® RT8863 Dual-Output, Phase Interleaved Synchronous Step-Down Controller with Tracking Control General Description Features The RT8863 is a high performance switching controller that drives all N-MOSFETs for dual-output synchronous step-down converters. Constant frequency current-mode architecture allows a phase-lockable frequency up to 640kHz. The TRCKx pins provide soft-start tracking function. Multiple RT8863s can be daisy chained in applications requiring more than two voltages to be tracked. The precision 0.8V reference and power good output indicator are compatible with a 7V to 28V input supply range, encompassing all battery chemistries. The RUN pins control their respective channels independently. The PLLIN pin selects among skip cycle mode and continuous current mode. Current foldback limits MOSFET dissipation during short-circuit conditions. Dual, 180 Degree Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise Start-Up Tracking for Both Outputs Constant Frequency Current-Mode Control ±1% Output Voltage Accuracy Foldback Output Current Limiting Wide Input Voltage Range : 7V to 28V Operation Power Good Indicator Adjustable Soft-Start Current Ramping No Reverse Current During Soft-Start Interval Dual N-MOSFET Synchronous Drive Phase Lockable Fixed Frequency 210kHz to 640kHz Output Over-Voltage Protection Small 32-Lead WQFN Package RoHS Compliant and Halogen Free Applications Telecom Infrastructure Server Power Supply Industry Equipment Simplified Application Circuit VIN C2 VIN Q3 RT8863 Q1 RSENSE1 TG1 TG2 SW1 SW2 BG1 BG2 L1 VOUT1 L2 RSENSE2 VOUT2 Q4 C6 Q2 PLLIN PGND R1 R2 SENSE2- SENSE1- Enable R3 R4 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 TRCK2 RUN1 RUN2 R9 SENSE2+ SENSE1+ VOSENSE1 C9 VOSENSE2 TRCK1 ITH2 R8 C13 CSS R10 GND is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8863 Ordering Information Pin Configurations (TOP VIEW) RT8863 SENSE1SENSE1+ ITH1 TRCK1 CLKOUT PGOOD BOOT1 TG1 Package Type QW : WQFN-32L 5x5 (W-Type) Lead Plating System G : Green (Halogen Free and Pb Free) 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. 24 2 23 3 22 4 RT8863GQW : Product Number YMDNN : Date Code 21 GND 5 20 19 6 7 33 18 17 8 9 Marking Information RT8863 GQW YMDNN 1 SW1 VIN INTVCC DRVCC PGND BG1 BG2 SW2 10 11 12 13 14 15 16 NC SENSE2SENSE2+ ITH2 VOSENSE2 RUN2 BOOT2 TG2 32 31 30 29 28 27 26 25 VOSENSE1 PLLFLTR RUN1 NC PLLIN SGND TRCK2 NC WQFN-32L 5x5 Functional Pin Description Pin No. Pin Name 1, 13 VOSENSE1 VOSENSE2 Pin Function 3, 14 RUN1 RUN2 Error Am plifier Feedback Voltage Input. It receives the remotely sensed feedback voltage for each controller from an external resistive divider across the output. Filter Connection for Phase Locked Loop. Alternatively, this pin can be driven with an AC or a DC voltage source to vary the frequency of the internal oscillator. Run Control Inputs. Forcing RUN pins below 1V would shut down the circuitry required for that channel. Forcing the RUN pins over 2V would turn on the IC. 4, 8, 9 NC No Internal Connection. 5 PLLIN External Synchronization Input to Phase Detector. Feeding an external clock signal will synchronize the chip to the external clock. 6 SGND Signal Ground. It is common to both controllers. It must be routed separately from high current grounds to the common () terminals of the output capacitors. 7, 29 TRCK2 TRCK1 Output Voltage Tracking Inputs. An internal 1.2A soft-start current always charges these pins. 10, 32 SENSE2 SENSE1 Negative inputs of the Differential Current Comparators. 2 PLLFLTR Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Pin No. Pin Name 11, 31 SENSE2+ SENSE1+ 12, 30 ITH2, ITH1 15, 26 BOOT2, BOOT1 16, 25 TG2, TG1 17, 24 SW2, SW1 18, 19 BG2, BG1 Pin Function Positive Input of the Differential Current Comparators. The ITHx pin voltage and controlled offsets between the SENSE and SENSE+ pins in conjunction with RSENSEx set the current trip threshold. Error Amplifier Output and Switching Regulator Compensation Point. Each associated channel’s current comparator trip point increases with this control voltage. Bootstrap Supplies for the High-Side Gate Drivers. Capacitors are connected between the BOOTx, and SWx pins and Schottky diodes are tied between the BOOT and INTVCC