RT8259 1.2A, 24V, 1.4MHz Step-Down Converter General Description Features The RT8259 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% 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 The chip also provides protection functions such as cycleby-cycle current limiting and thermal shutdown protection. The RT8259 is available in a SOT-23-6 and TSOT-23-6 packages. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 z z z z z Ordering Information RT8259 Applications Package Type E : SOT-23-6 J6 : TSOT-23-6 z z z Lead Plating System G : Green (Halogen Free and Pb Free) Note : z Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators WLED Drivers Pin Configurations Richtek products are : ` RoHS compliant and compatible with the current require- ` Suitable for use in SnPb or Pb-free soldering processes. (TOP VIEW) ments of IPC/JEDEC J-STD-020. PHASE VIN 6 Marking Information 5 4 2 3 BOOT GND For marking information, contact our sales representative directly or through a Richtek distributor located in your area. EN FB SOT-23-6/TSOT-23-6 Typical Application Circuit 深 VIN 4.5V to 24V 5 VIN C1 10µF Chip Enable Open = Automatic Startup DS8259-03 March 2011 BOOT RT8259 PHASE 6 4 EN VOUT 3.3V 1 CB 10nF D1 B230A L1 4.7µH R1 62k FB 3 GND 2 C2 22µF R2 19.6k www.richtek.com 1 RT8259 Table 1. Recommended Component Selection, C2 = 22μ μF V OUT (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 R1 (kΩ) 62 62 62 62 62 68 68 68 R2 (kΩ) 124 49.9 29.4 19.6 12 7.5 5.9 3.9 Functional Pin Description Pin Name Pin Function Bootstrap. A capacitor is connected between PHASE 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. 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 C1 ground path to prevent switching current spikes from inducing voltage noise into the part. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Pin No. 1 BOOT 2 GND 3 FB 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. 4 EN Chip Enable (Active High). If the EN pin is open, it will be pulled to high by internal circuit. 5 VIN Supply Voltage. Bypass VIN to GND with a suitable large capacitor to prevent large voltage spikes from appearing at the input. 6 PHASE Switch Output. Function Block Diagram VIN - X20 1µA Current Sense Amp EN 1.1V 3V FB - www.richtek.com 2 + Shutdown Reference Comparator S Q + EA - 深 BOOT Oscillator 1.4MHz 400k GND Ω 25mΩ Ramp Generator Regulator 10k + 30pF + Driver R PWM Comparator PHASE Bootstrap Control OC Limit Clamp 1pF DS8259-03 March 2011 RT8259 Absolute Maximum Ratings z z z z z z z z z z Supply Voltage, VIN -----------------------------------------------------------------------------------------------PHASE Voltage ----------------------------------------------------------------------------------------------------BOOT Voltage ------------------------------------------------------------------------------------------------------All Other Pins -------------------------------------------------------------------------------------------------------Output Voltage -----------------------------------------------------------------------------------------------------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) ----------------------------------------------------------------------------------------------- 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 z (Note 1) Recommended Operating Conditions z z z z z 26V −0.3V to (VIN + 0.3V) VPHASE + 6V 0.3V to 6V −0.3V to 15V 0.4W 250°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Voltage, VIN -----------------------------------------------------------------------------------------------Output Voltage, VOUT ---------------------------------------------------------------------------------------------EN Voltage, VEN ----------------------------------------------------------------------------------------------------Junction Temperature Range ------------------------------------------------------------------------------------Ambient