PD-97605 IR3863MPBF SupIRBuck TM 6A HIGHLY INTEGRATED WIDE-INPUT VOLTAGE, SYNCHRONOUS BUCK REGULATOR Features Description The IR3863 SupIRBuckTM is an easy-to-use, fully integrated and highly efficient DC/DC voltage regulator. The onboard constant on-time hysteretic controller and MOSFETs make IR3863 a space-efficient solution that delivers up to 6A of precisely controlled output voltage. Input Voltage Range: 3V to 21V Output Voltage Range: 0.5V to 12V Continuous 6A Load Capability Constant On-Time Control Compensation Loop not Required Excellent Efficiency at Very Low Output Currents Programmable Switching Frequency and Soft Start Thermally Compensated Over Current Protection Power Good Output Precision Voltage Reference (0.5V, +/-1%) Enable Input with Voltage Monitoring Capability Pre-bias Start Up Thermal Shut Down Under/Over Voltage Fault Protection Forced Continuous Conduction Mode Option Very Small, Low Profile 4mm x 5mm QFN Package Programmable switching frequency, soft start, and thermally compensated over current protection allows for a very flexible solution suitable for many different applications and an ideal choice for battery powered applications. Additional features include pre-bias startup, very precise 0.5V reference, over/under voltage shut down, power good output, and enable input with voltage monitoring capability. Applications Notebook and Desktop Computers Game Consoles Consumer Electronics – STB, LCD, TV, Printers General Purpose POL DC-DC Converters 8/8/2012 Rev3.2 1 IR3863MPBF ABSOLUTE MAXIMUM RATINGS (Voltages referenced to GND unless otherwise specified) • VIN, FF …………………………………………………. -0.3V to 25V • VCC, PGOOD, EN …………………………………..... -0.3V to 8.0V • BOOT …………………………………………………… -0.3V to 33V • PHASE …………………………………………………. -0.3V to 25V(DC), -5V(100ns) • BOOT to PHASE ………………………………………. -0.3V to 8V • ISET …………………………………………………….. -0.3V to 25V, 30mA • PGND to GND …………………………………………. -0.3V to +0.3V • All other pins …………………………………………… -0.3V to 3.9V • Storage Temperature Range ………………………… -65°C To 150°C • Junction Temperature Range ………………………… -40°C To 150°C • ESD Classification …………………………………….. JEDEC Class 1C • Moisture Sensitivity Level ……………………..……… JEDEC Level 2 @ 260°C (Note 2) Stresses beyond those listed under “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 are not implied. PACKAGE INFORMATION 4mm x 5mm POWER QFN JA 32o C / W J - PCB 2o C / W ORDERING INFORMATION PKG DESIG PACKAGE DESCRIPTION PIN COUNT PARTS PER REEL M IR3863MTRPbF 17 4000 M IR3863MTR1PbF 17 750 8/8/2012 Rev3.2 2 IR3863MPBF Simplified Block Diagram 8/8/2012 Rev3.2 3 IR3863MPBF Pin Description NAME NUMBER FCCM 1 ISET 2 PGOOD 3 GND 4,17 I/O LEVEL 3.3V DESCRIPTION Forced Continuous Conduction Mode (CCM). Ground this pin to enable diode emulation mode or discontinuous conduction mode (DCM). Pull this pin to 3.3V to operate in CCM under all load conditions. Connecting resistor to PHASE pin sets over current trip point. 