pins. Voltage swing at the BOOTx pins is from INTVCC to (VIN + INTVCC) . High Current Gate Drives for High-Side N-MOSFETs. These are the output of floating drivers with a voltage swing equal to INTVCC 0.5V superimposed on the switch node voltage SW. Switch Node Connections to Inductors. Voltage swing at these pins is from a Schottky diode (external) voltage drop below ground to VIN. To prev ent the spike happened during switching operation, placed a 10 between SW pin and MOSFET. High Current Gate Drives for Low-Side N-MOSFETs. Voltage swing at these pins is from ground to INTVCC. Power Ground. Connect to internal gate drivers’ ground. It also connects to negativ e terminal of input capacitors and the anodes of the Schottky rectifiers. External Power Input to Gate Drives. It can be connected with INTVCC pin and uses INTVCC as gate drives power supply. Output of the Internal 5V Linear Low Dropout Regulator. The driver and control circuits are powered from this voltage source. It must be decoupled to PGND pin with a minimum 4.7F tantalum or other low ESR capacitor. Main Supply Input. A bypass capacitor of 0.1F should be tied between this pin and the SGND pin. 20 PGND 21 DRVCC 22 INTVCC 23 VIN 27 PGOOD Open-Drain Logic Output of Power Good Indication. 28 CLKOUT Output Clock Signal. Available to daisy chained other controller ICs for additional MOSFET driver stages. GND Power Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 33 (Exposed Pad) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8863 Functional Block Description PHASE DET Duplicate For Second Controller Channel PLLFLTR Oscillator CLKOUT S R INTVCC BOOTx DROP OUT DET CLK1 CLK2 TGx SHDN Q 0.55V + MOSEFT Driver - PLLIN BGx Pll Detector PGND, GND PGOOD Current Sense Monitor Unit VOSENSE1 VOSENSE2 SWx INTVCC + 0.8VREF SENSEx+ Slope Comp SENSEx- DRVCC 5V LDO REG 2.4V VOSENSEx + INTVCC - Internal Supply VIN EA + 0.86V 1.2µA 0.8V TRCKx ITHx SoftStart SGND RUNx Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Operation The RT8863 is a dual, constant frequency, current mode synchronous Buck controller with embedded MOSFET drivers. The two controllers operate with 180 degrees out of phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current monitor unit resets the RS latch. The RT8863 uses an inductor peak current detect architecture. The inductor peak current level is controlled by ITHx pin voltage which is also the output of the error amplifier (EA). The main current monitor unit trips when the peak value of inductor current is sensed. The error amplifier compares the output voltage feedback signal at TRCKx pin to the internal reference voltage (0.8V). When the load current increased, it causes a slight voltage decrease in TRCKx pin. To rebalance the voltage, EA increases the ITHx voltage until the average inductor current matches the new current load. After the top MOSFET is turned off at each cycle, the bottom MOSFET is turned on until either the inductor current starts to reverse, or a new clock cycle is started. MOSFET drivers with another power supply. Put the bypass cap at DRVCC pin in this case to reject noise; and a small 1uF cap at INTVCC for loop compensation. Frequency Selection and Synchronization It is a trade-off between efficiency and component size when select a proper switching frequency. Low frequency operation increases efficiency by reducing MOSFET switching losses, but it requires larger inductance and/or capacitance to maintain low output ripple voltage. Sometimes system requires a particular switching frequency to avoid noise interference at some frequency band. RT8863 provides various switching frequency selection for users. When PLLFLTR pin is biased at different voltages range from 0 to 3V, the internal clock frequency is set from 250KHz to 700KHz. A built-in Phase-lock loop (PLL) can also synchronize the internal oscillator to an external clock source that feeds to PLLIN pin. A loop filter (series R-C) should be connected between PLLFLTR pin and SGND pin. Channels On/Off Control and Current Sharing Mode The two channels are independently controlled by RUNx pin. When both RUNx pin are pulled low, the whole chip is in shutdown mode. When either RUNx pin voltage reaches ~1V, its control