RT7285AGJ6

® RT7285/A 1.5A, 18V, 500kHz ACOTTM Synchronous Step-Down Converter General Description Features The RT7285/A is a synchronous step-down converter with Advanced Constant On-Time (ACOTTM) mode control. The  4.3V to 18V Input Voltage Range  1.5A Output Current Advanced Constant On-Time Control Fast Transient Response Support All Ceramic Capacitors Up to 95% Efficiency 500kHz Switching Frequency Adjustable Output Voltage from 0.6V to 8V Cycle-by-Cycle Current Limit Input Under-Voltage Lockout Hiccup Mode Under-Voltage Protection Thermal Shutdown Power Saving Mode for High Efficiency at Light Load RoHS Compliant and Halogen Free TM ACOT provides a very fast transient response with few external components. The low impedance internal MOSFET supports high efficiency operation with wide input voltage range from 4.3V to 18V. The proprietary circuit of the RT7285/A enables to support all ceramic capacitors. The output voltage can be adjusted between 0.6V and 8V.        Ordering Information  RT7285/A  Package Type E : SOT-23-6 J6 : TSOT-23-6   Lead Plating System G : Green (Halogen Free and Pb Free)  TSOT-23-6 SOT-23-6 Applications Note :  Richtek products are :    RoHS compliant and compatible with the current require-  ments of IPC/JEDEC J-STD-020.  Suitable for use in SnPb or Pb-free soldering processes.  Industrial and Commercial Low Power Systems Computer Peripherals LCD Monitors and TVs Green Electronics/Appliances Point of Load Regulation for High-Performance DSPs, FPGAs, and ASICs Pin Configurations Marking Information (TOP VIEW) RT7285GE SW VIN EN 6 5 4 2 3 0M=DNN 0M= : Product Code DNN : Date Code RT7285AGJ6 BOOT GND FB 0D=DNN T/SOT-23-6 0D= : Product Code DNN : Date Code Simplified Application Circuit VIN VIN RT7285/A BOOT SW Enable EN GND Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 VOUT FB is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT7285/A Functional Pin Description Pin No. Pin Name Pin Function Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic capacitor between the BOOT and SW pins. 1 BOOT 2 GND Power Ground. 3 FB Feedback Voltage Input. The pin is used to set the output voltage of the converter via a resistive divider. The converter regulates VFB to 0.6V 4 EN Enable Control Input. Connect EN to a logic-high voltage to enable the IC or to a logic-low voltage to disable. Do not leave this high impedance input unconnected. 5 VIN Power Input. The input voltage range is from 4.3V to 18V. Must bypass with a suitable large ceramic capacitor at this pin. 6 SW Switch Node. Connect to external L-C filter. Function Block Diagram BOOT VIN VIN Reg PVCC VIBIAS VREF Min off PVCC UGATE OC Control SW Driver LGATE UV & OV GND PVCC SW Ripple Gen. + Comparator FB GND SW VIN EN On-Time SW EN Operation The RT7285/A is a synchronous step-down converter with advanced constant on-time control mode. Using the ACOT control mode can reduce the output capacitance and fast transient response. It can minimize the component size without additional external compensation network. UVLO Protection Current Protection Thermal Shutdown The inductor current is monitored via the internal switches cycle-by-cycle. Once the output voltage drops under UV threshold, the RT7285/A will enter hiccup mode. When the junction temperature exceeds the OTP threshold value, the IC will shut down the switching operation. Once the junction temperature cools down and is lower than the OTP lower threshold, the converter will autocratically resume switching. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 To protect the chip from operating at insufficient supply voltage, the UVLO is needed. When the input voltage of VIN is lower than the UVLO falling threshold voltage, the device will be lockout. is a registered trademark of Richtek Technology Corporation. DS7285/A-01 October 2014 RT7285/A Absolute Maximum Ratings           (Note 1) VIN to GND ----------------------------------------------------------------------------------------------------SW to GND ---------------------------------------------------------------------------------------------------< 10ns ---------------------------------------------------------------------------------------------------------BOOT to GND ------------------------------------------------------------------------------------------------Other Pins -----------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C SOT-23-6 ------------------------------------------------------------------------------------------------------- 0.625W TSOT-23-6 ------------------------------------------------------------------------------------------------------ 0.625W Package Thermal Resistance (Note 2) SOT-23-6, θJA -------------------------------------------------------------------------------------------------- 160°C/W SOT-23-6, θJC ------------------------------------------------------------------------------------------------- 15°C/W TSOT-23-6, θJA ------------------------------------------------------------------------------------------------ 