RT8129A High Efficiency Single Synchronous Buck PWM Controller General Description Features The RT8129A is a high efficiency single phase synchronous buck controller with 5V/12V supply voltage. The RT8129A integrates a Constant-On-Time (COT) PWM controller and a MOSFET drivers with internal bootstrap diodes, which is specifically designed to improve converter efficiency at light load condition. At light load condition, it automatically operates in the diode emulation mode to reduce switching frequency and improve conversion efficiency. Other features include power good indication, enable/disable control and internal soft-start function. The RT8129A also provide protection functions including Over Voltage Protection (OVP), Under Voltage Protection (UVP), current limit and thermal shutdown. This device uses lossless low-side MOSFET RDS(ON) current sense technique for current limit with adjustable threshold set by connecting a resistor between the LGATE/OCSET and GND. With above functions, the RT8129A Wide Input Voltage Range : 2.5V to 25V High Light Load Efficiency Integrated High Driving Capability N-MOSFET Gate Drivers and Embedded Switching Boot Diode Single IC Supply Voltage : 4.5V to 13.2V Power-Good Indicator Enable/Disable Control Internal Soft-Start Programmable Current Limit Threshold Under Voltage Protection Over Voltage Protection Thermal Shutdown Applications Motherboard, Memory/Chip-set Power Graphic Card, GPU/Memory Core Power Low Voltage, High Current DC/DC Regulator Marking Information provides customers a cost-effective solution for high efficiency power conversion. The RT8129A is available in the WDFN-10L 3x3 package. 5N=YM DNN 5N= : Product Code YMDNN : Date Code Simplified Application Circuit VIN RT8129A VCC VCC BOOT UGATE PHASE VPGOOD PGOOD VOUT LGATE/ OCSET EN Enable FB GND Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8129A Ordering Information Pin Configurations RT8129A Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : BOOT PHASE UGATE LGATE/OCSET GND 1 2 3 4 5 GND (TOP VIEW) Package Type QW : WDFN-10L 3x3 (W-Type) 11 10 9 8 7 6 PGOOD NC FB EN VCC WDFN-10L 3x3 RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Functional Pin Description Pin No. Pin Name Pin Function BOOT Bootstrap Supply for High Side Gate Driver. Connect this pin to a power source VCC through a bootstrap diode, and connect a 0.1F or greater ceramic capacitor from this pin to the PHASE pin to supply the power for high side gate driver. 2 PHASE Switch Node. Connect this pin to the switching node of Buck converter. Connect this pin to the Source of high-side MOSFET together with the Drain of low-side MOSFET and the inductor. The PHASE voltage is sensed for zero current detection and over current protection when low side MOSFET is on. 3 UGATE High Side MOSFET Gate Driver Output. This pin provides the gate drive for the converter's high-side MOSFET. Connect this pin to the Gate of high-side MOSFET. 4 Low Side MOSFET Gate Driver Output. Connect this pin to the Gate of low side MOSFET. This pin is also used for current limit threshold setting. LGATE/OCSET Connect a resistor (ROCSET) from this pin to the GND pin to set the current limit threshold. 1 5, GND 11 (Exposed Pad) Ground. The Exposed Pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 6 VCC Supply Voltage Input. It is recommended to connect a 4.7F ceramic capacitor from this pin to the GND pin. VCC also powers the low side gate driver. 7 EN Enable Control Input. Drive EN higher than 2V to turn on the controller, lower than 0.8V to turn it off. If the EN pin is open, it will be pulled to high by internal circuit. 8 FB This pin is used for output voltage feedback input and it is also monitored for power good indication, over voltage and under voltage protections. Connect this pin to the converter output through voltage divider resistors for output voltage regulation. 