loop is enabled. RT8863 can be configured to an accurate current sharing mode: configure its two channels in parallel by tying RUN2 pin to GND and use RUN1 pin to enable both channels. In this configuration, ITHx pins needs to tie together; same for VOSENSEx pins and TRCKx pins. INTVCC and DRVCC as Power Pins RT8863 has an internal high voltage LDO that provides a 5V supply at INTVCC pin that can powers all internal 5V circuitry. DRVCC pin powers all the MOSFETs drivers. When DRVCC pin is connected to INTVCC pin, an external 5V power supply is no longer needed. The whole system can be powered by one input power at VIN. DRVCC pin can also provides flexibility is user want to power the Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8863 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN -----------------------------------------------------------------------------------Top-side Driver Voltage BOOT1, BOOT2 -----------------------------------------------------------------Switch Voltages, SW1, SW2 -------------------------------------------------------------------------------INTVCC ----------------------------------------------------------------------------------------------------------DRVCC, RUN1, RUN2 ---------------------------------------------------------------------------------------SENSE1+, SENSE2+, SENSE1−, SENSE2− ---------------------------------------------------------PLLIN, PLLFLTR, CLKOUT ---------------------------------------------------------------------------------TRCK1, TRCK2, PGODD ------------------------------------------------------------------------------------ITH1, ITH2 -------------------------------------------------------------------------------------------------------Power Dissipation, PD@ TA = 25°C WQFN-32L 5x5 ------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WQFN-32L 5x5, θJA -------------------------------------------------------------------------------------------WQFN-32L 5x5, θJC ------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ----------------------------------------------------------------------------------MM (Machine Model) ------------------------------------------------------------------------------------------ Recommended Operating Conditions −0.3V to 30V −0.3V to 36V −0.3V to 30V −0.3V to 7V −0.3V to (1.1) INTVCC V −0.3V to (1.1) INTVCC V −0.3V to (1.1) INTVCC V −0.3V to (1.1) INTVCC V −0.3V to 2.7V 3.64W 27.5°C/W 6°C/W 260°C 150°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VIN ------------------------------------------------------------------------------------ 7V to 28V Junction Temperature Range --------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range --------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 15V, TA = 25°C, unless otherwise specified) Parameter Regulated Feedback Voltage Feedback Current Reference Voltage Line Regulation Output Voltage Load Regulation Transconductance Amplifier gm Symbol VOSENSE1 VOSENSE2 IOSENSE1 IOSENSE2 Test Conditions VITHx = 1.2V Min Typ Max Unit 0.792 0.8 0.808 V -- -- 1 A -- 0.002 0.02 %/V 0.5 0.1 0.5 % VREFLNREG VIN = 7V to 28V VLOADREG Measured in servo loop, 0.7V VITH 2 gm1, gm2 VITHx = 1.2V, Sink/Source 5A -- 1.3 -- A/V Transconductance Amplifier GBW gmGBW1 gmGBW2 VITHx = 1.2V -- 3 -- MHz Input DC Supply Current IQ Normal Mode VIN = 15V, VRUNx = 3V -- 2 3 mA Shutdown Mode VIN = 15V, VRUNx = 0V -- 1 10 A Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Parameter Symbol Test Conditions Min Typ Max Unit Clock Input Current IPLLIN VPLLIN = 0.85V 5 2 0.5 A Under-Voltage Lockout VUVLO VIN pin ramping down -- 5.6 6 V Feedback Voltage Lockout Sense Pin Total Source Current VOVL Measure VOSENSEx 0.84 0.82 0.88 V ISENSE Each Channel VSENSEx = 0V 90 45 -- A Maximum Duty Factor DMAX 98 99.4 -- % Soft-Start Charge Current ITRCK1 ITRCK2 VTRCKx = 0.2V 2 1 -- A Run Pin ON Threshold VRUN1 VRUN2 VRUNx Rising 1.0 1.5 2.0 V Maximum Current Sense Threshold VSENSEx(MAX) VOSENSEx = 0.7V, VSENSEx(n) = 5V, VPLLIN < 0.5V 57 76 95 mV TG1, TG2 Rising Time tr -- 55 100 TG1, TG2 Falling Time tf -- 55 100 BG1, BG2 Rising Time tr -- 65 100 BG1, BG2 Falling Time tf -- 55 100 Top Gate Off to Bottom Gate On Delay, Synchronous Switch On Delay Time Bottom Gate Off to Top