RT5797A 3A, 1MHz, 5.5V CMCOT Synchronous Step-Down Converter General Description Features The RT5797A is a high efficiency synchronous stepdown DC/DC converter. Its input voltage range is from 2.7V to 5.5V and provides an adjustable regulated output voltage from 0.6V to 3.4V while delivering up to 3A of output current. Constant-On-time (CMCOT) operation with internal compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The RT5797A is available in the WDFN-8L 2x2 package. Ordering Information for Best Transient Response, Robust Loop Stability with Low-ESR (MLCC) COUT The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The Current Mode Efficiency Up to 95% RDS(ON) 100m HS / 70m LS VIN Range 2.7V to 5.5V VREF 0.6V with 2% Accuracy at 25C CMCOT ™ Control Loop Design Soft-Start 1.2ms Power Saving in Light Load Applications STB, Cable Modem, & xDSL Platforms LCD TV Power Supply & Metering Platforms General Purpose Point of Load (POL) Pin Configurations RT5797A FB PG VIN PGND Lead Plating System G : Green (Halogen Free and Pb Free) UVP Option H : Hiccup L : Latched-Off 1 2 3 4 PGND (TOP VIEW) Package Type QW : WDFN-8L 2x2 (W-Type) 9 8 7 6 5 SGND EN LX NC WDFN-8L 2x2 Note : Richtek products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Simplified Application Circuit RT5797A LX VIN VIN CIN February 2015 VOUT EN R1 FB PG SGND PGND R2 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 L COUT is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT5797A Marking Information RT5797ALGQW 2U : Product Code W : Date Code RT5797AHGQW 2V : Product Code W : Date Code 2UW 2VW Functional Pin Description Pin No. Pin Name Pin Function 1 FB Feedback Voltage Input. An external resistor divider from the output to GND, tapped to the FB pin, sets the output voltage. 2 PG Power Good Indicator. The output of this pin is an open-drain with external pull-up resistor. PG is pulled up when the FB voltage is within 90%, otherwise it is LOW. 3 VIN Supply Voltage Input. The RT5797A operates from a 2.7V to 5.5V input. 4, 9 PGND (Exposed Pad) Power Ground. The exposed pad must be soldered to a large PCB and connected to PGND for maximum power dissipation. 5 NC No Internal Connection. 6 LX Switch Node. 7 EN Enable Control Input. 8 SGND Signal GND. Function Block Diagram EN VIN UVLO Shut Down Control OTP - FB VREF Error Amplifier Ton Comparator + + RC CCOMP - Logic Control LX VIN Driver Current Limit Detector LX GND + Current Sense - LX PG Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Operation The RT5797A is a synchronous low voltage step-down converter that can support the input voltage range from 2.7V to 5.5V and the output current can be up to 3A. The RT5797A uses a constant on-time, current mode architecture. In normal operation, the high side PMOSFET is turned on when the switch controller is set by the comparator and is turned off when the Ton Enable Comparator comparator resets the switch controller. track the internal ramp voltage during soft-start interval. The typical soft-start time is 1.2ms. Low side MOSFET peak current is measured by internal RSENSE. The error amplifier EA adjusts COMP voltage by comparing the feedback signal (VFB) from the output voltage with the internal 0.6V reference. When the load current increases, it causes a drop in the feedback voltage relative to the reference, then the COMP voltage rises to allow higher inductor current to match the load current. UV Comparator If the feedback voltage (VFB) is lower than threshold voltage 0.2V, the UV comparator's output will go high and the switch controller will turn off the high side MOSFET. The output under voltage protection is designed to operate in Hiccup mode for RT5797AH, Latch mode for RT5797AL. PGOOD Comparator A logic-high enables the converter; a logic-low forces the IC into shutdown mode. Soft-Start (SS) An internal current source charges an internal capacitor to build the soft-start ramp voltage. The VFB voltage will Over Current Protection (OCP) The RT5797A provides over current protection by detecting low side MOSFET valley inductor current. If the sensed valley inductor current is over the current limit threshold (3.7A typ.), the OCP will be triggered. When OCP is tripped, the RT5797A will keep the over current threshold level then cause the UV protection. Thermal Shutdown (OTP) The device implements an internal thermal shutdown function when the junction temperature exceeds 150C. The thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal shutdown threshold. Once the die temperature decreases below the hysteresis of 20C, the device reinstates the power up sequence. When the feedback voltage (VFB) is higher than threshold voltage 0.54V, the PGOOD open drain output will be high impedance. The internal PG MOSFET is typical 10. The PGOOD signal delay time from EN is about 2ms. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT5797A Absolute Maximum Ratings (Note 1) Supply Input Voltage -------------------------------------------------------------------------------------------0.3V to 6.5V LX Pin Switch Voltage----------------------------------------------------------------------------------------- 0.3V to (VIN + 0.3V) <20ns -------------------------------------------------------------------------------------------------------------- 4.5V to 7.5V Power Dissipation, PD @ TA = 25C WDFN-8L 2x2 ----------------------------------------------------------------------------------------------------2.19W Package Thermal Resistance (Note 2) WDFN-8L 2x2, JA ----------------------------------------------------------------------------------------------45.5C/W Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------260C Junction Temperature ------------------------------------------------------------------------------------------40C to 150C Storage Temperature Range ---------------------------------------------------------------------------------65C to 150C ESD Susceptibility (Note 3) HBM (Human Body Model) -----------------------------------------------------------------------------------2kV Recommended Operating Conditions (Note 4) Supply Input Voltage -------------------------------------------------------------------------------------------2.7V to 5.5V Ambient Temperature Range---------------------------------------------------------------------------------40C to 85C Junction Temperature Range --------------------------------------------------------------------------------40C to 125C Electrical Characteristics (VIN = 3.6V, TA = 25C, unless otherwise specified) Parameter Symbol Min Typ Max Unit 2.7 -- 5.5 V 0.588 0.6 0.612 V VFB = 3.3V -- -- 1 A Active ,VFB = 0.63V, Not Switching -- 22 -- Shutdown -- -- 1 Switching Leakage Current -- -- 1 A Switching Frequency -- 1 -- MHz Input Voltage VIN Feedback Reference Voltage VREF Feedback Leakage Current IFB DC Bias Current Test Conditions A Switch On Resistance, Low RNMOS ISW = 0.3A -- 70 -- m Switch On Resistance, High RPMOS ISW = 0.3A -- 100 -- m Valley Current Limit ILIM 3.03 3.7 4.6 A Under-Voltage Lockout Threshold VUVLO VDD Rising -- 2.25 2.5 V VDD Falling -- 2 -- V -- 150 -- °C Over-Temperature Threshold Enable Input Voltage Logic-High VIH 1.5 -- -- Logic-Low VIL -- -- 0.4 Rising -- 90 -- Falling -- 85 -- PG Pin Threshold (relative to VOUT) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 V % is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Parameter Symbol Test Conditions Min Typ Max Unit -- -- 100 -- 1.2 -- ms Minimum Off Time -- 120 -- ns Output Discharge Switch On Resistance -- 1.8 -- k PG Open-Drain Impedance (PG = low) Soft-Start Time TSS 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. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. Note 3. Devices are ESD sensitive. Handling precaution recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT5797A Typical Application Circuit VIN 3 CIN 22μF 7 2 RT5797A 6 LX VIN L EN CFF* R1 VOUT COUT 22μF x 2 1 FB PG SGND PGND 8 4, 9 (Exposed Pad) R2 *CFF : Optional for performance fine-tune Table 1. Suggested Component Values VOUT (V) R1 (k) R2 (k) CIN (F) L (H) COUT (F) 3.3 90 20 22 1.5 22 x2 1.8 100 50 22 1.5 22 x2 1.5 100 66.6 22 1.5 22 x2 1.2 100 100 22 1.5 22 x2 1.05 100 133 22 1.5 22 x2 1 100 148 22 1.5 22 x2 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Typical Operating Characteristics Efficiency vs. Output Current 100 90 90 70 VIN = 5V, VOUT = 3.3V 80 VIN = 3.3V, VOUT = 1.2V 70 Efficiency (%) 80 Efficiency (%) Efficiency vs. Output Current 100 60 50 40 30 50 40 30 20 10 10 0 0.5 1 1.5 2 2.5 VIN = 3.3V, VOUT = 1.2V 60 20 0 VIN = 5V, VOUT = 3.3V 0 0.001 3 0.01 Output Voltage vs. Output Current 1 3.40 1.26 3.38 Output Current (V) 1.24 1.22 1.20 1.18 1.16 3.36 3.34 3.32 3.30 3.28 1.14 VIN = 3.3V VIN = 5V 1.12 3.26 0 0.5 1 1.5 2 2.5 3 0 0.5 Output Current (A) 1 1.5 2 2.5 3 Output Voltage (A) Output Voltage vs. Input Voltage Output Voltage vs. Input Voltage 1.26 3.40 3.38 Output Voltage (V) 1.24 Output Voltage (V) 10 Output Voltage vs. Output Current 1.28 Output Voltage (V) 0.1 Output Current (A) Output Current (A) IOUT = 0A 1.22 1.20 1.18 IOUT = 2A 1.16 IOUT = 0A 3.36 3.34 3.32 IOUT = 2A 3.30 3.28 3.26 1.14 3.24 VIN = 2.5V to 5.5V, V OUT = 1.2V 1.12 2.5 3 3.5 4 4.5 5 Input Voltage (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 February 2015 5.5 VIN = 4.5V to 5.5V, V OUT = 3.3V 3.22 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT5797A Switching Frequency vs. Input Voltage 1.5 0.64 1.4 Switcing Frequency (MHz) Reference Voltage (V) Reference Voltage vs. Input Voltage 0.65 0.63 0.62 0.61 0.60 0.59 0.58 0.57 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.56 0.6 0.55 0.5 2.5 3 3.5 4 4.5 5 IOUT = 0.6A 2.5 5.5 3 3.5 4.5 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 5 5.