Temperature Range ------------------------------------------------------------------------------------- 4.5V to 24V 0.8V to 15V 0V to 5.5V −40°C to 125°C −40°C to 85°C Electrical Characteristics (VIN = 12V, TA = 25° C unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit 0.784 0.8 0.816 V Feedback Reference Voltage V FB 4.5V ≤ VIN ≤ 24V 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 1.2 1.4 1.6 MHz -- 80 -- % -- 100 -- ns 3.9 4.2 4.5 V -- 200 -- mV V IH 1.4 -- -- V IL -- -- 0.4 -- 1 -- Oscillator Frequency fSW Maximum Duty Cycle Minimum On-Time tON 深 Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis Logic-High EN Input Voltage Logic-Low EN Pull Up Current VEN = 0V V μA To be continued DS8259-03 March 2011 www.richtek.com 3 RT8259 Parameter Symbol Test Conditions Min Typ Max Unit Shutdown Current ISHDN VEN = 0V -- 25 -- μA Quiescent Current IQ VEN = 2V, VFB = 1V (Not Switching) -- 0.55 1 mA Thermal Shutdown TSD -- 150 -- °C 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. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Note 2. θJA is measured in the natural convection at TA = 25°C on a high effective four layers thermal conductivity test board of JEDEC 51-7 thermal measurement standard. Note 3. Devices are ESD sensitive. Handling precaution is recommended. 深 Note 4. The device is not guaranteed to function outside its operating conditions. www.richtek.com 4 DS8259-03 March 2011 RT8259 Typical Operating Characteristics Efficiency vs. Load Current Efficiency vs. Load Current 100 100 VIN = 12V VIN = 24V 80 80 Efficiency (%) 70 60 50 40 30 VIN = 24V 70 60 50 40 30 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Efficiency (%) VIN = 12V 90 90 20 20 10 10 VOUT = 3.3V VOUT = 5V 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 1.2 Output Voltage vs. Load Current Output Voltage vs. Temperature 3.323 Output Voltage (V) 3.36 3.328 Output Voltage (V) 1.1 1.2 3.38 3.333 VIN = 24V 3.318 3.313 VIN = 12V 3.308 3.303 3.34 VIN = 24V 3.32 3.30 VIN = 12V 3.28 3.26 3.24 IOUT = 0A 3.22 3.298 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -50 1.1 1.2 -25 0 25 50 75 100 125 Temperature (°C) Load Current (A) Frequency vs. Temperature Frequency vs. Input Voltage 1.55 1.55 1.45 1.40 深 1.35 Frequency (MHz)1 1.50 1.50 Frequency (MHz) 1 Load Current (A) Load Current (A) 1.30 1.25 1.20 1.45 1.40 1.35 1.30 1.25 1.20 VOUT = 3.3V, IOUT = 0.3A 1.15 VIN = 12V, VOUT = 3.3V, IOUT = 0.3A 1.15 4 6.5 9 11.5 14 16.5 Input Voltage (V) DS8259-03 March 2011 19 21.5 24 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8259 Quiescent Current vs. Temperature 0.8 0.8 Quiescent Current (mA) 0.9 0.7 0.6 0.5 0.4 0.3 0.2 0.7 VIN = 24V 0.6 VIN = 12V 0.5 0.4 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Quiescent Current (mA) Quiescent Current vs. Input Voltage 0.9 0.3 0.2 VEN = 2V, VFB = 1V 0.1 4 6.5 9 11.5 14 16.5 19 21.5 24 -50 VOUT (5mV/Div) VPHASE (10V/Div) IL (1A/Div) 25 50 75 100 Load Transient Response Load Transient Response 125 VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.2A VOUT (50mV/Div) IOUT (500mA/Div) Time (50μs/Div) Time (50μs/Div) Switching Switching VOUT (5mV/Div) VPHASE (10V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.2A Time (250ns/Div) www.richtek.com 6 0 Temperature (°C) 深 IOUT (500mA/Div) -25 Input Voltage (V) VIN = 12V, VOUT = 3.3V, IOUT = 0.6A to 1.2A VOUT (50mV/Div) VEN = 2V, VFB = 1V 0.1 IL (1A/Div) VIN = 20V, VOUT = 3.3V, IOUT = 1.2A Time (250ns/Div) DS8259-03 March 2011 RT8259 Power Off from EN Power On from EN VIN = 12V, VOUT = 3.3V, IOUT = 1.2A VIN = 12V, VOUT = 3.3V, IOUT = 1.2A VEN (5V/Div) VOUT (1V/Div) I IN (500mA/Div) VOUT (1V/Div) I IN (500mA/Div) 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 VEN (5V/Div) Time (100μs/Div) 深 Time (250μs/Div) DS8259-03 March 2011 www.richtek.com 7 RT8259 Application Information The RT8259 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. Output Voltage Setting The resistive voltage divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. 