Gate On Delay, Top Switch On Delay Time CLOAD = 3300pF CLOAD = 3300pF ns ns t1D CLOAD = 3300pF at Each Driver -- 60 -- ns t2D CLOAD = 3300pF at Each Driver -- 80 -- ns tON(MIN) Tested with a Square wave -- 120 -- ns Internal VCC Voltage VINTVCC 7V < VIN < 30V 4.8 5 5.5 V INTVCC Load Regulation VLDO ICC = 0 to 20mA -- 0.5 2 % Minimum On-Time INTVCC Linear Regulator Oscillator and Phase Locked Loop Nominal Frequency f NOR VPLLFLTR = 1.7V 400 450 510 kHz Lowest Frequency f LOW VPLLFLTR = 0V 210 250 290 kHz Highest Frequency f HIGH VPLLFLTR 3V 520 580 640 kHz Phase Detector Output Current Sinking Capability IPLLFLTR VPLLFLTR = 1.7V, f PLLIN < f NOR -- 17 -- A Phase Detector Output Current Sourcing Capability IPLLFLTR VPLLFLTR = 1.7V, f PLLIN > f NOR -- 17 -- A PGOOD Voltage Low VPGOOD IPGOOD = 2mA -- 0.1 0.3 V PGOOD Leakage Current IPGOOD VPGOOD = 5V -- -- 1 A PGOOD Output Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8863 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. θJC is measured at the exposed pad 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 © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Typical Application Circuit VIN C1 C2 C3 23 21 VIN DRVCC D1 22 INTVCC 26 BOOT1 BOOT2 15 C5 C4 RSENSE1 Q1 R11 L1 VOUT1 D3 R13 C6 RT8863 25 TG1 Q2 SW2 17 24 SW1 19 BG1 BG2 31 SENSE1+ C7 R6 32 SENSE11 VOSENSE1 7 TRCK2 2 PLLFLTR 30 C11 R5 ITH1 C12 RUN1 14 RUN2 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 Q3 10 Q4 18 RSENSE2 L2 C9 R14 C15 R9 SENSE2+ 11 SENSE2- VOUT2 D4 C8 10 VOSENSE2 13 R10 CLKOUT 28 PGOOD 27 R7 VINTVCC TRCK1 29 3 Enable R12 PLLIN 5 20 PGND C14 C10 TG2 16 10 R1 R2 R3 R4 D2 ITH2 SGND 6 12 R8 C13 CSS GND 33 (Exposed Pad) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8863 Typical Operating Characteristics Efficiency vs. Output Current Efficiency vs. Input Voltage 100 100 IOUT = 3A 90 95 IOUT = 6A 80 Efficiency (%) Efficiency (%) 90 85 80 75 70 70 VIN VIN VIN VIN 60 50 40 30 10 Force Continuous Mode, VOUT = 5V, f = 300kHz 7 10 13 16 19 22 25 Force Continuous Mode, VOUT = 5V, f = 300kHz 0 0.001 60 28 0.01 0.1 1 10 Output Current (A) Input Voltage (V) Supply Current vs. Input Voltage Shutdown Current vs. Input Voltage 3.0 1.0 Shutdown Current (µA)1 2.5 Supply Current (mA) 7V 12V 15V 20V 20 65 2.0 1.5 1.0 0.5 0.0 0.8 0.6 0.4 0.2 0.0 7 10 13 16 19 22 25 28 7 10 13 16 Input Voltage (V) 19 22 25 28 Input Voltage (V) Sense Threshold vs. Input Voltage Sense Threshold vs. Input Voltage 60 89 59 VSENSE1 87 Sense Threshold (mV) Sense Threshold (mV) = = = = 85 83 81 79 VSENSE2 77 VSENSE1 58 57 56 55 54 VSENSE2 53 52 51 VPLLIN = 1.5V VPLLIN = 0V 50 75 7 9 11 13 15 17 19 21 23 25 Input Voltage (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 27 7 9 11 13 15 17 19 21 23 25 27 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Operation Frequency vs. PLLFLTR Voltage Oscillator Frequency vs. Temperature 650 VPLLFLTR = 2.4V 600 Operation Frequency (kHz)1 Oscillator Frequency (kHz)1 700 VPLLFLTR = 1.7V 500 VPLLFLTR = 1.2V 400 300 VPLLFLTR = 0V 200 600 550 500 450 400 350 300 250 200 100 -50 -25 0 25 50 75 100 0 125 1 3 4 PLLFLTR Voltage (V) Temperature (°C) Internal LDO Line Regulation Internal LDO Voltage vs. Temperature 6.0 6.0 5.5 5.5 Internal LDO Voltage (V) Internal LDO Line Regulation (V)1 2 5.0 4.5 4.0 3.5 3.0 5.0 4.5 4.0 3.5 3.0 2.5 2.5 2.0 2.0 VIN = 12V 7 10 13 16 19 22 25 -50 28 -25 0 Input Voltage (V) 25 50 75 100 125 Temperature (°C) UVLO vs. Temperature Internal LDO Load Regulation 5.20 6.0 INTVCC Voltage (V) 5.9 UVLO (V) 5.8 5.7 5.6 5.5 5.16 5.12 5.08 5.04 5.4 VIN = 12V 5.3 5.00 -50 -25 0 25 50 75 100 Temperature (°C) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 125 0 5 10 15 20 25 30 35 40 Output Current (mA) is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8863 Output Voltage vs. Output Current Output Voltage vs. Input Voltage 3.50 3.40 VIN VIN VIN VIN VIN 3.32 = = = = = 3.42 7V 10V 15V 20V 25V Output Voltage (V) Output Voltage (V) 3.36 IOUT IOUT IOUT IOUT IOUT IOUT IOUT 3.28 3.34 = 0.01A = 0.1A = 1A = 3A = 5A = 8A = 10A 3.26 3.18 3.24 VOUT = 3.3V, IOUT = 10mA to 10A VIN = 7V to 28V, VOUT = 3.3V 3.10 3.20 0 2 4 6 8 7 10 11.6 16.2 20.8 25.4 Output Current (A) Input Voltage (V) Power On from RUNx Power Off from