9 NC No Internal Connection. PGOOD Power Good Indication Output. This pin provides an open drain output. Connect this pin to a voltage source through a pull up resistor. The PGOOD voltage goes high to indicate the output voltage is in regulation. This pin can be left open if the power good indication function is not used. 10 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Function Block Diagram Trigger TON Generator 1-Shot VREF PHASE BOOT + R COMP S - + 125% VREF 75% VREF PGOOD + PGOOD Monitor ±10% VREF LDO& POR VCC EN UV Latch S1 Q - FB OV Latch S1 Q Thermal Shutdown PHASE Min TOFF 1-Shot Trigger VCC LGATE/OCSET GND ASM 10µA + SS REF UGATE Q - + gm - Sample and Hold VREF Operation The RT8129A integrates a Constant-On-Time (COT) PWM controller and MOSFET driver so that the external circuit is easily designed and the components are reduced. The controller provides the PWM signal which relies on the FB voltage comparing with internal reference voltage. The synchronous UGATE driver is turned on at the beginning of each cycle. After the internal one-shot timer expires, the UGATE driver will be turned off. The pulse width of this one-shot is determined by the controller's input voltage and the output voltage to keep the frequency fairly constant over the input voltage and output voltage range. Another one-shot sets a minimum off-time. The RT8129A remains in shutdown if the EN pin voltage is lower than 0.8V. When the EN pin voltage rises above the2V, the RT8129A will begin a new initialization and soft-start cycle. PGOOD The power good output is an open-drain architecture, and it requires a pull-up resistor. During soft-start Copyright © 2015 Richtek Technology Corporation. All rights reserved. October Soft-Start An internal current source charges an internal capacitor to build the soft-start ramp voltage. The output voltage will track the internal ramp voltage during soft-start interval. The typical soft-start time is 2ms. Current Limit Enable DS8129A-01 process, PGOOD is actively held low and is allowed to be pulled high after soft start process is completed and no protection occur. In addition, if the FB pin voltage is higher than 110% of VREF or lower than 90% of VREF during operation, PGOOD will be pulled low immediately. 2015 The current limit circuit employs a unique “valley” current sensing algorithm. If the magnitude of the current sense signal at PHASE is above the current limit threshold, the PWM is not allowed to initiate a new cycle. Thus, the current to the load exceeds the average output inductor current, the output voltage falls and eventually crosses the under-voltage protection threshold, inducing IC shutdown. is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8129A Over Voltage Protection (OVP) Under Voltage Protection (UVP) The FB voltage can be continuously monitored for over voltage protection. When the FB voltage exceeds The output voltage can be continuously monitored for under voltage protection. When the FB voltage is less 125% of the reference voltage, UGATE goes low and LGATE is forced high. The controller is latched until VCC is re-supplied and exceeds the POR rising threshold voltage. than 75% of the reference voltage, under voltage protection is triggered and then both UGATE and LGATE gate drivers are forced low. The controller is latched until VCC or EN pin voltage is re-supplied and exceeds the POR rising threshold voltage. There is a 5s delay built into the under voltage protection circuit to prevent false transitions. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 There is a 3s delay built into the under voltage protection circuit to prevent false transitions. is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Absolute Maximum Ratings (Note 1) VCC to GND ------------------------------------------------------------------------------------------------------------ 0.3V to 15V Other Pins --------------------------------------------------------------------------------------------------------------- 0.3V to 6.5V BOOT to PHASE DC--------------------------------------------------------------------------------------------------------------------------0.3V to 15V <100ns --------------------------------------------------------------------------------------------------------------------0.3V to 20V PHASE to GND DC------------------------------------------------------------------------------------------------------------------------- 5V to 25V <100ns ------------------------------------------------------------------------------------------------------------------- 10V to 30V BOOT to GND DC------------------------------------------------------------------------------------------------------------------------- 