RT8258 1.2A, 24V, 700kHz Step-Down Converter General Description Features The RT8258 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 1.2A. 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 1.2A Output Current 0.3Ω Ω Internal Power MOSFET Switch High Efficiency up to 92% 700kHz 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 The chip also provides protection functions such as cycleby-cycle current limiting and thermal shutdown protection. The RT8258 is available in a SOT-23-6 and TSOT-23-6 packages. z z z z Ordering Information z RT8258 Applications Package Type E : SOT-23-6 J6 : TSOT-23-6 z z Lead Plating System G : Green (Halogen Free and Pb Free) z z Note : Richtek products are : ` Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators WLED Drivers 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. PHASE VIN EN 6 Marking Information For marking information, contact our sales representative directly or through a Richtek distributor located in your area. 5 4 2 3 BOOT GND FB SOT-23-6/TSOT-23-6 Typical Application Circuit VIN 4.5V to 24V CIN 10µF Chip Enable 5 VIN BOOT RT8258 PHASE 6 4 EN GND 2 DS8258-02 March 2011 1 FB 3 CBOOT L 10nF 10µH D1 B230A VOUT 3.3V R1 100k R2 32.4k COUT 22µF www.richtek.com 1 RT8258 Table 1. Recommended Component Selection VOUT 1.2V 1.5V 1.8V 2.5V 3.3V 5V 8V 10V 15V R1 (kΩ) 100 91 91 100 100 91 91 91 120 R2 (kΩ) 200 100 75 47 32.4 17.4 10 7.87 6.8 L (μH) 3.6 3.6 4.7 6.8 10 15 22 22 33 COUT (μF) 22 22 22 22 22 22 22 22 22 Note : The value of R1 is related to the loop bandwidth of the RT8258. It is strongly recommended to follow the parameters in above table for the specific output voltage. Function Block Diagram VIN - X20 1µA Current Sense Amp EN 1.1V 3V - BOOT Oscillator 700kHz + Shutdown Reference Comparator S + EA - FB 25mΩ Ramp Generator Regulator 10k + 400k 30pF + - Driver Q R PHASE PWM Comparator Bootstrap Control OC Limit Clamp GND 1pF Functional Pin Description Pin No. Pin Name 1 BOOT 2 GND 3 FB 4 EN 5 VIN 6 PHASE www.richtek.com 2 Pin Function Gate Driver Bootstrap Input Pin. Connect a 10nF or greater capacitor between PHASE and BOOT pins to supply the MOSFET driver. Ground Pin. This pin should be connected to the (-) terminal of the output capacitor and it should be kept away from the D1 and input capacitor for noise prevention. Output Voltage Feedback Input Pin. 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. Chip Enable (Active High). If the EN pin is open, it will be pulled to high by internal circuit. Power Supply Input Pin. Bypass VIN to GND with a suitable large capacitor to prevent large voltage spikes from appearing at the input. Power Switching Output Pin. Connect this pin to the output inductor. DS8258-02 March 2011 RT8258 Absolute Maximum Ratings z z z z z z z z z z (Note 1) Supply Voltage, VIN -----------------------------------------------------------------------------------------------PHASE Voltage ----------------------------------------------------------------------------------------------------BOOT Voltage ------------------------------------------------------------------------------------------------------All Other Pins -------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C T/SOT-23-6 ----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) T/SOT-23-6, θJA -----------------------------------------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Mode) ---------------------------------------------------------------------------------------MM (Machine Mode) ----------------------------------------------------------------------------------------------- Recommended Operating Conditions z z z z z 26V −0.3V to (VIN + 0.3V) VPHASE + 6V 0.3V to 6V 0.4W 250°C/W 150°C 260°C −65°C to 150°C 2kV 200V (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 Test Conditions Min Typ Max Unit Feedback Reference Voltage V FB 4.5V ≤ VIN ≤ 24V 0.784 0.8 0.816 V Feedback Current IFB VFB = 0.8V -- 0.1 0.3 μA Switch On Resistance Switch Leakage RDS(ON) VEN = 0V, VPHASE = 0V --- 0.3 -- -10 Ω μA Current Limit ILIM VBOOT − VPHASE = 4.8V 1.6 2.1 -- A Oscillator Frequency fSW 600 700 800 kHz -- 90 -- % -- 100 -- ns 3.9 4.2 4.5 V -- 270 -- mV 1.4 -- -- -- -- 0.4 Maximum Duty Cycle Minimum On-Time tON Under Voltage Lockout Rising Threshold Voltage Under Voltage Lockout Threshold Hysteresis Logic High EN Input Voltage Logic Low V VEN = 0V -- 1 -- μA Shutdown Current ISHDN VEN = 0V -- 25 -- μA Quiescent Current IQ VEN = 2V, VFB = 1V (No Switching) -- 0.55 1 mA Thermal Shutdown T SD -- 150 -- °C EN Pull Up Current DS8258-02 March 2011 www.richtek.com 3 RT8258 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 the natural convection at TA = 25°C on a low effective single layer thermal conductivity test board of JEDEC 51-3 thermal measurement standard. 