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 ⎤⎥ 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. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 VOUT Inductor Selection R1 FB RT8259 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 RT8259. 深 5V BOOT RT8259 10nF PHASE Figure 2. External Bootstrap Diode For the ripple current selection, the value of ΔIL = 0.4(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 =⎢ × ⎢1− ⎥ ⎥ f × Δ I V L(MAX) ⎦ ⎣ IN(MAX) ⎦ ⎣ Inductor Core Selection 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. 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. Do not allow the core to saturate! www.richtek.com 8 DS8259-03 March 2011 RT8259 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. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Diode Selection 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. 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, 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 深 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. 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. The output ripple, ΔVOUT , is determined by : 1 ⎤ ΔVOUT ≤ ΔIL ⎡⎢ESR + 8fCOUT ⎦⎥ ⎣ DS8259-03 March 2011 www.richtek.com 9 RT8259 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 RT8259. 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). ` LX node is with high frequency voltage swing and should be kept at small area. Keep sensitive components away from the LX node to prevent stray capacitive noise pickup. 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 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 RT8259, 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 120°C/W on standard JEDEC 51-7 four- ` Place the feedback components to the FB pin as close as possible. ` Connect GND to a ground plane for noise reduction and thermal dissipation. layers thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : CB P D(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for T/SOT-23-6 packages The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA . For RT8259 packages, the Figure 3 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. V OUT R1 C OUT L1 D1 BOOT 1 6 PHASE GND 2 5 VIN FB 3 4 EN C IN GND 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 深 Maximum Power Dissipation (W) 0.50 R2 V OUT 0.10 0.05 0.00 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curves for RT8259 Packages www.richtek.com 10 DS8259-03 March 2011 RT8259 Table 2. Suggested Inductors for L1 Component Supplier Series Inductance (µH) TDK TAIYO YUDEN GOTERND GOTERND SLF7045 NR8040 GTSD53 GSSR2 DCR (mΩ) 4.7 4.7 4.7 4.7 Current Rating (A) Dimensions (mm) 30 18 45 18 2 4.7 1.87 5.7 7 x 7 x 4.5 8x8x4 5 x 5 x 2.8 10 x 10 x 3.8 Table 3. Suggested Capacitors for CIN and COUT Part No. Capacitance ( µF) Case Size MURATA GRM31CR61E106K 10 1206 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 Component Supplier TDK C3225X5R1E106K 10 1206 TAIYO YUDEN TMK316BJ106ML 10 1206 MURATA GRM31CR61C226M 22 1206 TDK C3225X5R1C226M 22 1206 TAIYO YUDEN EMK316BJ226ML 22 1206 Table 4. Suggested Diode for D1 Series VR RM (V) IOUT (A) Package DIODES DIODES PANJIT PANJIT B230A B330A SK23 SK33 30 30 30 30 2 3 2 3 DO-214AC DO-214AC DO-214AC DO-214AB 深 Component Supplier DS8259-03 March 2011 www.richtek.com 11 RT8259 Outline Dimension H D L B 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 C b A A1 e Dimensions In Millimeters Dimensions In Inches Symbol 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 深 SOT-23-6 Surface Mount Package www.richtek.com 12 DS8259-03 March 2011 RT8259 H D L B 07 55 圳 市 -2 深 鼎 38 圳 诚 市 15 国 富 5 际 25 佳 大 维 / 83 厦 电 28 21 子 05 44 室 50 C b A A1 e Dimensions In Millimeters Dimensions In Inches Symbol 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 深 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. DS8259-03 March 2011 www.richtek.com 13