RT7285CGE

Reference Design Design Tools Sample & Buy ® RT7285C 1.5A, 18V, 500kHz ACOTTM Synchronous Step-Down Converter General Description Features The RT7285C is a synchronous step-down converter with Advanced Constant On-Time (ACOTTM) mode control. The ACOTTM 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 RT7285C enables to support all ceramic capacitors. The output voltage can be adjusted between 0.6V and 8V. z z z z z z z z z Ordering Information z RT7285C z z Package Type E : SOT-23-6 J6 : TSOT-23-6 z Lead Plating System G : Green (Halogen Free and Pb Free) Note : ` RoHS compliant and compatible with the current require- z z z ments of IPC/JEDEC J-STD-020. ` Applications z Richtek products are : Suitable for use in SnPb or Pb-free soldering processes. Marking Information z 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 RT7285CGE (TOP VIEW) 0W= : Product Code 0W=DNN 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 RoHS Compliant and Halogen Free SW VIN EN DNN : Date Code 6 RT7285CGJ6 5 4 2 3 0B= : Product Code 0B=DNN BOOT GND FB DNN : Date Code SOT-23-6 / TSOT-23-6 Simplified Application Circuit VIN VIN RT7285C BOOT SW Enable VOUT FB EN GND Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS7285C-04 February 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT7285C Functional Pin Description Pin No. SOT-23-6 TSOT-23-6 Pin Name Pin Function 1 1 BOOT Bootstrap Supply for High-Side Gate Driver. Connect a 0.1µF ceramic capacitor between the BOOT and SW pins. 2 2 GND Power Ground. 3 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 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 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 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 RT7285C 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 RT7285C 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 © 2018 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. DS7285C-04 February 2018 RT7285C Absolute Maximum Ratings z z z z z z z z z z (Note 1) VIN to GND ----------------------------------------------------------------------------------------------------- −0.3V to 20V SW to GND ---------------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) < 10ns ----------------------------------------------------------------------------------------------------------- −5V to 25V BOOT to GND ------------------------------------------------------------------------------------------------- (VSW − 0.3V) to (VSW + 6V) Other Pins ------------------------------------------------------------------------------------------------------ −0.3V to 6V Power Dissipation, PD @ TA = 25°C SOT-23-6 / TSOT-23-6 --------------------------------------------------------------------------------------- 0.625W Package Thermal Resistance (Note 2) SOT-23-6 / TSOT-23-6, θJA --------------------------------------------------------------------------------- 160°C/W SOT-23-6 / 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 z z z (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 µA 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 3.55 3.9 4.25 V -- 340 -- mV Switch-On Resistance EN Input Threshold Logic-High VEN_H Logic-Low VIN Under-Voltage Lockout Threshold VEN_L VUVLO VIN Under-Voltage Lockout Threshold Hysteresis Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS7285C-04 February 2018 VIN Rising mΩ V is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT7285C Parameter Symbol Test Conditions Min Typ Max Unit Soft-Start Time tSS -- 800 -- µs Thermal Shutdown Threshold TSD -- 160 -- °C Thermal Shutdown Hysteresis ∆TSD -- 20 -- °C VOUT Discharge Resistance RDISCHG EN = 0V, VOUT = 0.5V -- 50 100 Ω UVP Detect 70 75 80 Hysteresis -- 10 -- -- 250 -- Output Under-Voltage Trip Threshold Output Under-Voltage Delay Time % µs 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 © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS7285C-04 February 2018 RT7285C Typical Application Circuit RT7285C VIN 4.3V to 18V 5 CIN 10µF BOOT VIN 1 SW 6 L CBOOT 100nF 3.6µH CFF 4 EN Enable FB 3 R1 10k R2 10k GND 2 VOUT 1.2V COUT 22µF Table 1. Suggested Component Values L (µH) COUT (µF) CFF (pF) 15 10 22 39 115 25.5 6.8 22 33 2.5 25.5 8.06 4.7 22 NC 1.2 10 10 3.6 22 NC VOUT (V) R1 (kΩ) 5 110 3.3 R2 (kΩ) Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS7285C-04 February 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT7285C Typical Operating Characteristics Efficiency vs. Load Current 100 90 90 80 80 70 VIN = VIN = VIN = VIN = 60 50 40 Efficiency (%) Efficiency (%) Efficiency vs. Load Current 100 5V 9V 12V 18V 30 20 VIN = 12V VIN = 15V VIN = 18V 70 60 50 40 30 20 10 10 VOUT = 1.2V 0 VOUT = 5V 0 0 0.3 0.6 0.9 1.2 1.5 0 0.3 