0.9 1.2 1.5 1.8 0 0.3 0.6 Load Current (A) Output Voltage vs. Load Current 1.2 0.815 3.331 0.810 3.326 3.321 3.316 3.311 1.8 0.805 0.800 0.795 0.790 VIN = 12V 3.306 VIN = 12V, IOUT = 0A 0.785 0 0.3 0.6 0.9 1.2 1.5 1.8 -50 -25 0 Load Current (A) 25 50 75 100 125 Temperature (°C) Current Limit vs. Temperature Current Limit vs. Duty Cycle 3.8 3.8 3.5 3.5 Current Limit (A) Peak Current (A) 1.5 Reference Voltage vs. Temperature 3.336 Reference Voltage (V) Output Voltage (V) 0.9 Load Current (A) 3.2 2.9 2.6 3.2 2.9 2.6 2.3 2.3 2.0 2.0 VIN = 12V, VOUT = 3.3V 0 16 32 48 Duty Cycle (%) DS8260A-03 March 2011 64 80 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8260A Quiescent Current vs. Temperature 0.8 1.45 0.7 Quiescent Current (mA) Switching Frequency (MHz)1 Switching Frequency vs. Temperature 1.50 1.40 1.35 1.30 1.25 0.6 0.5 0.4 0.3 VIN = 12V, IOUT = 0.3A VIN = 12V, VEN = 2V, VFB = 1V 0.2 1.20 -50 -25 0 25 50 75 100 -50 125 25 50 75 100 Temperature (°C) Load Transient Response Load Transient Response VOUT (100mV/Div) IOUT (1A/Div) IOUT (1A/Div) 125 VIN = 12V, VOUT = 3.3V, IOUT = 0.9A to 1.8A VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.8A Time (100μs/Div) Time (100μs/Div) Output Ripple Output Ripple VOUT (10mV/Div) VOUT (10mV/Div) VSW (10V/Div) VSW (5V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.8A Time (250ns/Div) www.richtek.com 6 0 Temperature (°C) VOUT (100mV/Div) IL1 (1A/Div) -25 IL1 (1A/Div) VIN = 24V, VOUT = 3.3V, IOUT = 1.8A Time (250ns/Div) DS8260A-03 March 2011 RT8260A Power Off from EN Power On from EN VEN (2V/Div) VEN (2V/Div) VOUT (2V/Div) VOUT (2V/Div) IL1 (2A/Div) IL1 (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.8A Time (250μs/Div) DS8260A-03 March 2011 VIN = 12V, VOUT = 3.3V, IOUT = 1.8A Time (50μs/Div) www.richtek.com 7 RT8260A Application Information The RT8260A is a high voltage buck converter that can support the input voltage range from 4.5V to 24V and the output current can be up to 1.8A. Output Voltage Setting The resistive voltage divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. VOUT R1 FB RT8260A R2 GND Figure 1. Output Voltage Setting For adjustable voltage mode, the output voltage is set by an external resistive voltage divider according to the following equation : VOUT = VFB ⎛⎜ 1 + R1 ⎞⎟ ⎝ R2 ⎠ where VFB is the feedback reference voltage (0.8V typ.). External Bootstrap Diode Connect a 10nF low ESR ceramic capacitor between the BOOT pin and SW pin. This capacitor provides the gate driver voltage for the high side MOSFET. It is recommended to add an external bootstrap diode between an external 5V and the BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty ratio is higher than 65%. The bootstrap diode can be a low cost one such as 1N4148 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT8260A. Note that the external boot voltage must be lower than 5.5V. 5V BOOT RT8260A 10nF SW Figure 2. External Bootstrap Diode www.richtek.com 8 Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current ΔIL increases with higher VIN and decreases with higher inductance. V V ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥ VIN ⎦ ⎣ f ×L ⎦ ⎣ Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. High frequency with small ripple current can achieve highest efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, the value of ΔIL = 0.24 (IMAX = 1.8) will be a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : ⎡ VOUT ⎤ ⎡ VOUT ⎤ L =⎢ ⎥ × ⎢1− ⎥ ⎣ f × ΔIL(MAX) ⎦ ⎣ VIN(MAX) ⎦ The inductor's current rating (defined by that which causes a temperature rise from 25°C ambient to 40°C) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Refer to Table 2 for the suggested inductor selection. Table 2. Suggested Inductors for Typical Application Circuit Component Series Dimensions (mm) Supplier TDK VLC6045 6 x 6 x 4.5 TDK TAIYO YUDEN SLF12565 NR8040 12.5 x 12.5 x 6.5 8x8x4 Diode Selection When the power switch turns off, the path for the current is through the diode connected between the switch output and ground. This forward biased diode must have a minimum voltage drop and recovery times. Schottky diode is recommended and it should be able to handle those current. The reverse voltage rating of the diode should be greater than the maximum input voltage, and current rating should be greater than the maximum load current. For more detail, please refer to Table 3. DS8260A-03 March 2011 RT8260A Table 3. Suggested Diode VRRM Component IOUT Series Package Supplier (V) (A) DIODES B220A 20 2 SMA DIODES B230A 30 2 SMA PANJIT SK22 20 2 DO-214AA PANJIT SK23 30 2 DO-214AA CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoidal current at the source of the top MOSFET. To prevent large ripple current, a low ESR input capacitor sized for the maximum RMS current should be used. The RMS current is given by : V VIN IRMS = IOUT(MAX) OUT −1 VIN VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. The selection of COUT is determined by the required Effective Series Resistance (ESR) to minimize voltage ripple. Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT , is determined by : Aluminum electrolytic capacitors have significantly higher ESR. However, it can be used in cost-sensitive applications for ripple current rating and long term reliability considerations. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD (ESR) also begins to charge or discharge COUT generating a feedback error signal for the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. 1 ⎤ ΔVOUT ≤ ΔIL ⎡⎢ESR + 8fCOUT ⎦⎥ ⎣ EMI Consideration The output ripple will be highest at the maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirement. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR value. However, it provides lower capacitance density than other types. Although Tantalum capacitors have the highest capacitance density, it is important to only use types that pass the surge test for use in switching power supplies. Since parasitic inductance and capacitance effects in PCB circuitry would cause a spike voltage on the SW pin when the high side MOSFET is turned-on/off, this spike voltage on SW may impact EMI performance in the system. In order to enhance EMI performance, there are two methods to suppress the spike voltage. One is to place an R-C snubber between SW and GND and place them as close as possible to the SW pin (see Figure 3). Another method is to add a resistor in series with the bootstrap capacitor, CBOOT. But this method will decrease the driving capability to the high side MOSFET. It is strongly recommended to DS8260A-03 March 2011 www.richtek.com 9 RT8260A reserve the R-C snubber during PCB layout for EMI improvement. Moreover, reducing the SW trace area and keeping the main power in a small loop will be helpful for EMI performance. For detailed PCB layout guide, please refer to the section on Layout Consideration. VIN 4.5V to 24V REN* Chip Enable CIN 10µF 3 VIN BOOT 7 RBOOT* CBOOT L 10nF 4.7µH RT8260A 4 EN SW 2 RS* CEN* 6, 9 (Exposed Pad) B230A R1 49.9k CS* GND VOUT 3.3V/1.8A COUT 22µF FB 5 R2 16k * : Optional Figure 3. Reference Circuit with Snubber and Enable Timing Control 0.90 Maximum Power Dissipation (W)1 Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula: PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications of the RT8260A, the maximum junction temperature is 125°C and TA is the ambient temperature. The junction to ambient thermal resistance, θJA, is layout dependent. For WDFN8L 2x2 packages, the thermal resistance, θJA, is 120°C/ W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for www.richtek.com 10 0.75 0.60 0.45 0.30 0.15 0.00 0 25 50 75 100 125 Ambient Temperature (°C) Figure 4. Derating Curves for RT8260A Packages Layout Consideration Follow the PCB layout guidelines for optimal performance of RT8260A. ` Keep the traces of the main current paths as short and wide as possible. ` Place the input capacitor as close as possible to the device pins (VIN and GND). ` SW node is with high frequency voltage swing and should be kept in a small area. Keep sensitive components away from the SW node to prevent stray capacitive noise pick-up. ` Place the feedback components as close to the FB pin as possible. ` Connect GND to a ground plane for noise reduction and thermal dissipation. WDFN-8L 2x2 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. For the RT8260A package, the derating curve in Figure 4 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Four-Layer PCB DS8260A-03 March 2011 RT8260A GND COUT L D The input capacitor must be placed as close to the IC as possible. The feedback components must be connected as close to the device as possible. VOUT RS CS CIN VIN 1 2 3 4 9 8 7 6 5 NC BOOT GND FB R1 R2 VOUT NC SW VIN EN GND SW GND SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. Figure 5. PCB Layout Guide Table 4. Suggested Capacitors for CIN and COUT Location Component Supplier Part No. Capacitance (µF) Case Size CIN MURATA GRM31CR61E106K 10 1206 CIN TDK C3225X5R1E106K 10 1206 CIN TAIYO YUDEN TMK316BJ106ML 10 1206 C OUT MURATA GRM31CR61C226M 22 1206 C OUT TDK C3225X5R1C226M 22 1206 C OUT TAIYO YUDEN EMK316BJ226ML 22 1206 DS8260A-03 March 2011 www.richtek.com 11 RT8260A Outline Dimension D2 D L E E2 1 e SEE DETAIL A b 2 1 2 1 A A1 A3 DETAIL A Pin #1 ID and Tie Bar Mark Options Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.200 0.300 0.008 0.012 D 1.950 2.050 0.077 0.081 D2 1.000 1.250 0.039 0.049 E 1.950 2.050 0.077 0.081 E2 0.400 0.650 0.016 0.026 e L 0.500 0.300 0.020 0.400 0.012 0.016 W-Type 8L DFN 2x2 Package Richtek Technology Corporation Richtek Technology Corporation Headquarter Taipei Office (Marketing) 5F, No. 20, Taiyuen Street, Chupei City 5F, No. 95, Minchiuan Road, Hsintien City Hsinchu, Taiwan, R.O.C. Taipei County, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611 Tel: (8862)86672399 Fax: (8862)86672377 Email: [email protected] Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek. www.richtek.com 12 DS8260A-03 March 2011