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 VEN = 0V VEN = 0V 0.0 0.0 2.5 3 3.5 4 4.5 5 -50 5.5 -25 0 25 50 75 100 125 Temperature (°C) Input Voltage (V) Quiescent Current vs. Input Voltage Quiescent Current vs. Temperature 30 40 35 25 Quiescent Current (µA) Quiescent Current (µA) 4.5 Shutdown Currrent vs. Temperature 5.0 Shutdown Current (μA)1 Shutdown Current (µA)1 Shutdown Current vs. Input Voltage 5.0 0.5 4 Input Voltage (V) Input Voltage (V) 20 15 10 5 30 VIN = 5V 25 20 VIN = 3.3V 15 10 5 VFB = 0.63V, LX no switch 0 0 2.5 3 3.5 4 4.5 5 Input Voltage (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 5.5 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Current Limit vs. Temperature 5 4 4 Inductor Current (A) Inductor Current (A) Current Limit vs. Input Voltage 5 3 2 1 3 2 1 VOUT = 1.2V VOUT = 1.2V 0 0 2.5 3 3.5 4 4.5 5 5.5 -50 -25 Input Voltage (V) 25 50 75 100 125 Temperature (°C) UVLO vs. Temperature Enable Voltage vs. Temperature 2.5 1.4 2.4 1.2 Enable Voltage (V) 2.3 Input Voltage (V) 0 Turn On 2.2 2.1 2.0 1.9 Turn Off 1.8 1.0 Enable On 0.8 Enable Off 0.6 0.4 1.7 0.2 1.6 VIN = 3.3V VEN = 3.3V 1.5 0.0 -50 -25 0 25 50 75 100 125 0 25 50 75 100 Load Transient Response Load Transient Response 125 VOUT (100mV/Div) IOUT (2A/Div) VIN = 3.3V, VOUT = 1.2V, IOUT = 0A to 3A Time (100μs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 -25 Temperature (°C) VOUT (100mV/Div) IOUT (2A/Div) -50 Temperature (°C) February 2015 VIN = 3.3V, VOUT = 1.2V, IOUT = 1A to 3A Time (100μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT5797A Voltage Ripple Voltage Ripple VIN = 3.3V, VOUT = 1.2V, IOUT = 1A VIN = 5V, VOUT = 3.3V, IOUT = 1A VOUT VOUT (10mV/Div) (10mV/Div) VLX (2V/Div) VLX (2V/Div) Time (1μs/Div) Time (1μs/Div) Power On from EN Power Off from EN VEN (2V/Div) VEN (2V/Div) VPGOOD (2V/Div) VPGOOD (2V/Div) VOUT VOUT (1V/Div) (1V/Div) IOUT IOUT (1A/Div) (1A/Div) VIN = 3.3V, VOUT = 1.2V, IOUT = 0A VIN = 3.3V, VOUT = 1.2V, IOUT = 0A Time (500μs/Div) Time (10ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Application Information The RT5797A is a single-phase step-down converter. It provides single feedback loop constant on-time, current mode control with fast transient response. An internal 0.6V reference allows the output voltage to be precisely regulated for low output voltage applications. A fixed switching frequency (1MHz) oscillator and internal compensation are integrated to minimize external component count. Protection features include over current protection, under voltage protection and over temperature protection. Output Voltage Setting Connect a resistive voltage divider at the FB between VOUT and GND to adjust the output voltage. The output voltage is set according to the following equation : VOUT = VREF 1 R1 R2 where VREF is the feedback reference voltage 0.6V (typ.). VOUT R1 During soft-start, the internal soft-start capacitor becomes charged and generates a linear ramping up voltage across the capacitor. This voltage clamps the voltage at the FB pin, causing PWM pulse width to increase slowly and in turn reduce the input surge current. The internal 0.6V reference takes over the loop control once the internal ramping-up voltage becomes higher than 0.6V. Over Voltage Protection (OVP) The RT5797AL provide Over Voltage Protection function when output voltage over 120%. The IC will be into Latch-off mode. UVLO Protection The RT5797A has input Under Voltage Lockout protection (UVLO). If the input voltage exceeds the UVLO rising threshold voltage (2.25V typ.), the converter resets and prepares the PWM for operation. If the input voltage falls below the UVLO falling threshold voltage during normal operation, the device will stop switching. The UVLO rising and falling threshold voltage has a hysteresis to prevent noisecaused reset. FB R2 GND Figure 1. Setting VOUT with a Voltage Divider Chip Enable and Disable The EN pin allows for power sequencing between the controller bias voltage and another voltage rail. The RT5797A remains in shutdown if the EN pin is lower than 400mV. When the EN pin rises above the VEN trip point, the RT5797A begins a new initialization and softstart cycle. Internal Soft-Start The RT5797A provides an internal soft-start function to