RT8259A 1.5A, 24V, 1.4MHz Step-Down Converter General Description Features The RT8259A is a monolithic step-down switch mode converter with a built-in power MOSFET. It achieves 1.5A 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. The chip also provides protection functions such as cycle-bycycle current limiting and thermal shutdown protection. z z z z z z z z Ordering Information z RT8259A z Package Type S : SOP-8 Lead Plating System G : Green (Halogen Free and Pb Free) Note : z Applications z z Richtek products are : ` RoHS compliant and compatible with the current require- 1.5A Output Current 0.3Ω Ω Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors Up to 92% Efficiency Fixed 1.4MHz Frequency Thermal Shutdown Cycle-By-Cycle Over Current Protection Wide 4.5V to 24V Operating Input Range Output Adjustable from 0.8V to 15V Available in SOP-8 Package RoHS Compliant and Halogen Free z z Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators WLED Drivers ments of IPC/JEDEC J-STD-020. ` Suitable for use in SnPb or Pb-free soldering processes. Pin Configurations (TOP VIEW) 8 BOOT VIN 2 7 NC NC 3 6 GND EN 4 5 FB PHASE SOP-8 Typical Application Circuit 1N4148 VIN 4.5V to 5.5V 8 2 VIN BOOT C1 10µF/25V RT8259A PHASE 1 Chip Enable 4 EN Open = Automatic Startup GND 6 VOUT 3.3V L1 CB 10nF 4.7µH D1 B230A R1 49.9k FB 5 R2 16k Figure 1. Input Voltage 4.5V to 5.5V DS8259A-02 March 2011 VIN 5.5V to 24V 8 2 BOOT VIN C1 10µF/25V RT8259A PHASE 1 Chip Enable C2 22µF 6.3V 4 EN Open = Automatic Startup GND 6 VOUT 3.3V L1 CB 10nF 4.7µH D1 B230A R1 49.9k FB 5 R2 16k C2 22µF 6.3V Figure 2. Input Voltage 5.5V to 24V www.richtek.com 1 RT8259A Recommended Component Selection VOUT 1.2V 1.8V 2.5V 3.3V 5V 8V 10V 15V L1 (μH) 2 2 3.6 4.7 6.8 10 10 15 R2 (kΩ) 100 39 24 16 8.2 5.23 4.42 2.61 R1 (kΩ) 49.9 48.7 51 49.9 43 47 51 46.4 Functional Pin Description Pin No. Pin Name Pin Function 1 PHASE Switch Output. 2 VIN Supply Voltage. The RT8259 operates from a 4.5V to 24V unregulated input. C1 is needed to prevent large voltage spikes from appearing at the input. 3, 7 NC No Internal connection. 4 EN 5 FB 6 GND 8 BOOT 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 pins sets the output 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. 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. Function Block Diagram VIN X20 1µA Current Sense Amp EN 3V FB 25mOhm Ramp Generator Regulator 10k + BOOT + Shutdown Reference Comparator 1V Oscillator 1.4MHz S Q + EA - 400k 30pF + - Driver R PHASE PWM Comparator Bootstrap Control OC Limit Clamp GND 1pF www.richtek.com 2 DS8259A-02 March 2011 RT8259A Absolute Maximum Ratings z z z z z z z z z z z (Note 1) Supply Voltage, VIN -----------------------------------------------------------------------------------------------PHASE Voltage ----------------------------------------------------------------------------------------------------BOOT Voltage ------------------------------------------------------------------------------------------------------All Other Pins -------------------------------------------------------------------------------------------------------Output Voltage -----------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C SOP-8 ----------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) SOP-8, 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 −0.3V to 15V 0.833W 120°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 Voltage VFB 4.5V ≤ VIN ≤ 24V Feedback Current IFB VFB = 0.8V -- 0.1 0.3 μA Switch On Resistance Switch Leakage RDS(ON) VEN = 0V, VSW = 0V --- 0.3 -- -10 Ω μA Current Limit ILIM 1.8 2.4 -- A Oscillator Frequency fSW 1.2 1.4 1.6 MHz -- 80 -- % -- 100 -- ns 3.9 4.2 4.5 V VBOOT − VPHASE = 4.5V Maximum Duty Cycle Minimum On-Time Under Voltage Lockout Threshold Under Voltage Lockout tON Rising Threshold Hysteresis EN Input Low Voltage -- 200 -- mV -- -- 0.4 V EN Input High Voltage 1.4 -- 5.5 V -- 1 -- μA EN Pull Up Current To be continued DS8259A-02 March 2011 www.richtek.com 3 RT8259A 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. 