0.3V to 40V <100ns ------------------------------------------------------------------------------------------------------------------- 0.3V to 45V UGATE to GND DC------------------------------------------------------------------------------------------------------------------------- 0.3V to 40V <100ns ------------------------------------------------------------------------------------------------------------------- 10V to 45V UGATE to PHASE DC------------------------------------------------------------------------------------------------------------------------- 0.3V to 15V <40ns --------------------------------------------------------------------------------------------------------------------- 5V to 20V LGATE to GND DC------------------------------------------------------------------------------------------------------------------------- 0.3V to 15V <100ns ------------------------------------------------------------------------------------------------------------------- 5V to 20V Power Dissipation, PD @ TA = 25C WDFN-10L 3x3 -------------------------------------------------------------------------------------------------------- 3.27W Package Thermal Resistance (Note 2) WDFN-10L 3x3, JA -------------------------------------------------------------------------------------------------- 30.5C/W WDFN-10L 3x3, JC -------------------------------------------------------------------------------------------------- 7.5C/W Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------- 260C Junction Temperature ------------------------------------------------------------------------------------------------ 150C Storage Temperature Range --------------------------------------------------------------------------------------- 65C to 150C ESD Susceptibility HBM (Human Body Model) ----------------------------------------------------------------------------------------- 2kV (Note 3) Recommended Operating Conditions (Note 4) Power Input Voltage, VIN ------------------------------------------------------------------------------------------- 2.5V to 25V Control Voltage, VCC ------------------------------------------------------------------------------------------------ 4.5V to 13.2V Ambient Temperature Range--------------------------------------------------------------------------------------- 40C to 85C Junction Temperature Range -------------------------------------------------------------------------------------- 40C to 125C Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8129A Electrical Characteristics (TA = 25C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit VCC rising -- -- 4.4 VCC falling 3.9 -- -- -- 0.8 -- V 1 -- 1 % 0.8 -- 3.3 V 270 300 330 kHz PWM Controller VCC POR Threshold Reference Voltage VREF Reference and Error Amplifier Excluding External Resistive Divider Tolerance FB Error Comparator Threshold Output Voltage Range (Note 5) V PWM Frequency FSW Minimum On-Time TON(MIN) -- 70 -- ns Minimum Off-Time TOFF(MIN) -- 300 -- ns -- 10 40 A Logic-High VENH 2 -- -- Logic-Low -- -- 0.8 EN Threshold EN Internal Pull Migh Current EN Input Voltage VEN = 0V VENL V PGOOD Over-Voltage Until PGOOD Goes Low Measured at FB, with respect to reference, no load -- 880 902 mV Under-Voltage Until PGOOD Goes Low Measured at FB, with respect to reference, no load -- 720 -- mV Fault Propagation Delay Falling edge, FB forced below PGOOD trip threshold -- 1 -- s Output Low Voltage ISINK = 1mA -- -- 0.4 V ILEAK High state, forced to 5V -- -- 1 A UGATE Gate Driver Source RUGATEsr VBOOT − VPHASE = 12V, ISOURCE = 100mA -- 1.5 3 UGATE Gate Driver Sink RUGATEsk VBOOT − VPHASE = 12V, ISINK = 10mA -- 2.25 4 LGATE Gate Driver Source RLGATEsr VCC = 12V, ISOURCE = 100mA -- 1.5 3 LGATE Gate Driver Sink RLGATEsk VCC = 12V, ISOURCE = 10mA -- 1 2 From UG falling to LG rising, PHASE = 1.5V 5 20 -- From LG falling to UG rising 5 20 -- VCC to BOOT, 10mA -- -- 80 Leakage Current Driver Dead Time Internal Boot Charging Switch on Resistance Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 ns is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Parameter Symbol Test Conditions Min Typ Max Unit 9.5 10 10.5 A -20 -- 20 mV 0.95 1 1.03 V -- 5 -- s 0.57 0.6 0.63 V 1.2 2 2.8 ms 145 -- 165 C Protection Current Limit Setting