0.6 Load Current (A) 1.2 Referecnec Voltage vs. Input Voltage Reference vs. Temperature Reference Voltage (V) 0.610 0.605 0.600 0.595 0.605 VIN = 12V 0.600 0.595 VIN = 4.5V to 18V, VOUT = 1.2V, IOUT = 0A 0.590 IOUT = 0A 0.590 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.5 -50 -25 0 Input Voltage (V) 25 50 75 100 125 Temperature (°C) Output Voltage vs. Load Current Switching Frequency vs. Input Voltage 600 1.230 1.226 1.218 1.214 Switcing Frequency (kHz)1 VIN = 18V VIN = 12V VIN = 9V VIN = 5V VIN = 4.5V 1.222 Output Voltage (V) 1.5 Load Current (A) 0.610 Referecnec Voltage (V) 0.9 1.210 1.206 1.202 1.198 1.194 580 560 540 520 VIN = 4.5V to 18V, VOUT = 1.2V 1.190 VOUT = 1.2V, IOUT = 0A 500 0 0.3 0.6 0.9 1.2 Load Current (A) Copyright © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 1.5 4 6 8 10 12 14 16 18 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. DS7285C-04 February 2018 RT7285C Current Limit vs. Temperature 3.0 630 2.8 610 2.6 VIN = 6V VIN = 12V VIN = 18V VIN = 4.5V 590 570 550 Current Limit (A) Switching Frequency (kHz)1 Switching Frequency vs. Temperature 650 530 510 490 2.4 VIN = 6V VIN = 12V VIN = 18V 2.2 2.0 1.8 1.6 1.4 470 VOUT = 1.2V 450 1.2 1.0 -50 -25 0 25 50 75 100 125 -50 25 50 75 100 Temperature (°C) Load Transient Response Load Transient Response VOUT (20mV/Div) IOUT (1A/Div) IOUT (1A/Div) Time (100µs/Div) Time (100µs/Div) Switching Switching VOUT (20mV/Div) VOUT (20mV/Div) VSW (10V/Div) VSW (10V/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (1µs/Div) Copyright © 2018 Richtek Technology Corporation. All rights reserved. February 2018 125 VIN = 12V, VOUT = 1.2V, IOUT = 0.75A to 1.5A VIN = 12V, VOUT = 1.2V, IOUT = 0A to 1.5A DS7285C-04 0 Temperature (°C) VOUT (20mV/Div) IL (1A/Div) -25 IL (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 0.75A Time (1µs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT7285C Power Off from VIN Power On from VIN VIN (10V/Div) VOUT (1V/Div) VSW (10V/Div) VIN (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 (2.5ms/Div) Time (2.5ms/Div) Power On from EN Power Off from EN VEN (2V/Div) VOUT (1V/Div) VSW (10V/Div) VEN (2V/Div) VOUT (1V/Div) VSW (10V/Div) I SW (1A/Div) I SW (1A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (2.5ms/Div) Copyright © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 VIN = 12V, VOUT = 1.2V, IOUT = 1.5A Time (5ms/Div) is a registered trademark of Richtek Technology Corporation. DS7285C-04 February 2018 RT7285C 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 (∆IL) 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 ∆IL as some percentage of that current. L= VOUT × ( VIN − VOUT ) VIN × fSW × ∆IL Once an inductor value is chosen, the ripple current (∆IL) is calculated to determine the required peak inductor current. VOUT × ( VIN − VOUT ) ∆IL = VIN × fSW × L ∆I IL(PEAK) = IOUT(MAX) + L 2 ∆IL IL(VALLY) = IOUT(MAX) − 2 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 © 2018 Richtek Technology Corporation. All rights reserved. DS7285C-04 February 2018 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 RT7285C 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 RT7285C Output Capacitor Selection The RT7285C 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 × 5mΩ = 2.3mV VRIPPLE(C) = 0.46A = 5.227mV 8 × 22µF × 500kHz 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 = ∆IOUT 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 : VOUT tON and DMAX = VIN × fSW tON + tOFF(MIN) VRIPPLE = 2.3mV + 5.227mV = 7.527mV tON = Output Transient Undershoot and Overshoot 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 ) 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 © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 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. DS7285C-04 February 2018 RT7285C Feed-forward Capacitor (Cff) Enable Operation (EN) The RT7285C 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 RT7285C CEN VOUT R1 GND Cff Figure 2. External Timing Control FB RT7285C R2 GND VIN REN 100k RT7285C Q1 Enable Figure 1. Cff Capacitor Setting ` EN GND Cff can be calculated base on below equation : Cff = 1 2 × 3.1412 × R1× BW × 0.8 Internal Soft-Start (SS) The RT7285C soft-start uses an internal soft-start time 800µs. Following below equation to get the minimum capacitance range in order to avoid UV occur. Figure 3. Digital Enable Control Circuit VIN REN1 EN REN2 RT7285C GND Figure 4. Resistor Divider for Lockout Threshold Setting COUT × VOUT × 0.6 × 1.2 (ILIM − Load Current) × 0.8 T ≤ 800µs T= Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS7285C-04 February 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT7285C 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 (