5V Power good open drain output – pull up with a resistor to 3.3V Reference Bias return and signal reference. FB 5 3.3V Inverting input to PWM comparator, OVP / PGOOD sense. SS 6 3.3V Soft start/shutdown. This pin provides user programmable softstart function. Connect an external capacitor from this pin to GND to set the startup time of the output voltage. The converter can be shutdown by pulling this pin below 0.3V. NC 7 - - 3VCBP 8 3.3V For internal LDO. Bypass with a 1.0µF capacitor to AGND. A resistor in series with the bypass capacitor may be required in single-ground plane designs. Refer to Layout Recommendations for details. NC 9 - - VCC 10 5V VCC input. Gate drive supply. A minimum of 1.0µF ceramic capacitor is required. PGND 11 Reference Power return. PHASE 12 VIN Phase node (or switching node) of MOSFET half bridge. VIN 13 VIN Input voltage for the system. BOOT 14 VIN +VCC Bootstrapped gate drive supply – connect a capacitor to PHASE. FF 15 VIN Input voltage feed forward – sets on-time with a resistor to VIN. EN 16 5V Enable pin to turn on and off the device. Use two external resistors to set the turn on threshold (see Electrical Specifications) for input voltage monitoring. 8/8/2012 Rev3.2 4 IR3863MPBF Recommended Operating Conditions Symbol Definition Min Max Unit V VIN VCC Input Voltage Supply Voltage 3 4.5 21* 5.5 VOUT Output Voltage 0.5 12 IOUT Output Current 0 6 A Fs Switching Frequency N/A 750 kHz TJ Junction Temperature -40 125 oC * PHASE pin must not exceed 25V. Electrical Specifications Unless otherwise specified, these specification apply over VIN = 12V, 4.5V<VCC<5.5V, 0oC ≤ TJ ≤ 125oC. PARAMETER CONTROL LOOP Reference Accuracy, VREF NOTE TEST CONDITION VFB = 0.5V MIN TYP MAX UNIT 0.495 0.5 0.505 V On-Time Accuracy RFF = 180K, TJ = 65oC 280 300 320 ns Min Off Time Soft-Start Current DCM Comparator Offset EN = High Measure at VPHASE 8 -4.5 500 10 -2.5 12 0 ns µA mV SUPPLY CURRENT VCC Supply Current (standby) VCC Supply Current (dynamic) FF Shutdown Current EN = Low, No Switching 23 µA EN = High, Fs = 300kHz 6 mA EN = Low 2 µA FORCED CONTINUOUS CONDUCTION MODE (FCCM) FCCM Start Threshold FCCM Stop Threshold 8/8/2012 Rev3.2 2 V 0.6 V 5 IR3863MPBF Electrical Specifications (continued) Unless otherwise specified, these specification apply over VIN = 12V, 4.5V<VCC<5.5V, 0oC ≤ TJ ≤ 125oC. PARAMETER GATE DRIVE Deadtime NOTE 1 BOOTSTRAP PFET Forward Voltage UPPER MOSFET Static Drain-to-Source OnResistance LOWER MOSFET Static Drain-to-Source OnResistance FAULT PROTECTION ISET Pin Output Current ISET Pin Output Current Temperature Coefficient 1 Under Voltage Threshold Under Voltage Hysteresis Monitor body diode conduction on PHASE pin 1 5 MAX UNIT 30 ns VCC = 5V, ID = 5A, TJ = 25oC 21.1 26 mΩ VCC = 5V, ID = 5A, TJ = 25oC 20.1 25 mΩ 17 19 4400 21 0.37 0.4 0.43 µA ppm/ oC V On the basis of 25oC On the basis of 25oC Rising VFB VCC Turn-off Threshold mV 7.5 mV 0.586 0.625 0.655 V 7.5 mV Falling VFB -40oC to 125oC VCC Turn-on Threshold TYP 300 Rising VFB & Monitor PGOOD 1 MIN I(BOOT) = 10mA Falling VFB & Monitor PGOOD Over Voltage Threshold Over Voltage Hysteresis TEST CONDITION 3.9 4.2 4.5 V 3.6 3.9 4.2 V VCC Threshold Hysteresis 300 -40oC EN Rising Threshold to 125oC 1.1 EN Hysteresis 1.25 mV 1.45 400 EN Input Current EN = 3.3V V mV 15 µA 50 Ω PGOOD Pull Down Resistance 25 PGOOD Delay Threshold (VSS) 1 V 140 oC 20 oC Thermal Shutdown Threshold 1 Thermal Shutdown Threshold Hysteresis 1 125 Note 1: Guaranteed by design, not tested in production Note 2: Upgrade to industrial/MSL2 level applies from date codes 1227 (marking explained on application note AN1132 page 2). Products with prior date code of 1227 are qualified with MSL3 for Consumer Market. 