(R) RT8251 5A, 24V, 570kHz Step-Down Converter General Description Features The RT8251 is a monolithic step-down switch mode converter with a built-in internal power MOSFET. It achieves 5A continuous output current over a wide input supply range with excellent load and line regulation. Current mode operation provides fast transient response and eases loop stabilization. z Wide Operating Input Voltage Range : 4.75V to 24V z Adjustable Output Voltage Range : 0.8V to 15V Output Current up to 5A 25 A Low Shutdown Current Internal Power MOSFET : 70m The RT8251 provides protection functions such as cycle-by-cycle current limiting and thermal shutdown. In shutdown mode, the regulator draws 25A of supply current. Programmable soft-start minimizes the inrush supply current and the output overshoot at initial startup. The RT8251 requires a minimum number of external components. The RT8251 is available in WQFN-16L 3x3 and SOP-8 (Exposed Pad) packages. z z z z z z z z z Applications z z z Pin Configurations High Efficiency up to 95% 570kHz Fixed Switching Frequency Stable with Low ESR Output Ceramic Capacitors Thermal Shutdown Protection Cycle-By-Cycle Over Current Protection RoHS Compliant and Halogen Free z Distributed Power Systems Battery Charger DSL Modems Pre-regulator for Linear Regulators (TOP VIEW) VIN VIN SW SW Ordering Information RT8251 16 15 14 13 VIN VIN VIN GND 12 1 2 SW SW 9 BOOT 11 GND 3 10 17 4 6 7 Lead Plating System G : Green (Halogen Free and Pb Free) 8 FB COMP EN SS 5 Package Type QW : WQFN-16L 3x3 (W-Type) SP : SOP-8 (Exposed Pad-Option 1) SW Note : Richtek products are : WQFN-16L 3x3 RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. 8 BOOT VIN 2 SW 3 GND 4 GND SS 7 EN 6 COMP 5 FB 9 Suitable for use in SnPb or Pb-free soldering processes. SOP-8 (Exposed Pad) Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8251 Marking Information RT8251GQW RT8251GSP JM= : Product Code RT8251GSP : Product Number RT8251 GSPYMDNN YMDNN : Date Code JM=YM DNN YMDNN : Date Code Typical Application Circuit 1, 2, 3, 15, 16 VIN 4.75V to 24V CIN 10F x 2 BOOT VIN RT8251 7 EN 8 SS Chip Enable CSS 10nF 4, Exposed Pad (17) 9 SW 10 to 14 FB 5 GND CBOOT L 100nF 4.7H COMP D B540C CC 2.2nF 6 VOUT 3.3V/5A R1 30.9k RC 22k COUT 22F x 2 R2 10k CP (Open) Figure 1. Typical Application Circuit for WQFN-16L 3x3 2 VIN 4.75V to 24V CIN 10F x 2 Chip Enable BOOT VIN RT8251 7 EN CSS 10nF 8 SS 4, Exposed Pad(9) GND 1 SW 3 CBOOT 100nF D B540C L 4.7H R1 30.9k FB 5 COMP 6 CC 2.2nF RC 22k VOUT 3.3V/5A COUT 22F x 2 R2 10k CP NC Figure 2. Typical Application Circuit for SOP-8 (Exposed Pad) Table 1. Recommended Component Selection VOUT (V) R1 (k) R2 (k) RC (k) CC (nF) L1 (H) COUT (F) 15 182 10 51 1 22 44 10 115 10 43 1.2 10 44 8 91 10 39 1.5 10 44 5 3.3 52.3 30.9 10 10 30 22 1.5 2.2 6.8 4.7 44 44 2.5 21.5 10 16 2.2 4.7 44 1.8 1.2 12.4 4.99 10 10 13 13 2.2 2.2 2.2 2.2 44 44 Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Functional Pin Description Pin No. WQFN-16L 3x3 Pin Name SOP-8 (Exposed Pad) 1, 2, 3, 15, 16 2 VIN 4, 4, GND 17 (Exposed Pad) 9 (Exposed Pad) 5 5 FB 6 6 COMP 7 7 EN 8 8 SS 9 1 BOOT 10, 11, 12, 13, 14 3 SW Pin Function Power Input Pin. VIN supplies the power to the IC, as well as the step-down converter switches. Connect VIN with a 4.75V to 24V power source. Connect VIN to GND with a capacitor that the capacitance is large enough to eliminate noise on the input to the IC. Ground. This pin is the voltage reference for the regulated output voltage. For this reason, care must be taken in its layout. This node should be placed outside of the D1 to CIN ground path to prevent switching current spikes from inducing voltage noise into the part. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. Feedback Input. An external resistor divider from the output to GND, tapped to the FB pin, sets the output voltage. Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 Compensation Node. This node is the output of the transconductance error amplifier and the input to the current comparator. Frequency compensation is done at this node by connecting a series R-C to ground. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN higher than 1.4V to turn on the regulator, lower than 0.4V to turn it off. For automatic startup, leave EN unconnected. Soft-Start Control Input. SS controls the soft start period. Connect a capacitor ( 10nF) from SS to GND to set the soft-start period. A 10nF capacitor sets the Soft-Start period to 1ms. Bootstrap. This capacitor CBOOT is needed to drive the power switch's gate above the supply voltage. It is connected between the SW and BS pins to form a floating supply across the power switch driver. The voltage across CBOOT is about 5V and is supplied by the internal +5V supply when the SW pin voltage is low. 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 BOOT to power the high-side switch. is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8251 Function Block Diagram VIN VCC Internal Regulator Current Sense Slope Comp Amplifier + Oscillator 570kHz 1A EN VA VCC 10k Foldback Control 1.1V 3V + Shutdown Comparator 0.4V BOOT + UV Comparator VCC + Current Comparator 10A SS VA - Logic SW GND + +EA Gm = 820A/V 0.8V FB Absolute Maximum Ratings COMP (Note 1) Supply Voltage, VIN ---------------------------------------------------------------------------------------- -0.3V to 26V z Switching Voltage, VSW ----------------------------------------------------------------------------------- -0.3V to (VIN + 0.3V) z BOOT Voltage, VBOOT -------------------------------------------------------------------------------------- (VSW - 0.3V) to (VSW + 6V) z All Other Pins ------------------------------------------------------------------------------------------------ -0.3V to 6V z Power Dissipation, PD @ TA = 25C WQFN-16L 3x3 ---------------------------------------------------------------------------------------------- 1.471W SOP-8 (Exposed Pad) ------------------------------------------------------------------------------------- 1.333W z Package Thermal Resistance (Note 2) WQFN-16L 3x3, JA ---------------------------------------------------------------------------------------- 68C/W WQFN-16L 3x3, JC ---------------------------------------------------------------------------------------- 7.5C/W SOP-8 (Exposed pad), JA ------------------------------------------------------------------------------- 75C/W SOP-8 (Exposed Pad), JC ------------------------------------------------------------------------------- 15C/W z Junction Temperature -------------------------------------------------------------------------------------- 150C z Lead Temperature (Soldering, 10 sec.) ---------------------------------------------------------------- 260C z Storage Temperature Range ------------------------------------------------------------------------------ -65C to 150C z ESD Susceptibility (Note 3) HBM (Human Body Mode) -------------------------------------------------------------------------------- 2kV MM (Machine Mode) --------------------------------------------------------------------------------------- 200V z Recommended Operating Conditions z z z z (Note 4) Supply Voltage, VIN ---------------------------------------------------------------------------------------- 4.75V to 24V Enable Voltage, VEN ---------------------------------------------------------------------------------------- 0V to 5.5V Junction Temperature Range ----------------------------------------------------------------------------- -40C to 125C Ambient Temperature Range ----------------------------------------------------------------------------- -40C to 85C Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Electrical Characteristics (VIN = 12V, TA = 25C unless otherwise specified) Parameter Symbol Min Typ Max Unit 0.784 0.8 0.816 V --- 70 15 --- m --- -6.8 10 -- A A -- 4.6 -- A/V -420 920 570 -720 A/V kHz --- 185 85 --- kHz % -- 100 -- ns UVLO Threshold Rising -- 4.1 -- V UVLO Threshold Hysteresis -- 200 -- mV Feedback Reference Voltage VFB High-Side Switch-On Resistance Low-Side Switch-On Resistance RDS(ON)1 RDS(ON)2 Test Conditions 4.75V VIN 24V High-Side Switch Leakage Current Limit ILIM VEN = 0V, VSW = 0V Duty = 85%; VBOOT-SW = 4.8V Current Sense Transconductance GCS Output Current to VCOMP Error Amplifier Tansconductance Oscillator Frequency Gm fSW I C = 10A Short Circuit Oscillation Frequency Maximum Duty Cycle DMAX Minimum On-Time VFB = 0V VFB = 0.7V tON EN Threshold Logic Low VIL -- -- 0.4 Voltage Logic High VIH 1.4 -- 5.5 --- 1 25 --- A A V Enable Pull Up Current Shutdown Current ISHDN VEN = 0V VEN = 0V Quiescent Current Soft-Start Current IQ ISS VEN = 2V, VFB = 1V VSS = 0V --- 0.8 10 1 -- mA A CSS = 10nF -- 1 -- ms -- 150 -- C Soft-Start Period Thermal Shutdown TSD Note 1. Stresses beyond those listed "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and 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 may affect device reliability. Note 2. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. JC is measured at 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. Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8251 Typical Operating Characteristics Efficiency vs. Load Current Efficiency vs. Load Current 100 100 VIN = 12V 90 VIN = 24V 80 VIN = 24V 70 Efficiency(%) Efficiency (%) 80 VIN = 12V 90 60 50 40 30 70 60 50 40 30 20 20 10 10 VOUT = 3.3V 0 VOUT = 5V 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 Load Current (A) 1 VIN = 12V VIN = 5V 0.5 0 -0.5 -1 -1.5 0.1 1 1.5 4.5 5 IOUT = 3A IOUT = 0A 1 0.5 0 -0.5 -1 -1.5 -2 4 10 6.5 9 11.5 14 16.5 19 21.5 24 Input Voltage (V) Load Current (A) Quiescent Current vs. Temperature Reference Voltage vs. Temperature 1.2 0.816 1 0.811 Reference Voltage (V) Quiescent Current (mA) 4 VOUT = 3.3V VOUT = 3.3V 0.01 3.5 IOUT = 5A VIN = 24V -2 0.001 3 Output Voltage Deviation vs. Input Voltage 2 Output Voltage Deviation (%)1 Output Voltage Deviation (%)1 1.5 2.5 Load Current (A) Output Voltag Deviation vs. Load Current 2 2 0.8 0.6 0.4 0.2 0.806 0.801 0.796 0.791 VIN = 12V 0 VIN = 12V 0.786 -50 -25 0 25 50 75 100 Temperature (C) Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 125 -50 -25 0 25 50 75 100 125 Temperature (C) is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Switching Frequency vs. Temperature Switching Frequency vs. Input Voltage 630 Switching Frequenc (kHz)1 11 Switcing Frequency (kHz)1 610 600 590 580 570 560 VOUT = 3.3V, IOUT = 1A 550 4 6.5 9 11.5 14 16.5 19 21.5 24 610 590 VIN = 12V 570 550 VIN = 24V 530 VOUT = 3.3V, IOUT = 1A 510 -50 Input Voltage (V) -25 0 25 50 75 100 125 Temperature (C) Output Ripple Current Limit vs. Duty Cycle 9.3 VOUT (10mV/Div) Peak Current (A) 8.7 8.1 VSW (10V/Div) 7.5 6.9 6.3 VIN = 12V VOUT = 3.3V IOUT = 5A ISW (2A/Div) 5.7 0 10 20 30 40 50 60 70 80 90 100 Time (1s/Div) Duty Cycle (%) Load Transient Response Load Transient Response VIN = 12V, VOUT = 3.3V IOUT = 0A to 5A VIN = 12V, VOUT = 3.3V IOUT = 2.5A to 5A VOUT (200mV/Div) VOUT (200mV/Div) IOUT (2A/Div) IOUT (2A/Div) Time (100s/Div) Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 Time (100s/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8251 Power On from EN Power Off from EN VEN (5V/Div) VEN (5V/Div) VOUT (2V/Div) VOUT (2V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 5A Time (250s/Div) Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 VIN = 12V, VOUT = 3.3V, IOUT = 5A Time (25s/Div) is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Application Information The RT8251 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75V to 24V and the output current can be up to 5A. Output Voltage Setting The resistive divider allows the FB pin to sense the output voltage as shown in Figure 3. Soft-Start The RT8251 contains an external soft-start clamp that gradually raises the output voltage. The soft-start timming can be set by the external capacitor between SS pin and GND. The chip provides a 10A charge current for the external capacitor. If 10nF capacitor is used to set the soft-start time, its period will be 1ms (typ.). VOUT Chip Enable Operation R1 FB RT8251 R2 GND Figure 3. Output Voltage Setting The output voltage is set by an external resistive 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 100nF 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 cycle 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 RT8251. 5V BOOT RT8251 100nF SW Figure 4. External Bootstrap Diode Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 The EN pin is the chip enable input. Pull the EN pin low (<0.4V) will shutdown the device. During shutdown mode, the RT8251 quiescent current drops to lower than 25A. Drive the EN pin to high ( >1.4V, < 5.5V) will turn on the device again. If the EN pin is open, it will be pulled to high by internal circuit. For external timing control (e.g.RC), the EN pin can also be externally pulled to High by adding a100k or greater resistor from the VIN pin (see Figure 5). Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current IL increases with higher VIN and decreases with higher inductance. V V IL = OUT x 1- OUT VIN f xL 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 IL = 0.24(IMAX) 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 = x 1- f x IL(MAX) VIN(MAX) The inductor 's current rating (caused a 40C temperature rising from 25C ambient) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Please see Table 2 for the inductor selection reference. is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8251 Table 2. Suggested Inductors for Typical Application Circuit Component Supplier Series Dimensions (mm) TDK TAIYO YUDEN TDK SLF10165 NR10050 VLF12060 10.1x10.1x7 10x9.8x5 12x11.7x6 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 4. CIN and COUT Selection The input capacitance, C IN , is needed to filter the trapezoidal current at the source of the high side 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 I RMS = I OUT /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. For the input capacitor, two 10F low ESR ceramic capacitors are recommended. For the recommended capacitor, please refer to table 3 for more detail. The selection of COUT is determined by the required 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. Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 The output ripple, VOUT , is determined by : 1 VOUT IL ESR + 8fC OUT The output ripple will be highest at the maximum input voltage since IL 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. 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 ILOAD (ESR) also begins to charge or discharge COUT generating a feedback error signal for the regulator is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 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. snubber between SW and GND and make them as close as possible to the SW pin (see Figure 5). 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 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 on EMI performance. For detailed PCB layout guide, please refer to the section of Layout Consideration. EMI Consideration Since parasitic inductance and capacitance effects in PCB circuitry would cause a spike voltage on the SW pin when high-side MOSFET is turned-on/off, this spike voltage on SW may impact on 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 2 VIN 4.75V to 24V REN* CIN 10F x 2 Chip Enable BOOT VIN 1 RBOOT* CBOOT L 100nF 4.7H RT8251 SW 3 7 EN CS* 8 SS CSS 4, 10nF Exposed Pad(9) GND D B540C RS* CEN* VOUT 3.3V/5A R1 30.9k COUT 22F x 2 FB 5 COMP * : Optional 6 CC 2.2nF RC 22k R2 10k CP NC Figure 5. Reference Circuit with Snubber and Enable Timing Control Thermal Considerations For continuous operation, do not exceed the maximum operation junction temperature 125C. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = (TJ(MAX) - TA ) / JA Where T J(MAX) is the maximum operation junction temperature , TA is the ambient temperature and the JA is the junction to ambient thermal resistance. For recommended operating conditions specification of RT8251, the maximum junction temperature is 125C. The junction to ambient thermal resistance JA is layout dependent. For PSOP-8 and WQFN packages, the thermal resistance JA are 75C/W and 68C/W on the standard JEDEC 51-7 four-layers thermal test board. The maximum power dissipation at TA = 25C can be calculated by following formula : Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 PD(MAX) = (125C - 25C) / (75C/W) = 1.333W for PSOP-8 PD(MAX) = (125C - 25C) / (68C/W) = 1.471W for WQFN (min.copper area PCB layout) PD(MAX) = (125C - 25C) / (49C/W) = 2.04W for PSOP-8 (70mm2copper area PCB layout) The thermal resistance JA of SOP-8 (Exposed Pad) is determined by the package architecture design and the PCB layout design. However, the package architecture design had been designed. If possible, it's useful to increase thermal performance by the PCB layout copper design. The thermal resistance JA can be decreased by adding copper area under the exposed pad of SOP-8 (Exposed Pad) package. As shown in Figure 6, the amount of copper area to which the SOP-8 (Exposed Pad) is mounted affects thermal performance. When mounted to the standard is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8251 SOP-8 (Exposed Pad) pad (Figure 6a), JA is 75C/W. Adding copper area of pad under the SOP-8 (Exposed Pad) (Figure 6.b) reduces the JA to 64C/W. Even further, increasing the copper area of pad to 70mm2 (Figure 6.e) reduces the JA to 49C/W. The maximum power dissipation depends on operating ambient temperature for fixed T J(MAX) and thermal resistance JA. For RT8251 packages, the derating curves in Figure 7 and Figure 8 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation allowed. 