160°C/W TSOT-23-6, θJC ------------------------------------------------------------------------------------------------ 15°C/W Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------ 260°C Junction Temperature ---------------------------------------------------------------------------------------- 150°C Storage Temperature Range ------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Model) --------------------------------------------------------------------------------- 2kV Recommended Operating Conditions    −0.3V to 20V −0.3V to (VIN + 0.3V) −5V to 25V (VSW − 0.3V) to (VSW + 6V) −0.3V to 6V (Note 4) Supply Input Voltage, VIN ---------------------------------------------------------------------------------- 4.3V to 18V 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 Shutdown Current ISHDN VEN = 0V -- -- 4 A Quiescent Current IQ VEN = 2V, VFB = 1V -- 0.5 -- mA High-Side RDS(ON)_H VBOOTSW = 4.8V -- 230 -- Low-Side RDS(ON)_L VIN = 5V -- 130 -- Current Limit ILIM Valley Current 1.7 2.2 2.8 A Oscillator Frequency f SW -- 500 -- kHz Maximum Duty Cycle DMAX -- 90 -- % Minimum On-Time tON -- 60 -- ns Feedback Threshold Voltage VFB 591 600 609 mV 1.5 -- -- -- -- 0.4 Switch-On Resistance EN Input Threshold Logic-High VEN_H Logic-Low VEN_L Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 m V is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT7285/A Parameter VIN Under-Voltage Lockout Threshold VIN Under-Voltage Lockout Threshold Hysteresis Soft-Start Time Symbol Min Typ Max Unit 3.55 3.9 4.25 V -- 340 -- mV tSS -- 800 -- s Thermal Shutdown Threshold T SD -- 160 -- C Thermal Shutdown Hysteresis TSD -- 20 -- C VOUT Discharge Resistance RDISCHG VEN = 0V, VOUT = 0.5V -- 50 100  UVP Detect 70 75 80 Hysteresis -- 10 -- -- 250 -- V UVLO Output Under-Voltage Trip Threshold Test Conditions VIN Rising Output Under-Voltage Delay Time % s 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. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. The case position of θJC is on the top 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 © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS7285/A-01 October 2014 RT7285/A Typical Application Circuit RT7285/A VIN 4.3V to 18V 5 CIN 10µF BOOT VIN SW 6 4 EN Enable 1 CBOOT 100nF L 2µH R1 10k FB 3 R2 10k GND 2 VOUT 1.2V COUT 22µF Table 1. Suggested Component Values VOUT (V) R1 (k) R2 (k) L (H) COUT (F) CFF (pF) 5 110 15 10 22 29 3.3 115 25.5 6.8 22 33 2.5 25.5 8.06 4.7 22 NC 1.2 10 10 3.6 22 NC Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT7285/A Typical Operating Characteristics Reference Voltage vs. Input Voltage Efficiency vs. Load Current 100 0.610 90 0.608 Reference Voltage (V) Efficiency (%) 80 70 VIN = VIN = VIN = VIN = 60 50 40 5V 9V 12V 18V 30 20 0.605 0.603 0.600 0.598 0.595 0.593 10 VOUT = 1.2V 0 0.001 0.01 0.1 1 VIN = 4V to 18V, IOUT = 1A 0.590 4 10 5 6 7 8 Input Voltage (V) Load Current (A) Switching Frequency vs. Input Voltage Reference Voltage vs. Temperature 600 Switching Frequency (kHz)1 Reference Voltage (V) 0.610 0.605 VIN = 9V VIN = 12V VIN = 18V 0.600 0.595 VOUT = 1.2V, IOUT = 0.5A 590 580 570 560 550 540 530 520 510 VOUT = 1.2V, IOUT = 0.5A 500 0.590 -50 -25 0 25 50 75 100 4 125 5 6 7 8 Switching Frequency vs. Temperature Current Limit vs. Temperature 550 3.0 540 2.8 530 2.6 520 Current Limit (A) VIN = 6V VIN = 12V VIN = 18V 510 9 10 11 12 13 14 15 16 17 18 Input Voltage (V) Temperature (°C) Switching Frequency (kHz)1 9 10 11 12 13 14 15 16 17 18 500 490 480 470 2.4 VIN = 6V VIN = 12V VIN = 18V 2.2 2.0 1.8 1.6 1.4 460 VOUT = 1.2V, IOUT = 0.5A 450 -50 -25 0 25 50 75 100 Temperature (°C) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 125 1.2 1.0 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS7285/A-01 October 2014 RT7285/A Load Transient Response Load Transient Response VOUT (20mV/Div) VOUT (20mV/Div) IOUT (1A/Div) IOUT (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 0A to 1.5A Time (250μs/Div) Time (250μs/Div) Switching Power On from EN VOUT (10mV/Div) VEN (2V/Div) VOUT (1V/Div) VSW (5V/Div) IL (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 0.75A to 1.5A VSW (10V/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A I SW (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (1μs/Div) Time (2.5ms/Div) Power Off from EN Power On from VIN VEN (2V/Div) VIN (10V/Div) VOUT (1V/Div) VSW (10V/Div) VOUT (1V/Div) VSW (10V/Div) I SW (1A/Div) I SW (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (5ms/Div) Time (2.5ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT7285/A Power Off from VIN VIN (10V/Div) VOUT (1V/Div) VSW (10V/Div) I SW (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (2.5ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS7285/A-01 October 2014 RT7285/A Application information Inductor Selection Selecting an inductor involves specifying its inductance and also its required peak current. The exact inductor value is generally flexible and is ultimately chosen to obtain the best