Current IOCSET Current Limit Threshold Offset Over Voltage Protection Threshold VOVP OVP latch delay Under Voltage Protection Threshold VUVP Voltage Ramp Soft-Start Time Thermal Shutdown Threshold From FB 0% to FB 100% TSD 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. JC is measured at the exposed pad of the package. Note 3. Devices are ESD sensitive. Handling precaution recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Note 5. No production tested. Test condition VIN = 7V, VOUT = 1.25V, IOUT = 10A using application circuit. Typical Application Circuit VIN DBOOT RT8129A R1 6 VCC VCC C4 VPGOOD BOOT 1 3 UGATE 2 PHASE C1 RPGOOD 10 7 PGOOD LGATE/ 4 OCSET DS8129A-01 2015 Q2 R3 R2 C3 C6 COUT RFB1 C2 8 GND 5, 11 (Exposed Pad) October LOUT VOUT RLGATE EN Copyright © 2015 Richtek Technology Corporation. All rights reserved. CIN Q1 RUGATE ROCSET FB Enable C5 RBOOT CBOOT C7 RFB2 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8129A Typical Operating Characteristics Efficiency vs. Load Current 100 90 90 80 80 70 VIN = 5V 60 VIN = 12V 50 VIN = 19V Efficiency (%) Efficiency (%) Efficiency vs. Load Current 100 40 30 70 VIN = 5V 60 VIN = 12V 50 VIN = 19V 40 30 20 20 10 10 VCC = 5V, VOUT = 1.05V, VNN 0 0.01 0.1 1 VCC = 5V, VOUT = 1.2V, DDRIV 0 0.01 10 0.1 90 90 80 80 VIN = 5V 60 VIN = 12V 50 VIN = 19V 40 30 70 VIN = 5V 60 VIN = 12V 50 VIN = 19V 40 30 20 20 10 10 VCC = 5V, VOUT = 1.35V, DDRIII-L 0 0.01 0.1 1 VCC = 5V, VOUT = 1.5V, DDRIII 0 0.01 10 0.1 1 10 100 Load Current (A) Load Current (A) Frequency vs. Load Current Frequency vs. Load Current 350 350 VCC = 5V, VOUT = 1.05V, VNN VCC = 5V, VOUT = 1.2V, DDRIV 300 Frequency (kHz)1 300 Frequency (kHz)1 10 Efficiency vs. Load Current 100 Efficiency (%) Efficiency (%) Efficiency vs. Load Current 100 70 1 Load Current (A) Load Current (A) 250 VIN = 5V 200 VIN = 12V VIN = 19V 150 100 250 200 VIN = 5V 150 VIN = 12V VIN = 19V 100 50 50 0 0 0.01 0.1 1 Load Current (A) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 10 0.01 0.1 1 10 Load Current (A) is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Frequency vs. Load Current Frequency vs. Load Current 350 350 VCC = 5V, VOUT = 1.5V, DDRIII VCC = 5V, VOUT = 1.35V, DDRIII-L 300 Frequency (kHz)1 Frequency (kHz)1 300 250 200 150 VIN = 5V 100 250 200 150 100 VIN = 5V VIN = 12V 50 VIN = 12V 50 VIN = 19V VIN = 19V 0 0 0.01 0.1 1 0.01 10 Frequency vs. Load Current 1 10 Output Voltage vs. Load Current 350 1.060 VIN = 12V, VOUT = 1.2V 300 VCC = 5V VCC = 12V Output Voltage (V) Frequency (kHz)1 0.1 Load Current (A) Load Current (A) 250 200 150 100 1.055 VIN = 19V VIN = 12V VIN = 5V 1.050 1.045 50 VCC = 5V, VOUT = 1.05V, R1 = 2.49k, R2 = 7.87k, VNN 0 1.040 0.01 0.1 1 10 0.01 Load Current (A) 0.1 1 10 Load Current (A) Output Voltage vs. Load Current Output Voltage vs. Load Current 1.205 1.365 1.204 Output Voltage (V) Output Voltage (V) 1.203 1.202 1.201 1.200 VIN = 19V 1.199 VIN = 12V 1.198 VIN = 5V 1.363 1.361 VIN = 19V VIN = 12V 1.359 VIN = 5V 1.357 1.197 VCC = 5V, VOUT = 1.35V, R1 = 2.49k, R2 = 3.57k, DDRIII-L VCC = 5V, VOUT = 1.2V, R1 = 1k, R2 = 2k, DDRIV 1.196 1.195 0.01 0.1 1 Load Current (A) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 10 1.355 0.01 0.1 1 10 Load Current (A) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8129A VREF vs. Temperature Output Voltage vs. Load Current 0.804 1.530 1.525 0.802 VREF (V) Output Voltage (V) 0.803 VIN = 19V 1.520 VIN = 12V VIN = 5V VCC = 12V 0.801 VCC = 5V 0.800 0.799 1.515 0.798 VCC = 5V, VOUT = 1.5V, R1 = 1k, R2 = 1.1k, DDRIII VIN = 12V, No Load 0.797 1.510 0.01 0.1 1 -50 10 -25 0 50 75 100 125 Temperature (°C) Load Current (A) Quiescent Current vs. VCC Shutdown Current vs. VCC 2.5 Shutdown Current (mA)1 6 Quiescent Current (mA) 25 5 4 3 2 1 2.0 1.5 1.0 0.5 VIN = 12V, VOUT = 1.2V, DDR IV, No Load VIN = 12V, VOUT = 1.2V, DDR IV, No Load 0.0 0 0 2.5 5 7.5 10 12.5 0 15 7.5 10 Power On from EN Power Off from EN 12.5 15 EN (5V/Div) VOUT VOUT (500mV/Div) (500mV/Div) PHASE (10V/Div) PHASE (10V/Div) PGOOD (10V/Div) VIN = 12V, VCC = 5V, VOUT = 1.2V, No Load Time (1ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 5 VCC (V) EN (5V/Div) PGOOD (10V/Div) 2.5 VCC (V) VIN = 12V, VCC = 5V, VOUT = 1.2V, Load = 100mA Time (5ms/Div) is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Power