2.2 (a) Copper Area = (2.3 x 2.3) mm2, JA = 75C/W Four Layer PCB Power Dissipation (W) 2.0 1.8 Copper Area 70mm2 50mm2 30mm2 10mm2 Min.Layout 1.6 1.4 1.2 1.0 0.8 (b) Copper Area = 10mm2, JA = 64C/W 0.6 0.4 0.2 0.0 0 25 50 75 100 125 (c) Copper Area = 30mm2 , JA = 54C/W Ambient Temperature (C) Figure 7. Derating Curves for PSOP-8 Package Maximum Power Dissipation (W)1 1.6 Four Layer PCB 1.4 1.2 (d) Copper Area = 50mm2 , JA = 51C/W 1.0 WQFN-16L 3x3 0.8 0.6 0.4 0.2 0.0 0 15 30 45 60 75 90 105 120 Ambient Temperature (C) Figure 8. Derating Curves for WQFN Package Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 135 (e) Copper Area = 70mm2 , JA = 49C/W Figure 6. Themal Resistance vs. Copper Area Layout Design is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Layout Consideration Connect feedback network behind the output capacitors. Keep the loop area small. Place the feedback components near the RT8251. Follow the PCB layout guidelines for optimal performance of the RT8251. Keep the traces of the main current paths as short and wide as possible. Connect all analog grounds to a command node and then connect the command node to the power ground behind the output capacitors. Examples of PCB layout guide are shown in Figure 9 and Figure 10 for reference. Put the input capacitor as close as possible to the device pins (VIN and GND). LX node is with high frequency voltage swing and should be kept at small area. Keep analog components away from the LX node to prevent stray capacitive noise pickup. Input capacitor must be placed as close as to the IC as possible. COUT GND CIN CS D RS VIN VIN SW SW L VOUT 16 15 14 13 R1 SW SW should be connected SW GND to inductor by Wide and 10 SW 17 short trace. Keep sensitive C BOOT 9 BOOT components away from this trace. 5 6 7 8 12 1 11 2 3 4 FB COMP EN SS VIN VIN VIN GND R2 RC VOUT CSS CP CC GND The feedback components must be connected as close to the device as possible. Figure 9. PCB Layout Guide for WQFN Package Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8251 VIN GND The feedback components must be connected as close to the device as possible. SW GND CBOOT Input capacitor must be placed as close to the IC as possible. CIN BOOT D CS COUT CSS RS VIN 2 SW 3 GND 4 GND CC 8 SS 7 EN 6 COMP 5 FB 9 CP R1 R2 L RC VOUT VOUT GND SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. Figure 10. PCB Layout Guide for PSOP-8 Package Table 3. Suggested Capacitors for CIN and COUT Location Component Supplier Part No. Capacitance (F) Case Size CIN MURATA GRM31ER61E226K 22 1210 CIN TDK C4535X5R1E226M 22 1812 CIN TAIYO YUDEN TMK325BJ226MM 22 1210 COUT MURATA GRM32ER61C476M 47 1210 COUT MURATA GRM31CR60J476M 47 1206 COUT TDK C3216X5R0J476M 47 1206 COUT TAIYO YUDEN LMK316BJ476MM 47 1206 Table 4. Suggested Diode Component Supplier Part No. V RRM (V) I OUT (A) Package DIODES B540C 40 5 SMC ON MBRS540T3 40 5 SMC Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8251-03 January 2012 RT8251 Outline Dimension D SEE DETAIL A D2 L 1 E E2 b e A A1 1 1 2 2 DETAIL A Pin #1 ID and Tie Bar Mark Options A3 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 0.180 0.300 0.007 0.012 D 2.950 3.050 0.116 0.120 D2 1.300 1.750 0.051 0.069 E 2.950 3.050 0.116 0.120 E2 1.300 1.750 0.051 0.069 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 16L QFN 3x3 Package Copyright (c) 2012 Richtek Technology Corporation. All rights reserved. DS8251-03 January 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8251 H A M EXPOSED THERMAL PAD (Bottom of Package) Y J X B F C I D Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 4.801 5.004 0.189 0.197 B 3.810 4.000 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.510 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.170 0.254 0.007 0.010 I 0.000 0.152 0.000 0.006 J 5.791 6.200 0.228 0.244 M 0.406 1.270 0.016 0.050 X 2.000 2.300 0.079 0.091 Y 2.000 2.300 0.079 0.091 X 2.100 2.500 0.083 0.098 Y 3.000 3.500 0.118 0.138 Option 1 Option 2 8-Lead SOP (Exposed Pad) Plastic Package Richtek Technology Corporation 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. www.richtek.com 16 DS8251-03 January 2012