8/8/2012 Rev3.2 6 IR3863MPBF TYPICAL OPERATING DATA Tested with demoboard shown in Figure 7, VIN = 12V, VCC = 5V, VOUT = 1.05V, Fs = 300kHz, T A = 25oC, no airflow, unless otherwise specified 95% 95% 90% 85% 90% 12VIN 16VIN 85% Efficiency Efficiency 80% 7VIN 75% 70% 65% 60% 80% 50% 55% 1 VOUT = 1.05V; L = 2.2uH, 11.2mΩ 65% 60% 0.1 VOUT = 1.5V; L = 3.3uH, 19.9mΩ 70% 55% 45% 0.01 VOUT = 3.3V; L = 4.7uH, 23mΩ 75% 50% 0.01 10 0.1 Load Current (A) Figure 1. Efficiency vs. Load Current for VOUT = 1.05V 10 Figure 2. Efficiency vs. Load Current for VIN = 12V 350 1400 300 1200 250 1000 RFF (kOhm) Switching Frequency (kHz) 1 Load Current (A) 200 150 100 5.0 Vout 4.0 3.0 2.0 1.0 4.5 3.5 2.5 1.5 0.5 800 600 400 50 200 0 0 1 2 3 4 5 0 200 250 300 350 400 450 500 550 600 650 700 750 Switching Frequency (kHz) 6 Load Current (A) Figure 4. RFF vs. Switching Frequency 1.080 1.080 1.075 1.075 Output Voltage (V) Output Voltage (V) Figure 3. Switching Frequency vs. Load Current 16VIN 12VIN 7VIN 1.070 1.065 1.060 1.055 1.050 1.070 1.065 1.060 1.055 1.050 1.045 1.045 0 1 2 3 4 5 Load Current (A) Figure 5. Output Voltage Regulation 8/8/2012 Rev3.2 6 7 8 9 10 11 12 13 14 15 Input Voltage (V) Figure 6. Line Regulation at IOUT = 6A 7 16 IR3863MPBF TYPICAL APPLICATION CIRCUIT +3 .3V V CC T P1 V INS R1 10 K V IN R2 10 K T P2 V IN C1 1u F EN T P4 EN C2 22 uF + C3 68 uF 4 3 FCCM R3 20 0K T P5 P GND C4 0.22u F R4 10 K 5 FB T P13 SS SS 6 7 T P14 +3 .3V 13 V IN 14 15 B OOT 16 P GOOD IR3863 GND1 P HA SE C7 op en C8 op en C9 15 0uF C10 47 uF C11 op en C12 0.1uF R13 op en 12 FB T P10 P GND C24 op en T P24 P GNDS SS NC1 8 C20 0.1uF C6 op en C15 op en C16 op en C17 op en C18 op en C19 op en C26 op en C27 op en P GND 4 T P7 V OUT C13 op en 11 3 V OUT R6 op en ISE T V CC P GOOD T P11 P GOOD V SW U1 IR3 863 FCCM 10 2 3V CBP 1 FF GND EN 17 L1 2.2uH +3 .3V R5 10 K T P23 V OUTS T P6 P GNDS ISE T NC2 V SW 9 1 2 S W1 E N / FCCM +3 .3V 2 T P28 V ID C22 op en 9 8 10 +V out2s -Vo ut2 s 5 +V out1s -Vo ut1 s 4 7 R8 2.55K Q1 op en 1 3 R10 op en T P18 V OLT A GE S E NS E V OUT R9 op en 3 R11 20 +3 .3 V T P25 B 2 T P27 A 1 C23 op en +V i ns V CC +V dd2 s -Vd d2s -Vi ns R7 2.80K +V dd1 s -Vd d1s C14 op en V CC T P17 P GND C25 1u F V IN T P16 V CC R12 4.99 6 C21 1u F T P26 A GND Figure 7. Typical Application Circuit for VOUT = 1.05V, Fs = 300kHz Demoboard Bill of Materials QTY REF DESIGNATOR VALUE 3 C1, C21, C25 1.00uF 1 C10 47uF 2 C12, C20 0.100uF 1 C2 22.0uF 1 C3 68uF 1 C4 0.22uF 1 C9 150uF 1 L1 2.2uH 4 R1, R2, R4, R5 10.0K 1 R11 20 1 R12 4.99 1 R3 200K 1 R7 2.80K 1 R8 2.55K 1 SW1 SPST 1 U1 IR3863 8/8/2012 Rev3.2 DESCRIPTION capacitor, X7R, 1.00uF, 25V, 0.1, 0603 capacitor, 47uF, 6.3V, 805 capacitor, X7R, 0.100uF, 25V, 0.1, 603 capacitor, X5R, 22.0uF, 16V, 20%, 1206 capacitor, electrolytic, 68uF, 25V, 0.2, SMD capacitor, X5R, 0.22uF, 10V, 0.1, 0603 capacitor, tantalum polymer, 150uF, 6.3V, 20%, 7343 inductor, ferrite, 2.2uH, 8.0A, 11.2mOhm, SMT resistor, thick film, 10.0K, 1/10W, 0.01, 0603 resistor, thick film, 20, 1/10W, 0.01, 603 resistor, thick film, 4.99, 1/8W, 0.01, 603 resistor, thick film, 200K, 1/10W, 0.01, 603 resistor, thick film, 2.80K, 1/10W, 0.01, 603 resistor, thick film, 2.55K, 1/10W, 0.01, 0603 switch, DIP, SPST, 2 position, SMT 4mm X 5mm QFN MANUFACTURER PART NUMBER Murata GRM188R71E105KA12D