RUNx VIN (5V/Div) VOUT (2V/Div) VIN (5V/Div) VOUT (2V/Div) RUNx (5V/Div) RUNx (5V/Div) VIN = 15V, VOUT = 5V, VRUNx = 0V to 3V 30 VIN = 15V, VOUT = 5V, VRUNx = 3V to 0V Time (25ms/Div) Time (10ms/Div) Load Transient Response Switching VOUT (10mV/Div) VOUT (100mV/Div) IL (2A/Div) TGx (20V/Div) IOUT (1A/Div) BGx (5V/Div) VIN = 15V, VOUT = 3.3V, IOUT = 0A to 2A Time (100μs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 VIN = 15V, VOUT = 3.3V, IOUT = 2A Time (1μs/Div) is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 Coincident Tracking (1V/Div) Ratiometric Tracking V OUT1 V OUT1 V OUT2 V OUT2 VIN = 12V, VOUT1 = 5V, VOUT2 = 3.3V Time (2.5ms/Div) (1V/Div) VIN = 12V, VOUT1 = 5V, VOUT2 = 3.3V Time (2.5ms/Div) Internal Soft-Start V OUT1 V OUT2 (1V/Div) VIN = 12V, VOUT1 = 5V, VOUT2 = 3.3V Time (2.5ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8863 Application Information The RT8863 allows users to program how the channel outputs ramp up either as coincidentally or ratiometrically tracking as shown as in Figure 1. The TRCKx pins act as clamps on the channels' reference voltages. VOUT is referenced to the TRCKx voltage when the TRCKx < 0.8V and to the internal precision reference when TRCKx > 0.8V. Output Voltage VOUT1 VOUT1 R1 R3 To TRCKSS2 pin VOUT2 To VOSENSE1 pin R4 R3 To VOSENSE2 pin R4 R2 Figure 2. (a) Coincidentally Tracking Setting VOUT1 VOUT2 VOUT2 R1 R3 To TRCKSS2 pin To VOSENSE1 pin To VOSENSE2 pin R2 R4 Time Figure 1. (a) Coincident Tracking Output Voltage VOUT1 Figure 2. (b) Ratiometrically Tracking Setting Figure 2. Setups for Different Output Voltage Tracking RSENSEx Selection For Output Current VOUT2 RSENSEx is chosen based on the required output current. The current comparator has a maximum threshold of 76mV, yielding a maximum average output current IMAXx. The RSENSEx design could be followed as : Time Figure 1. (b) Ratiometrically Tracking Figure 1. Two Different ways of Output Voltage Tracking RSENSEx = 76mV / IMAXx When using the controller in very low dropout conditions, the maximum output current level will be reduced. Feedback and Compensation To implement the tracking in Figure 1, connect an extra resistive divider to the output of the master channel and connect its midpoint to the slave channel’s TRCKx pin. The ratio of this divider should be selected the same as that of channel 2’s feedback divider (Figure 2). In this tracking mode, the master channel’s output must be set higher than slave channel’s output. To implement the ratiometric tracking in Figure 2b, no extra divider is needed; simply connect one of TRCKx pins to the other. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 The RT8863 allows the output voltage of the DC/DC converter to be adjusted from 0.8V to 85% of VIN supply via an external resistor divider. It will try to maintain the VOSNESEx pin at internal reference voltage (0.8V). VOUT R1 FB R2 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 According to the resistor divider network above, the output voltage is set as : VREF R2 = R1 VOUT VREF The RT8863 is a current-mode controller and requires external compensation to have an accurate output voltage regulation with fast transient response. If the output voltage have been detected over-voltage and made FB voltage exceed 0.85V, the over-voltage protection of RT8863 will be triggered. In this case, the high-side MOSFET is turned off and the bottom MOSFET is turned on until the over-voltage condition is cleared. Operating Frequency The IC uses a constant frequency phase-lockable architecture with the frequency determined by an internal capacitor. This capacitor is charged by a fixed current plus an additional current which is proportional to the voltage applied to the PLLFLTR pin. When PLLIN = 0V, the internal frequency depends on PLLFLTR voltage. The IC has a phase-locked loop comprised of an internal voltage controlled oscillator and phase detector. The frequency range of the voltage controlled oscillator is ±30% around the center frequency. A voltage applied to the PLLFLTR pin of 1.7V corresponds to a frequency of approximately 450kHz. The operating frequency range of the IC is 210kHz to 640kHz. INTVCC Regulator An internal low dropout regulator produces 5V at the INTVCC pin from the VIN supply pin. INTVCC powers the drivers and internal circuitry within the IC. The INTVCC pin regulator can supply a peak current of 50mA and must be bypassed to ground with low ESR type capacitor. A 1μF ceramic capacitor placed directly adjacent to the INTVCC and PGND pins is highly recommended. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between channels. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 RUNx and Soft-Start The RT8863 RUNx pins shut down their respective channels independently. The RT8863 is put in a low quiescent current state (~1μA) if both RUN pin voltages are below 1V. The TRCKx pins are actively pulled to ground in this shutdown state. Once the RUNx pin voltages are above 1.5V, the respective channel of the RT8863 is powered up. The RT8863 has the ability to either softstart by itself with an external soft-start capacitor or tracking the output of the other channel or supply. When the device is configured to soft-start by itself, an external soft-start capacitor should be connected to the TRCKx pins. A softstart current of 1.2μA is to charge the soft-start capacitor CSS. The total soft-start time can be estimated as : tSoft-Start = 0.8V x CSS / 1.2μA The RT8863 is designed such that the TRCKx pins are actively pulled down if the channels are shut down. Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that each controller is capable of turning on the high-side MOSFET. It is determined by internal timing delays and the gate charge required to turn on the high-side MOSFET. Low duty cycle applications may approach this minimum on-time limit. If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. The minimum on-time for each controller is approximately 120ns. Inductor Selection The inductor plays an important role in step-down converters because it stores the energy from the input power rail and then releases the energy to the load. From the viewpoint of efficiency, the dc resistance (DCR) of the inductor should be as small as possible to minimize the conduction loss. In addition, because the inductor takes up a significant portion of the board space, its size is also important. Low profile inductors can save board space especially when there is a height limitation. However, low is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8863 DCR and low profile inductors are usually not cost effective. Additionally, larger inductance results in lower ripple current, which means lower power loss. However, the inductor current rising time increases with inductance value. This means the transient response will be slower. Therefore, the inductor design is a compromise between performance, size and cost. In general, the inductance is designed such that the ripple current ranges between 20% to 40% of the full load current. The inductance can be calculated using the following equation : LMIN = VIN VOUT V OUT fSW k IOUT_rated VIN where k is the inductor ripple current. Input Capacitor Selection Voltage rating and current rating are the key parameters in selecting an input capacitor. For a conservatively safe design, an input capacitor should generally have a voltage rating 1.5 times greater than the maximum input voltage. The input capacitor is used to supply the input RMS current, which is approximately calculated using the following equation : IRMS = IOUT VOUT VOUT 1 VIN VIN The next step is to select a proper capacitor for RMS current rating. Placing more than one capacitor with low Equivalent Series Resistance (ESR) in parallel to form a capacitor bank is a good design. Also, placing ceramic capacitor close to the Drain of the high-side MOSFET is helpful in reducing the input voltage ripple at heavy load. When the chip is used in a hazard environment where long trace line to Input capacitor and output can be hard shorted to ground, a bigger capacitor up to 150μF should be used to reduce the voltage ripple on supply line. 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 that flows into or out of the capacitor results in ripple voltage. The output voltage ripples contains two components, VOUT_ESR and VOUT_C. 