On from VCC VCC (5V/Div) Power Off from VCC VCC (5V/Div) VOUT VOUT (500mV/Div) (500mV/Div) PHASE (10V/Div) PGOOD (10V/Div) PHASE (10V/Div) PGOOD (10V/Div) VIN = 12V, VOUT = 1.2V, No Load Time (1ms/Div) Time (5ms/Div) Load Transient Response Load Transient Response ILoad (10A/Div) ILoad (10A/Div) VIN = 12V, VCC = 5V VOUT VOUT (30mV/Div) VIN = 12V, VCC = 5V (30mV/Div) PHASE (10V/Div) PHASE (10V/Div) LGATE (10V/Div) LGATE (10V/Div) VOUT = 1.2V, Load = 0.1 to 10A Time (20µs/Div) OVP UVP PGOOD (10V/Div) FB (500mV/Div) (500mV/Div) VOUT PHASE (10V/Div) VIN = 12V, VCC = 5V, VOUT = 1.2V, No Load Time (50µs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 VOUT = 1.2V, Load = 10 to 0.1A Time (20µs/Div) PGOOD (10V/Div) PHASE (10V/Div) LGATE (10V/Div) VIN = 12V, VOUT = 1.2V, Load = 100mA October 2015 LGATE (10V/Div) VIN = 12V, VCC = 5V, VOUT = 1.2V Time (20µs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8129A OCP VIN = 12V, VCC = 5V, VOUT = 1.2V, ROCSET = 13k, ILoad (10A/Div) VOUT (500mV/Div) PHASE (10V/Div) LGATE (10V/Div) RDS,ON(VGS=4.5V) = 7.4m Time (50µs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Application Information The RT8129A is a single-phase synchronous buck current is also reduced, and eventually comes to the PWM controller with integrated drivers which is optimized for high performance graphic microprocessor and computer applications. A COT (Constant-On-Time) PWM controller and two MOSFET drivers with internal bootstrap diodes are integrated so that the external circuit is easily designed and the component count is point that its valley touches zero current, which is the boundary between continuous conduction and discontinuous conduction modes. By emulating the behavior of diodes, the low-side MOSFET allows only partial of negative current when the inductor freewheeling current reach negative level. As the load reduced. current is further decreased, it takes longer and longer to discharge the output capacitor to the level that requires the next “ON” cycle. In reverse, when the output current increases from light load to heavy load, the switching frequency increases to the preset value as the inductor current reaches the continuous condition. The topology solves the poor load transient timing problems of fixed-frequency current-mode PWM and avoids the problems caused by widely varying switching frequencies in conventional constant-on-time and constant off-time PWM schemes. RT8129A also features complete fault protection functions including OVP, UVP and Current Limit. PWM Operation The switching waveforms may appear noisy and asynchronous when light loading causes diode-emulation operation, but this is a normal The RT8129A integrates a Constant-On-Time PWM controller, and the controller provides the PWM signal which relies on the FB voltage comparing with internal reference voltage as shown in Figure 1. Referring to the function block diagram of TON generator, the synchronous UGATE driver will be turned on at the operating condition that results in high light-load efficiency. Trade-offs in DEM noise vs. light-load beginning of each cycle. After the internal one-shot timer expires, the UGATE driver will be turned off. The pulse width of this one shot is determined by the converter's input voltage and the output voltage to keep the frequency fairly constant over the input voltage range. Another one-shot sets a minimum off-time. resistance remains fixed) and less output voltage ripple. The disadvantages for using higher inductor values include larger physical size and degrade load-transient response (especially at low input-voltage levels). efficiency is made by varying the inductor value. Generally, low inductor values produce a broader efficiency vs. load curve, while higher values result in higher full-load efficiency (assuming that the coil Enable and Disable The EN pin allows for power sequencing between the controller bias voltage and another voltage rail. The VFB VPEAK VFB VVALLEY RT8129A remains in shutdown if the EN pin is lower than 800mV. When EN pin rises above the 2V, the RT8129A will begin a new initialization and soft-start cycle. VREF t tON Figure 1. Constant On-Time PWM Control Diode-Emulation Mode In diode-emulation mode, the RT8129A automatically reduces switching