TDK C2012X5R0J476M TDK C1608X7R1E104K Taiyo Yuden EMK316BJ226ML-T Panasonic EEV-FK1E680P TDK C1608X5R1A224K Sanyo 6TPC150M Cyntec PCMB065T-2R2MS KOA RK73H1J1002F KOA RK73H1JLTD20R0F KOA RK73H1J4R99F KOA RK73H1JLTD2003F KOA RK73H1JLTD2801F KOA RK73H1JLTD2551F C&K Components SD02H0SK IRF IR3863MTRPBF 8 IR3863MPBF TYPICAL OPERATING DATA Tested with demoboard shown in Figure 7, VIN = 12V, VCC = 5V, VOUT = 1.05V, Fs = 300kHz, T A = 25oC, no airflow, unless otherwise specified EN EN PGOOD PGOOD SS SS VOUT VOUT 5V/div 5V/div 1V/div 500mV/div 5ms/div 5V/div 5V/div 1V/div 500mV/div 1ms/div Figure 9: Shutdown Figure 8: Startup VOUT VOUT PHASE PHASE iL iL 20mV/div 10V/div 500mA/div 5µs/div Figure 10: DCM (IOUT = 0.1A) 20mV/div 10V/div 2A/div 2µs/div Figure 11: CCM (IOUT = 5A) PGOOD PGOOD VOUT FB SS VOUT iL IOUT 5V/div 1V/div 1V/div 5A/div 500us/div Figure 12: Over Current Protection (tested by shorting VOUT to PGND) 8/8/2012 Rev3.2 5V/div 1V/div 500mV/div 2A/div 50µs/div Figure 13: Over Voltage Protection (tested by shorting FB to VOUT) 9 IR3863MPBF TYPICAL OPERATING DATA Tested with demoboard shown in Figure 7, VIN = 12V, VCC = 5V, VOUT = 1.05V, Fs = 300kHz, T A = 25oC, no airflow, unless otherwise specified VOUT VOUT PHASE PHASE iL iL 50mV/div 10V/div 2A/div 50µs/div Figure 14: Load Transient 0-2A 50mV/div 10V/div 2A/div 50µs/div Figure 15: Load Transient 3-5A FCCM FCCM PHASE PHASE VOUT VOUT iL iL 5V/div 10V/div 500mV/div 5A/div 10µs/div 2V/div 10V/div 500mV/div 5A/div 5µs/div Figure 16: DCM/FCCM Transition Figure 17: FCCM/DCM Transition Figure 18: Thermal Image at VIN = 12V, IOUT = 6A (IR3863: 60oC, Inductor: 45oC, PCB: 32oC) Figure 19: Thermal Image at VIN = 16V, IOUT = 6A (IR3863: 61oC, Inductor: 46oC, PCB: 33oC) 8/8/2012 Rev3.2 10 IR3863MPBF CIRCUIT DESCRIPTION PWM COMPARATOR The PWM comparator initiates a SET signal (PWM pulse) when the FB pin falls below the reference (VREF) or the soft start (SS) voltage. ON-TIME GENERATOR The PWM on-time duration is programmed with an external resistor (RFF) from the input supply (VIN) to the FF pin. The simplified equation for RFF is shown in equation 1. The FF pin is held to an internal reference after EN goes HIGH. A copy of the current in RFF charges a timing capacitor, which sets the on-time duration, as shown in equation 2. RFF VOUT 1V 20 pF FSW (1) TON RFF 1V 20 pF VIN (2) CONTROL LOGIC The control logic monitors input power sources, sequences the converter through the soft-start and protective modes, and initiates an internal RUN signal when all conditions are met. PGOOD The PGOOD pin is open drain and it needs to be externally pulled high. High state indicates that output is in regulation. The PGOOD logic monitors EN_DELAY, SS_DELAY, and under/over voltage fault signals. PGOOD is released only when EN_DELAY and SS_DELAY = HIGH and output voltage is within the OV and UV thresholds. PRE-BIAS STARTUP IR3863 is able to start up into pre-charged output, which prevents oscillation and disturbances of the output voltage. With constant on-time control, the output voltage is compared with the soft start voltage (SS) or Vref, depending on which one is lower, and will not start switching unless the output voltage drops below the reference. This scheme prevents discharge of a pre-biased output voltage. SHUTDOWN The IR3863 will shutdown