prevent large inrush current and output voltage overshoot when the converter starts up. The soft-start (SS) automatically begins once the chip is enabled. Input Capacitor Selection High quality ceramic input decoupling capacitor, such as X5R or X7R, with values greater than 22F are recommended for the input capacitor. The X5R and X7R ceramic capacitors are usually selected for power regulator capacitors because the dielectric material has less capacitance variation and more temperature stability. Voltage rating and current rating are the key parameters when selecting an input capacitor. Generally, selecting an input capacitor with voltage rating 1.5 times greater than the maximum input voltage is a conservatively safe design. The input capacitor is used to supply the input RMS current, which can be approximately calculated using the following equation : IIN_RMS = ILOAD VOUT V 1 OUT VIN VIN The next step is selecting a proper capacitor for RMS Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT5797A current rating. One good design uses more than one capacitor with low equivalent series resistance (ESR) in parallel to form a capacitor bank. to the average inductor current. The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be approximately calculated using the following equation : must be large enough not to saturate at the peak inductor current (IPEAK) : VIN IOUT(MAX) VOUT V = 1 OUT CIN fSW VIN VIN Output Capacitor Selection The output capacitor and the inductor form a low pass filter in the Buck topology. In steady state condition, the ripple current flowing into/out of the capacitor results in ripple voltage. The output voltage ripple (VP-P) can be calculated by the following equation : 1 VP_P = LIR ILOAD(MAX) ESR + 8 COUT fSW When load transient occurs, the output capacitor supplies the load current before the controller can respond. Therefore, the ESR will dominate the output voltage sag during load transient. The output voltage undershoot (VSAG) can be calculated by the following equation : VSAG = ILOAD ESR For a given output voltage sag specification, the ESR value can be determined. Another parameter that has influence on the output voltage sag is the equivalent series inductance (ESL). The rapid change in load current results in di/dt during transient. Therefore, the ESL contributes to part of the voltage sag. Using a capacitor with low ESL can obtain better transient performance. Generally, using several capacitors connected in parallel can have better transient performance than using a single capacitor for the same total ESR. Find a low loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. The core IPEAK = ILOAD(MAX) + LIR ILOAD(MAX) 2 The calculation above serves as a general reference. To further improve transient response, the output inductor can be further reduced. This relation should be considered along with the selection of the output capacitor. Inductor saturation current should be chosen over IC’s current limit. Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) TA) / JA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and JA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125C. The junction to ambient thermal resistance, JA, is layout dependent. For WDFN-8L 2x2 package, the thermal resistance, JA, is 45.5C/W on a standard four-layer thermal test board. The maximum power dissipation at TA = 25C can be calculated by the following formula : PD(MAX) = (125C 25C) / (45.5C/W) = 2.19W for WDFN-8L 2x2 package Inductor Selection The switching frequency (on-time) and operating point (% ripple or LIR) determine the inductor value as shown below : L= fSW VOUT VIN VOUT LIR ILOAD(MAX) VIN The maximum power dissipation depends on the operating ambient temperature for fixed TJ(MAX) and thermal resistance, JA. The derating curve in Figure 2 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. where LIR is the ratio of the peak-to-peak ripple current Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015 RT5797A Maximum Power Dissipation (W)1 2.5 Four-Layer PCB 2.0 1.5 1.0 0.5 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 2. Derating Curve of Maximum Power Dissipation Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS5797A-00 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT5797A Outline Dimension Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.200 0.300 0.008 0.012 D 1.950 2.050 0.077 0.081 D2 1.000 1.250 0.039 0.049 E 1.950 2.050 0.077 0.081 E2 0.400 0.650 0.016 0.026 e L 0.500 0.300 0.020 0.400 0.012 0.016 W-Type 8L DFN 2x2 Package Richtek Technology Corporation 14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS5797A-00 February 2015