frequency at light-load conditions to maintain high efficiency. As the output current decreases from heavy-load condition, the inductor Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 Power-On Reset (POR), UVLO Power-on reset (POR) occurs when VCC rises above to approximately 4.4V (typical), the RT8129A will reset the fault latch and preparing the PWM for operation. Below 4V (typical), the VCC under voltage-lockout (UVLO) circuitry inhibits switching by keeping UGATE and LGATE low. is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8129A VIN Detection Power-Good Output (PGOOD) When VCC exceeds its POR rising threshold, LGATE will be forced low UGATE and UGATE will output The power good output is an open drain architecture, and it requires a pull-up resistor. During soft-start, continuous pulses (~25kHz, 100ns), for input voltage VIN detection. If the PHASE pin voltage exceeds 1V for 3 consecutive cycles when the UGATE is turned on, VIN is recognized as ready. The controller will initiate soft-start operation. PGOOD is actively held low and is allowed to transition high after soft start is completed. In addition, if the FB pin voltage is higher than 110% of VREF or lower than 90% of VREF, PGOOD will go low immediately. Soft-Start The RT8129A provides an internal soft-start function. The RT8129A provides cycle-by-cycle current limit control by detecting the PHASE voltage drop across The soft-start function is used to prevent large inrush current and output voltage overshoot while the the low-side MOSFET when it is turned on. The current limit circuit employs a unique “valley” current sensing converter is being powered-up. The soft-start function automatically begins after the chip is enabled. algorithm. If the magnitude of the current sense signal at PHASE is above the current limit threshold, the PWM is not allowed to initiate a new cycle. When soft-start process starts, an internal current source charges the internal soft-start capacitor such that the internal soft-start voltage ramps up uniformly. The FB voltage will track the internal soft-start voltage during the soft-start interval. The PWM pulse width increases gradually to limit the input current. After the internal soft-start voltage exceeds the reference voltage, the FB voltage no longer tracks the soft-start voltage but rather follows the reference voltage. Therefore, both the duty cycle of the UGATE and the input current are limited during the soft-start interval. If the protection is not triggered during soft-start process, the soft-start process is finished until the signal Internal SSOK goes high, Figure 2 shows the internal soft-start sequence. VCC POR Threshold VCC EN Current Limit In an over-current condition, the current to the load exceeds the average output inductor current. Thus, the output voltage falls and eventually crosses the under-voltage shutdown. 2V Internal SS threshold, inducing IC Current Limit Threshold Setting Current limit threshold is externally programmed by adding a resistor (ROCSET) between LGATE and GND. Once VCC exceeds the POR threshold, an internal current source IOCSET flows through ROCSET. The voltage across ROCSET is stored as the over current protection threshold VOCSET. After that, the current source is switched off. ROCSET can be determined using the following equation : 0.8V FB protection ROCSET IVALLEY RLGDS(ON) IOCSET Where IVALLEY represents the desired inductor limit current (valley inductor current) and IOCSET is current limit setting current. Internal SSOK PGOOD UGATE LGATE POR VIN Detection OCP Programming Soft Start Normal operation Off Diode Emulation with LGATE turns on to Ultrasonic Mode discharge output voltage (Load Current Dependent) if the phase voltage >1V If ROCSET is not present, there is no current path for IOCSET to build the OCP threshold. In this situation, the OCP threshold is internally preset to 640mV. The recommended range for ROCSET is 5k to 60k which means the threshold voltage range is 50mV to 600mV. Figure 2. Soft-Start Sequence Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A Output Over-Voltage Protection (OVP) VOUT The voltage on the FB pin is monitored for over-voltage protection. When the FB voltage exceeds than 1V (typically 125% x VREF), over voltage protection is