if VCC is below its UVLO limit. The IR3863 can be shutdown by pulling the EN pin below its lower threshold. Alternatively, the output can be shutdown by pulling the soft start pin below 0.3V. VCC and 3VCBP pins are continuously monitored, and the IR3863 will be disabled if the voltage of either pin drops below the falling thresholds. EN_DELAY will become HIGH when VCC and 3VCBP are in the normal operating range and the EN pin = HIGH. SOFT START With EN = HIGH, an internal 10µA current source charges the external capacitor (CSS) on the SS pin to set the output voltage slew rate during the soft start interval. The soft start time (tSS) can be calculated from equation 3. t SS CSS 0.5V 10A (3) The feedback voltage tracks the SS pin until SS reaches the 0.5V reference voltage (Vref), then feedback is regulated to Vref. CSS will continue to be charged, and when SS pin reaches VSS (see Electrical Specification), SS_DELAY goes HIGH. With EN_DELAY = LOW, the capacitor voltage and SS pin is held to the FB pin voltage. A normal startup sequence is shown in Figure 20. 8/8/2012 Rev3.2 Figure 20. Normal Startup 11 IR3863MPBF CIRCUIT DESCRIPTION UNDER/OVER VOLTAGE MONITOR The IR3863 monitors the voltage at the FB node through a 350ns filter. If the FB voltage is below the under voltage threshold, UV# is set to LOW holding PGOOD to be LOW. If the FB voltage is above the over voltage threshold, OV# is set to LOW, the shutdown signal (SD) is set to HIGH, MOSFET gates are turned off, and PGOOD signal is pulled low. Toggling VCC or EN will allow the next start up. Figure 21 and 22 show PGOOD status change when UV/OV is detected. The over voltage and under voltage thresholds can be found in the Electrical Specification section. OVER CURRENT MONITOR The over-current circuitry monitors the output current during each switching cycle. The voltage across the lower MOSFET, VPHASE, is monitored for over current and zero crossing. The OCP circuit evaluates VPHASE for an over current condition typically 270ns after the lower MOSFET is gated on. This delay functions to filter out switching noise. The minimum lower gate interval allows time to sample VPHASE. The over current trip point is programmed with a resistor from the ISET pin to PHASE pin, as shown in equation 4. When over current is detected, the MOSFET gates are tri-state and SS voltage is pulled to 0V. This initiates a new soft start cycle. If there is a total of four OC events, the IR3863 will disable switching. Toggling VCC or EN will allow the next start up. RSET RDSON IOC 19 A (4) * typical filter delay Figure 21. Under/Over Voltage Monitor Figure 23. Over Current Protection * typical filter delay UNDER VOLTAGE LOCK-OUT The IR3863 has VCC and EN under voltage lockout (UVLO) protection. When either VCC or EN is below their UVLO threshold, IR3863 is disabled. IR3863 will restart when both VCC and EN are above their UVLO thresholds. Figure 22. Over Voltage Protection 8/8/2012 Rev3.2 12 IR3863MPBF CIRCUIT DESCRIPTION OVER TEMPERATURE PROTECTION When the IR3863 exceeds its over temperature threshold, the MOSFET gates are tri-state and PGOOD is pulled low. Switching resumes once temperature drops below the over temperature hysteresis level. GATE DRIVE LOGIC The gate drive logic features adaptive dead time, diode emulation, and a minimum