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 sag can be calculated using the following equation : VOUT_sag = ESR x ΔIOUT 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). A rapid change in load current results in di/dt during transient. Therefore, ESL contributes to part of the voltage sag. Use a capacitor that has low ESL to obtain better transient performance. Generally, using several capacitors in parallel will have better transient performance than using single capacitor for the same total ESR. Unlike the electrolytic capacitor, the ceramic capacitor has relative low ESR and can reduce the voltage deviation during load transient. However, the ceramic capacitor can only provide low capacitance value. Therefore, use a mixed combination of electrolytic capacitor and ceramic capacitor for better transient performance. MOSFET Selection The majority of power loss in the step-down power conversion is due to the loss in the power MOSFETs. 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 applications. 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, MOSFETs with low RDS(ON) are preferred in 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. VOUT_ESR IL ESR VOUT_C IL 1 8 COUT fSW Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS8863-01 February 2015 RT8863 INTVCC Regulator and DRVCC Thermal Considerations RT8863 features an internal NPN linear regulator that supplies power to INTVCC from Vin supply. The INTVCC regulator regulates INTVCC voltage to 5V and can supply 50mA current. A 1μF capacitor is needed to ensure the regulator is stable. DRVCC pin powers the MOSFETs driver circuitries. It should either connect to INTVCC pin, or an external power supply. This pin must have a 4.7μF tantalum, 10μF special polymer, or low ESR type of electrolytic capacitor. A 1μF ceramic capacitor placed directly adjacent to the DRVCC and GND IC pin is highly recommended. This bypassing capacitor is necessary to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between channels. 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 : The Maximum rating for the INTVCC pin is 50mA. TO prevent maximum junction temperature from being exceeded, the input supply current must be checked operating in continuous mode at maximum Vin. Undervoltage Lockout The RT8863 recommended minimum supply voltage is 7V, where INVVCC will drop below 5V when VIN is below 6V. To protect the controller in case of undervoltage conditions, the RT8863 has another 2 functions implemented. An UVLO comparator always monitor the INTVCC voltage to ensure that an adequate gate driver voltage is present. It locks out the switching actions when INTVCC is below 4.5V. Another way to detect an undervoltage condition is to monitor the VIN supply voltage. When VIN is below 3.5V, the internal reference voltage is pulled low and same for the linear regulator for INTVCC pin. In this case, even when RUNx pin is pulled above 1V (its turn on threshould voltage), the IC is still in turn off mode. where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For WQFN-32L 5x5 packages, the thermal resistance, θJA, is 27.5°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (125°C − 25°C) / (27.5°C/W) = 3.64W for WQFN-32L 5x5 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 3 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 4.0 Maximum Power Dissipation (W)1 For higher input voltage applications where large MOSFETs are being driven at high frequencies, power dissipation need to be checked carefully to avoid the maximum junction temperature rating for the IC to be exceeded. PD(MAX) = (TJ(MAX) − TA) / θJA Four-Layer PCB 3.6 3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curve of Maximum Power Dissipation Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8863-01 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 17 RT8863 Outline Dimension D2 D SEE DETAIL A L 1 E E2 e b 1 2 DETAIL A Pin #1 ID and Tie Bar Mark Options A A1 1 2 A3 Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. 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 4.950 5.050 0.195 0.199 D2 3.400 3.750 0.134 0.148 E 4.950 5.050 0.195 0.199 E2 3.400 3.750 0.134 0.148 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 32L QFN 5x5 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. www.richtek.com 18 DS8863-01 February 2015