triggered and low-side MOSFET is forced on. This activates low-side MOSFET to discharge the output capacitor. The RT8129A is latched once OVP is triggered and can only be released by VCC power-on reset. A 5s delay is used in OVP detection circuit to prevent false trigger. Output Under-Voltage Protection (UVP) The voltage on the FB pin is monitored for under voltage protection. When the FB voltage is less than 0.6V (typically 75% x VREF) during normal operation, under voltage protection is triggered and then UGATE and LGATE gate drivers are forced low. The RT8129A is latched once UVP is triggered and can only be released by VCC or EN power-on reset. There is a 3s RFB1 FB RFB2 Figure 4. Setting VOUT with a Resistive Voltage Divider MOSFET Gate Driver The RT8129A integrates high current gate drivers for the MOSFET to obtain high efficiency power conversion in synchronous buck topology. A dead time is used to prevent the crossover conduction for high side and low side MOSFET. Because both the two gate signals are off during the dead time, the inductor current freewheels through the body diode of the low side MOSFET. The freewheeling current and the forward voltage of the body diode contribute to the power loss. The RT8129A employs adaptive dead time The output voltage waveform is shown as Figure 3, control scheme to ensure safe operation without sacrificing efficiency. Furthermore, elaborate logic circuit is implemented to prevent short through conduction. For high output current applications, two or more power MOSFET are usually paralleled to reduce RDS(ON). which can be adjusted from 0.8V to 3.3V by setting the feedback resistors, RFB1 and RFB2 (see Figure 4). The gate driver needs to provide more current to switch on/off these paralleled MOSFET. The gate driver with Choose RFB2 to be approximately 10k and solve for RFB1 using the equation below : lower source/sink current capability result in longer rising/ falling time in gate signals, and therefore higher R VOUT VREF 1 FB1 R FB2 switching loss. The RT8129A embeds high current gate drivers to obtain high efficiency power conversion. where the VREF is 0.8V (typical). Inductor Selection delay built into the UVP circuit to prevent false transitions. During soft-start, the UVP blanking time is equal to PGOOD blanking time. Output Voltage Setting Inductor plays an importance role in step-down converters because the energy from the input power VOUT ΔVOUT VOUT VVALLEY t tON Figure 3. Output Voltage Waveform rail is stored in it and then released to the load. From the viewpoint of efficiency, the dc resistance (DCR) of inductor should be as small as possible to minimize the copper loss. In addition, because inductor cost most of the board space, its size is also important. Low profile inductors can save board space especially when the height has limitation. However, low DCR and low profile inductors are usually cost ineffective. Additionally, larger inductance results in lower ripple current, which means the lower power loss. However, the inductor current rising time increases with Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8129A inductance value. This means the transient response will be slower. Therefore, the inductor design is a trade-off between performance, size and cost. MOSFET Selection In general, inductance is designed such that the ripple current ranges between 20% ~ 40% of full load current. The inductance can be calculated using the following equation. For low-voltage high-current applications, the duty cycle of the high-side MOSFET is small. Therefore, the switching loss of the high-side MOSFET is of concern. Power MOSFETs with lower total gate charge are preferred in such kind of application. LMIN VIN VOUT V OUT FSW k IOUT_rated VIN where k is the ratio between inductor ripple current and rated output current. Input Capacitor Selection The majority of power loss in the step-down power conversion is due to the loss in the power MOSFET. However, the small duty cycle means the low-side MOSFET is on for most of the switching cycle. Therefore, the conduction loss tends to dominate the total power loss of the converter. To improve the overall efficiency, the MOSFET with low RDS(ON) are preferred Voltage rating and current rating are the key parameters in