mix of cost, physical size, and circuit efficiency. Lower inductor values benefit from reduced size and cost and they can improve the circuit's transient response, but they increase the inductor ripple current and output voltage ripple and reduce the efficiency due to the resulting higher peak currents. Conversely, higher inductor values increase efficiency, but the inductor will either be physically larger or have higher resistance since more turns of wire are required and transient response will be slower since more time is required to change current (up or down) in the inductor. A good compromise between size, efficiency, and transient response is to use a ripple current (ΔIL) about 20% to 40% of the desired full output load current. Calculate the approximate inductor value by selecting the input and output voltages, the switching frequency (fSW), the maximum output current (IOUT(MAX)) and estimating a ΔIL as some percentage of that current. VOUT   VIN  VOUT  VIN  fSW  IL Once an inductor value is chosen, the ripple current (ΔIL) is calculated to determine the required peak inductor current. L= VOUT   VIN  VOUT  VIN  fSW  L I IL(PEAK) = IOUT(MAX)  L 2 IL IL(VALLY) = IOUT(MAX)  2 IL = Considering the Typical Operating Circuit for 1.2V output at 1.5A and an input voltage of 12V, using an inductor ripple of 0.6A (40%), the calculated inductance value is : L= 1.2  12  1.2  = 3.6μH 12  500kHz  0.6 Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 The ripple current was selected at 0.6A and, as long as we use the calculated 3.6μH inductance, that should be the actual ripple current amount. The ripple current and required peak current as below : IL = 1.2  12  1.2  = 0.6A 12  500kHz  3.6μH and IL(PEAK) = 1.5A  0.6 = 1.8A 2 Inductor saturation current should be chosen over IC's current limit. Input Capacitor Selection The input filter capacitors are needed to smooth out the switched current drawn from the input power source and to reduce voltage ripple on the input. The actual capacitance value is less important than the RMS current rating (and voltage rating, of course). The RMS input ripple current (IRMS) is a function of the input voltage, output voltage, and load current : V IRMS = IOUT(MAX)  OUT VIN VIN 1 VOUT Ceramic capacitors are most often used because of their low cost, small size, high RMS current ratings, and robust surge current capabilities. However, take care when these capacitors are used at the input of circuits supplied by a wall adapter or other supply connected through long, thin wires. Current surges through the inductive wires can induce ringing at the RT7285/A input which could potentially cause large, damaging voltage spikes at VIN. If this phenomenon is observed, some bulk input capacitance may be required. Ceramic capacitors (to meet the RMS current requirement) can be placed in parallel with other types such as tantalum, electrolytic, or polymer (to reduce ringing and overshoot). Choose capacitors rated at higher temperatures than required. Several ceramic capacitors may be paralleled to meet the RMS current, size, and height requirements of the application. The typical operating circuit use 10μF and one 0.1μF low ESR ceramic capacitors on the input. is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT7285/A Output Capacitor Selection The RT7285/A is optimized for ceramic output capacitors and best performance will be obtained using them. The total output capacitance value is usually determined by the desired output voltage ripple level and transient response requirements for sag (undershoot on positive load steps) and soar (overshoot on negative load steps). Output Ripple Output ripple at the switching frequency is caused by the inductor current ripple and its effect on the output capacitor's ESR and stored charge. These two ripple components are called ESR ripple and capacitive ripple. Since ceramic capacitors have extremely low ESR and relatively little capacitance, both components are similar in amplitude and both should be considered if ripple is critical. VRIPPLE = VRIPPLE(ESR)  VRIPPLE(C) VRIPPLE(ESR) = IL  RESR VRIPPLE(C) = IL 8  COUT  fSW For the Typical Operating Circuit for 1.2V output and an inductor ripple of 0.46A, with 1 x 22μF output capacitance each with about 5mΩ ESR including PCB trace resistance, the output voltage ripple components are : VRIPPLE(ESR) = 0.46A  5m = 2.3mV VRIPPLE(C) = 0.46A = 5.227mV 8  22μF  500kHz VRIPPLE = 2.3mV  5.227mV = 7.527mV Output Transient Undershoot and Overshoot In addition to voltage ripple at the switching frequency, the output capacitor and its ESR also affect the voltage sag (undershoot) and soar (overshoot) when the load steps up and down abruptly. The ACOT transient response is very quick and output transients are usually small. However, the combination of small ceramic output capacitors (with little capacitance), low output voltages (with little stored charge in the output capacitors), and low duty cycle applications (which require high inductance to get reasonable ripple currents with high input voltages) increases the size of voltage variations in response to very quick load changes. Typically, load changes occur slowly with respect to the IC's 500kHz switching frequency. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 But some modern digital loads can exhibit nearly instantaneous load changes and the following section shows how to calculate the worst-case voltage swings in response to very fast load steps. The output voltage transient undershoot and overshoot each have two components : the voltage steps caused by the output capacitor's ESR, and the voltage sag and soar due to the finite output capacitance and the inductor current slew rate. Use the following formulas to check if the ESR is low enough (typically not a problem with ceramic capacitors) and the output capacitance is large enough to prevent excessive sag and soar on very fast load step edges, with the chosen inductor value. The amplitude of the ESR step up or down is a function of the load step and the ESR of the output capacitor : VESR _STEP = ΔIOUT x RESR The amplitude of the capacitive sag is a function of the load step, the output capacitor value, the inductor value,the input-to-output voltage differential, and the maximum duty cycle. The maximum duty cycle during a fast transient is a function of the on-time and the minimum off-time since the ACOTTM control scheme will ramp the current using on-times spaced apart with minimum off-times, which is as fast as allowed. Calculate the approximate on-time (neglecting parasitics) and maximum duty cycle for a given input and output voltage as : tON = VOUT tON and DMAX = VIN  fSW tON  tOFF(MIN) The actual on-time will be slightly longer as the IC compensates for voltage drops in the circuit, but we can neglect both of these since the on-time increase compensates for the voltage losses. Calculate the output voltage sag as : L  (IOUT )2 VSAG = 2  COUT   VIN(MIN)  DMAX  VOUT  The amplitude of the capacitive soar is a function of the load step, the output capacitor value, the inductor value and the output voltage : VSOAR = L  (IOUT )2 2  COUT  VOUT is a registered trademark of Richtek Technology Corporation. DS7285/A-01 October 2014 RT7285/A Feed-forward Capacitor (Cff) Enable Operation (EN) The RT7285/A is optimized for ceramic output capacitors and for low duty cycle applications. However for high-output voltages, with high feedback attenuation, the circuit's response becomes over-damped and transient response can be slowed. In high-output voltage circuits (VOUT > 3.3V) transient response is improved by adding a small “feedforward” capacitor (Cff) across the upper FB divider resistor (Figure 1), to increase the circuit's Q and reduce damping to speed up the transient response without affecting the steady-state stability of the circuit. Choose a suitable capacitor value that following below step. For automatic start-up the EN pin can be connected to VIN, through a 100kΩ resistor. Its large hysteresis band makes EN useful for simple delay and timing circuits. EN can be externally pulled to VIN by adding a resistorcapacitor delay (REN and CEN in Figure 2). Calculate the delay time using EN's internal threshold where switching operation begins (1.4V, typical).  Get the BW the quickest method to do transient response form no load to full load. Confirm the damping frequency. The damping frequency is BW. An external MOSFET can be added to implement digital control of EN when no system voltage above 2V is available (Figure 3). In this case, a 100kΩ pull-up resistor, REN, is connected between VIN and the EN pin. MOSFET Q1 will be under logic control to pull down the EN pin. To prevent enabling circuit when VIN is smaller than the VOUT target value or some other desired voltage level, a resistive voltage divider can be placed between the input voltage and ground and connected to EN to create an additional input under voltage lockout threshold (Figure 4). EN BW VIN REN EN RT7285/A CEN GND VOUT Figure 2. External Timing Control R1 Cff FB RT7285/A R2 GND VIN REN 100k EN Q1 Enable RT7285/A GND Figure 1. Cff Capacitor Setting  Cff can be calculated base on below equation : 1 Cff  2  3.1412  R1 BW  0.8 Figure 3. Digital Enable Control Circuit VIN Internal Soft-Start (SS) The RT7285/A soft-start uses an internal soft-start time 800μs. Following below equation to get the minimum capacitance range in order to avoid UV occur. REN1 REN2 EN RT7285/A GND Figure 4. Resistor Divider for Lockout Threshold Setting COUT  VOUT  0.6  1.2 (ILIM  Load Current)  0.8 T  800μs T Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS7285/A-01 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT7285/A Output Voltage Setting Set the desired output voltage using a resistive divider from the output to ground with the midpoint connected to FB. The output voltage is set according to the following equation : VOUT = 0.6 x (1 + R1 / R2) VOUT turn-on can be slowed by placing a small (