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. Note 4. The device is not guaranteed to function outside its operating conditions. www.richtek.com 4 DS8259A-02 March 2011 RT8259A Typical Operating Characteristics Efficiency vs. Load Current Efficiency vs. Load Current 100 100 VIN = 12V 90 60 50 40 30 VIN = 12V 80 VIN = 24V 70 Efficiency (%) Efficiency (%) 80 90 VIN = 24V 70 60 50 40 30 20 20 10 10 VOUT = 5V 0.1 0.3 0.5 0.7 0.9 1.1 1.3 VOUT = 3.3V 0 0 0.1 1.5 0.3 0.5 Load Current (A) 0.9 1.1 1.3 1.5 Peak Current Limit vs. Duty Cycle Output Voltage vs. Output Current 3.34 3.5 3.33 3.0 3.32 VIN = 24V 3.31 VIN = 12V 3.30 3.29 Current Limit (A) Output Voltage (V) 0.7 Load Current (A) 2.5 2.0 1.5 1.0 3.28 0.5 3.27 0 0.25 0.5 0.75 1 1.25 0 1.5 20 40 Output Current (A) 60 80 100 Duty Cycle (%) Output Voltage vs. Temperature Reference Voltage vs. Input Voltage 3.36 0.820 3.34 0.816 Output Voltage (V) Reference Voltage (V) 0.818 0.814 0.812 0.810 0.808 0.806 0.804 3.32 VIN = 24V 3.30 3.28 VIN = 12V 3.26 0.802 0.800 3.24 5 7.5 10 12.5 15 17.5 Input Voltage (V) DS8259A-02 March 2011 20 22.5 25 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8259A Frequency vs. Temperature 1.60 1.55 1.55 1.50 1.50 Frequency (MHz) Frequency (MHz) Frequency vs. Input Voltage 1.60 1.45 1.40 1.35 1.30 1.25 1.45 1.40 1.35 1.30 1.25 VOUT = 3.3V, IOUT = 0.3A VIN = 12V, VOUT = 3.3V, IOUT = 0.3A 1.20 1.20 5 7.5 10 12.5 15 17.5 20 22.5 25 -50 -25 0 50 100 125 Quiescent Current vs. Temperature Quiescent Current vs. Input Voltage 1.0 0.9 0.9 Quiescent Current (mA) 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.8 0.7 0.6 VIN = 24V 0.5 VIN = 12V 0.4 0.3 0.2 0.1 0.1 75 Temperature (°C) Input Voltage (V) Quiescent Current (mA) 25 VEN = 2V, VFB = 1V VEN = 2V, VFB = 1V 0.0 0.0 5 7.5 10 12.5 15 17.5 20 22.5 25 -50 0 25 50 75 100 Temperature (°C) Load Transient Response Load Transient Response VIN = 12V, VOUT = 3.3V, IOUT = 0.75A to 1.5A VOUT (50mV/Div) IOUT (1A/Div) IOUT (1A/Div) Time (100μs/Div) 125 VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.5A VOUT (50mV/Div) www.richtek.com 6 -25 Input Voltage (V) Time (100μs/Div) DS8259A-02 March 2011 RT8259A Output Ripple Output Ripple VOUT (5mV/Div) VOUT (5mV/Div) VLX (10V/Div) VLX (20V/Div) ILX (1A/Div) ILX (1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.5A VIN = 24V, VOUT = 3.3V, IOUT = 1.5A Time (500ns/Div) Time (500ns/Div) Power On from EN Power Off from EN VIN = 12V, VOUT = 3.3V, IOUT = 1.5A VEN (5V/Div) VEN (5V/Div) VOUT (1V/Div) VOUT (1V/Div) I IN (500mA/Div) I IN (500mA/Div) Time (250μs/Div) DS8259A-02 March 2011 VIN = 12V, VOUT = 3.3V, IOUT = 1.5A Time (100μs/Div) www.richtek.com 7 RT8259A Application Information The Typical Application Circuit shows the basic RT8259A application circuit. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. 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 ⎤⎥ 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 = 0.4(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− VIN(MAX) ⎥ f × Δ I L(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. www.richtek.com 8 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! 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. 