selecting input capacitor. Generally, input capacitor has a voltage rating 1.5 times greater than the maximum input voltage is a conservatively safe design. in the circuit design. In some cases, more than one MOSFET are connected in parallel to further decrease the on-state resistance. However, this depends on the low-side MOSFET driver capability and the budget. The input capacitor is used to supply the input RMS For continuous operation, do not exceed absolute current, which can be approximately calculated using the following equation. maximum junction temperature. The maximum power Thermal Considerations dissipation depends on the thermal resistance of the VOUT V IRMS IOUT 1 OUT VIN VIN IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient The next step is to select proper capacitor for RMS current rating. Use more than one capacitor with low equivalent series resistance (ESR) in parallel to form a capacitor bank is a good design. Besides, placing ceramic capacitor close to the drain of the high-side temperature. The maximum power dissipation can be MOSFET is helpful in reducing the input voltage ripple at heavy load. TA is the ambient temperature, and JA is the junction Output Capacitor Selection For recommended operating condition specifications, The output filter capacitor must have ESR low enough to meet output ripple and load transient requirement, yet have high enough ESR to satisfy stability requirements. Also, the capacitance must be high enough to absorb the inductor energy going from a full load to no load condition without triggering the OVP circuit. Organic semiconductor capacitor(s) or special polymer capacitor(s) are recommended. the maximum junction temperature is 125C. The calculated by the following formula : PD(MAX) = (TJ(MAX) TA) / JA where TJ(MAX) is the maximum junction temperature, to ambient thermal resistance. junction to ambient thermal resistance, JA, is layout dependent. For WDFN-10L 3x3 package, the thermal resistance, JA, is 30.5C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25C can be calculated by the following formula : PD(MAX) = (125C 25C) / (30.5C/W) = 3.27W for WDFN-10L 3x3 package Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015 RT8129A The maximum power dissipation depends on the Layout Consideration operating ambient temperature for fixed TJ(MAX) and thermal resistance, JA. The derating curve in Figure 5 Layout is very important in high frequency switching allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Maximum Power Dissipation (W)1 3.5 converter design. If designed improperly, the PCB could radiate excessive noise and contribute to the converter instability. Certain points must be considered before starting a layout for RT8129A. Four-Layer PCB 3.0 Connect RC low pass filter as close as possible VCC pin. 2.5 Keep current protection setting network as close as 2.0 possible to the IC. Routing of the network should 1.5 avoid coupling to high-voltage switching node. 1.0 Connections from the drivers to the respective gate of the high-side or the low-side MOSFET should be 0.5 as short as possible to reduce stray inductance. 0.0 0 25 50 75 100 125 All sensitive analog traces and components such as Ambient Temperature (°C) FB, EN, PGOOD, and VCC should be placed away Figure 5. Derating Curve of Maximum Power from high-voltage switching nodes such as PHASE, Dissipation LGATE, UGATE, or BOOT nodes to avoid coupling. Use internal layer(s) as ground plane(s) and shield the feedback trace from power traces and components. Power sections should connect directly to ground plane(s) using multiple vias as required for current handling (including the chip power ground connections). Power components should be placed to minimize loops and reduce losses. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8129A-01 October 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 17 RT8129A 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 0.180 0.300 0.007 0.012 D 2.950 3.050 0.116 0.120 D2 2.300 2.650 0.091 0.104 E 2.950 3.050 0.116 0.120 E2 1.500 1.750 0.059 0.069 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 10L DFN 3x3 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 18 is a registered trademark of Richtek Technology Corporation. DS8129A-01 October 2015