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 DS8258-02 March 2011 RT8258 Typical Operating Characteristics Efficiency vs. Output Current 100 90 90 VIN = 24V 80 70 VIN = 12V 80 VIN = 12V Efficiency (%) Efficiency (%) Efficiency vs. Output Current 100 60 50 40 30 20 VIN = 24V 70 60 50 40 30 20 10 10 VOUT = 5V 0 0 0.2 0.4 0.6 0.8 1 VOUT = 3.3V 0 1.2 0 0.2 0.4 Output Current (A) 0.8 1 1.2 Output Voltage vs. Temperature Output Voltage vs. Output Current 3.366 3.366 3.344 3.344 VIN = 24V 3.322 Output Voltage (V) Output Voltage (V) 0.6 Load Current (A) VIN = 12V 3.300 3.278 3.278 3.256 3.234 3.234 0.2 0.4 0.6 0.8 1 VIN = 24V 3.300 3.256 0 VIN = 12V 3.322 1.2 IOUT = 0A -50 -25 0 25 50 75 100 125 Temperature (°C) Output Current (A) Frequency vs. Temperature Quiescent Current vs. Temperature 600 750 Quiescent Current (μA) Frequency (kHz)1 575 700 650 600 550 VIN = 24V 525 500 VIN = 12V 475 450 425 VIN = 12V, VOUT = 3.3V, IOUT = 0A 550 VEN = 2V, VFB = 1V 400 -50 -25 0 25 50 Temperature (°C) DS8258-02 March 2011 75 100 125 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8258 Power Off from EN Power On from EN VIN = 12V, VOUT = 3.3V, IOUT = 2A VEN (2V/Div) VEN (2V/Div) VOUT (1V/Div) VOUT (1V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.2A Time (100μs/Div) Time (50μs/Div) Output Ripple Voltage Output Ripple Voltage VOUT (10mV/Div) VOUT (10mV/Div) VPHASE (10V/Div) VPHASE (10V/Div) IL (1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.2A Time (500ns/Div) Load Transient Response Load Transient Response VOUT (0.1V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 0.6A to 1.2A Time (50μs/Div) www.richtek.com 6 VIN = 24V, VOUT = 3.3V, IOUT = 1.2A Time (500ns/Div) VOUT (0.1V/Div) IOUT (0.5A/Div) IL (1A/Div) IOUT (0.5A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.2A Time (250μs/Div) DS8258-02 March 2011 RT8258 Application Information The RT8258 is an asynchronous high voltage buck converter that can support the input voltage range from 4.5V to 24V and the output current can be up to 1.2A. Inductor Selection Output Voltage Setting and decreases with higher inductance. The resistive voltage divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. VOUT R1 FB RT8258 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 ⎠ 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 V V ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥ f × L VIN ⎦ ⎣ ⎦ ⎣ 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 = 0.34(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 : Where VFB is the feedback reference voltage (0.8V typ.). ⎡ VOUT ⎤ ⎡ VOUT ⎤ L =⎢ ⎥ × ⎢1− VIN(MAX) ⎥ f × Δ I L(MAX) ⎣ ⎦ ⎣ ⎦ External Bootstrap Diode Inductor Core Selection Connect a 10nF low ESR ceramic capacitor between the BOOT pin and PHASE pin. This capacitor provides the gate driver voltage for the high side MOSFET. The inductor type must be selected once the value for L is known. Generally speaking, high efficiency converters can not afford the core loss found in low cost powdered iron cores. So, the more expensive ferrite or mollypermalloy cores will be a better choice. 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 RT8268. 5V BOOT RT8258 10nF PHASE Figure 2. External Bootstrap Diode DS8258-02 March 2011 The selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. As the inductance increases, core losses decrease. Unfortunately, increase of the inductance requires more turns of wire and therefore the copper losses will increase. Ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. So, design goals can focus on the reduction of copper loss and the saturation prevention. Ferrite core material saturates “hard”, which means that inductance collapses abruptly when the peak design current is exceeded. The previous situation results in an abrupt increase in inductor ripple current and consequent output voltage ripple. www.richtek.com 7 RT8258 Do not allow the core to saturate! Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy. However, they are usually more expensive than the similar powdered iron inductors. The rule for inductor choice mainly depends on the price vs. size requirement and any radiated field/EMI requirements. 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. 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 : www.richtek.com 8 1 ⎤ ΔVOUT ≤ ΔIL ⎡⎢ESR + 8fCOUT ⎥⎦ ⎣ 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. 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) and 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. DS8258-02 March 2011 RT8258 Thermal Considerations Layout Consideration For continuous operation, do not exceed the maximum operation junction temperature 125°C. 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 : Follow the PCB layout guidelines for optimal performance of RT8258. PD(MAX) = (TJ(MAX) − TA ) / θJA ` Keep the traces of the main current paths as short and wide as possible. ` Put the input capacitor as close as possible to the device pins (VIN and GND). ` PHASE node is with high frequency voltage swing and should be kept at small area. Keep sensitive components away from the PHASE node to prevent stray capacitive noise pick-up. ` Place the feedback components to the FB pin as close as possible. ` Connect the GND to a ground plane for noise reduction and thermal dissipation. where T J(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating conditions specification of the RT8258, the maximum junction temperature of the die is 125°C. The junction to ambient thermal resistance θJA is layout dependent. For T/SOT-23-6 package, the thermal resistance θJA is 250°C/W on standard JEDEC 51-3 single COUT layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : VOUT The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA . For RT8258 package, the Figure 3 of derating curve allows the designer to see the effect of rising ambient temperature on the maximum power dissipation allowed. Maximum Power Dissipation (W) 0.50 L CB PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for T/SOT-23-6 package GND D1 BOOT 1 6 PHASE GND 2 5 VIN FB 3 4 EN CIN R2 VOUT R1 Figure 4. PCB Layout Guide Single Layer PCB 0.45 0.40 0.35 0.30 T/SOT-23-6 0.25 0.20 0.15 0.10 0.05 0.00 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curve for RT8258 Package DS8258-02 March 2011 www.richtek.com 9 RT8258 Table 2. Suggested Inductors for L Component Supplier Series Dimensions (mm) TDK SLF12555T 12.5x12.5x5.5 TAIYO YUDEN NR8040 8x8x4 TDK SLF12565T 12.5x12.5x6.5 Table 3. Suggested Capacitors for CIN and COUT Capacitance Case Size (μF) GRM31CR61E106K 10 1206 Location Component Supplier Part No. CIN MURATA CIN TDK C3225X5R1E106K 10 1206 CIN TAIYO YUDEN TMK316BJ106ML 10 1206 COUT MURATA GRM31CR61C226M 22 1206 COUT TDK C3225X5R1C226M 22 1206 COUT TAIYO YUDEN EMK316BJ226ML 22 1206 Table 4. Suggested Diode for D1 Component Supplier Series DIODES DIODES PANJIT PANJIT B230A B330A SK23 SK33 www.richtek.com 10 VRRM (V) 30 30 30 30 IOUT (A) 2 3 2 3 Package DO-214AC DO-214AC DO-214AC DO-214AB DS8258-02 March 2011 RT8258 Outline Dimension H D L C B b A A1 e Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 0.889 1.295 0.031 0.051 A1 0.000 0.152 0.000 0.006 B 1.397 1.803 0.055 0.071 b 0.250 0.560 0.010 0.022 C 2.591 2.997 0.102 0.118 D 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 SOT-23-6 Surface Mount Package DS8258-02 March 2011 www.richtek.com 11 RT8258 H D L C B b A A1 e Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 0.700 1.000 0.028 0.039 A1 0.000 0.100 0.000 0.004 B 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 C 2.591 3.000 0.102 0.118 D 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 TSOT-23-6 Surface Mount 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 DS8258-02 March 2011