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 IRMS = IOUT(MAX) OUT VIN VIN −1 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 : 1 ⎤ ΔVOUT ≤ ΔIL ⎡⎢ESR + 8fCOUT ⎥⎦ ⎣ The output ripple will be highest at maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and DS8259A-02 March 2011 RT8259A 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. External Bootstrap Diode When the operating input voltage is lower than 5.5V, it is recommended to add an external bootstrap diode for efficiency improvement. The bootstrap diode can be a low cost one such as IN4148 or BAT54. For higher operating input voltage between 5.5V and 24V, the external diode must be removed. VIN 4.5V to 5.5V Output Voltage Setting The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 4. VOUT R1 FB RT8259A R2 GND Figure 4. Setting the Output Voltage For adjustable voltage mode, the output voltage is set by an external resistive divider according to the following equation : VOUT = VREF ⎛⎜ 1 + R1 ⎞⎟ ⎝ R2 ⎠ Where VREF is the internal reference voltage (0.8V typ.). 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. Thermal Considerations 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 : PD(MAX) = ( TJ(MAX) - TA ) / θJA BOOT RT8259A PHASE Figure 3 DS8259A-02 March 2011 10nF where T J(MAX) is the maximum operation junction temperature 125°C, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating conditions specification of RT8259A, where T J(MAX) is the maximum junction temperature of the die (125°C) and TA is the maximum www.richtek.com 9 RT8259A ambient temperature. The junction to ambient thermal resistance θJA is layout dependent. For SOP-8 package, the thermal resistance θJA is 120°C/W on standard JEDEC 51-7 four-layers thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : PD(MAX) = (125°C -25°C) / 120°C/W = 0.833W (SOP-8) The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA . For RT8259A packages, the Figure 5 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. Maximum Power Dissipation (W)1 1.0 Layout Consideration Follow the PCB layout guidelines for optimal performance of RT8259A } 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 small area. Keep analog components away from LX node to prevent stray capacitive noise pick-up. } Connect feedback network behind the output capacitors. Keep the loop area small. Place the feedback components near the RT8259A. } Connect all analog grounds to a command node and then connect the command node to the power ground behind the output capacitors. } An example of PCB layout guide is shown in Figure 6 for reference. Four Layer PCB 0.8 0.6 0.4 C OUT V 0.2 V IN OUT L1 D2 CB D1 0.0 0 25 50 75 100 125 8 BOOT VIN 2 7 NC NC 3 6 GND EN 4 5 FB PHASE Ambient Temperature (°C) CIN Figure 5. Derating Curves for RT8259A Packages GND R2 R1 V OUT Figure 6 Table 1. Suggested Inductors for L1 Component Supplier Series Inductance (µH) DCR (mΩ) Current Rating (A) Dimensions (mm) TDK SLF7045 4.7 30 2 7x7x4.5 TAIYO YUDEN NR8040 4.7 18 4.7 8x8x4 GOTERND GTSD53 4.7 45 1.87 5x5x2.8 GOTERND GSSR2 4.7 18 5.7 10x10x3.8 Table 2. Suggested Capacitors for CIN and COUT Component Capacitance Case Part No. Supplier (µF) Size MURATA GRM31CR61E106K 10 1206 TDK C3225X5R1E106K TAIYO TMK316BJ106ML YUDEN MURATA GRM31CR61C226M 10 1206 10 1206 22 1206 TDK C3225X5R1C226M 22 1206 TAIYO YUDEN EMK316BJ226ML 22 1206 www.richtek.com 10 Table 3. Suggested Diode for D1 Component VRRM IOUT Series Supplier (A) (V) D IODES B230A 30 2 D IODES B330A 30 3 PANJIT SK23 30 2 PANJIT SK33 30 3 Package DO-214AC DO-214AC DO-214AC DO-214AB DS8259A-02 March 2011 RT8259A Outline Dimension H A M J 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 3.988 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.508 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.170 0.254 0.007 0.010 I 0.050 0.254 0.002 0.010 J 5.791 6.200 0.228 0.244 M 0.400 1.270 0.016 0.050 8-Lead SOP Plastic 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. DS8259A-02 March 2011 www.richtek.com 11