lower gate interval. An adaptive dead time prevents the simultaneous conduction of the upper and lower MOSFETs. The lower gate voltage must be below approximately 1V after PWM goes HIGH before the upper MOSFET can be gated on. Also, the differential voltage between the upper gate and PHASE must be below approximately 1V after PWM goes LOW before the lower MOSFET can be gated on. The upper MOSFET is gated on after the adaptive delay for PWM = HIGH and the lower MOSFET is gated on after the adaptive delay for PWM = LOW. When FCCM = LOW, the lower MOSFET is driven ‘off’ when the ZCROSS signal indicates that the inductor current is about to reverse direction. The ZCROSS comparator monitors the PHASE voltage to determine when to turn off the lower MOSFET. The lower MOSFET stays ‘off’ until the next PWM falling edge. When the lower peak of the inductor current is above zero, IR3863 operates in continuous conduction mode. The continuous conduction mode can also be selected for all load current levels by pulling FCCM to HIGH. Whenever the upper MOSFET is turned ‘off’, it stays ‘off’ for the Min Off Time denoted in the Electrical Specifications. This minimum duration allows time to recharge the bootstrap capacitor and allows the over current monitor to sample the PHASE voltage. COMPONENT SELECTION Selection of components for the converter is an iterative process which involves meeting the specifications and tradeoffs between performance and cost. The following sections will guide one through the process. Inductor Selection Inductor selection involves meeting the steady state output ripple requirement, minimizing the switching loss of the upper MOSFET, meeting transient response specifications and minimizing the output capacitance. The output voltage includes a DC voltage and a small AC ripple component due to the low pass filter which has incomplete attenuation of the switching harmonics. Neglecting the inductance in series with the output capacitor, the magnitude of the AC voltage ripple is determined by the total inductor ripple current flowing through the total equivalent series resistance (ESR) of the output capacitor bank. ΔI One can use equation 5 to find the required inductance. ΔI is defined as shown in Figure 24. The main advantage of small inductance is increased inductor current slew rate during a load transient, which leads to a smaller output capacitance requirement as discussed in the Output Capacitor Selection section. The draw back of using smaller inductances is increased switching power loss in the upper MOSFET, which reduces the system efficiency and increases the thermal dissipation. Input Current IOUT ΔI TS Figure 24. Typical Input Current Waveform TON VIN VOUT (5) 2L 8/8/2012 Rev3.2 13 IR3863MPBF COMPONENT SELECTION Input Capacitor Selection The main function of the input capacitor bank is to provide the input ripple current and fast slew rate current during the load current step up. The input capacitor bank must have adequate ripple current carrying capability to handle the total RMS current. Figure 24 shows a typical input current. Equation 6 shows the RMS input current. The RMS input current contains the DC load current and the inductor ripple current. As shown in equation 5, the inductor ripple current is unrelated to the load current. The maximum RMS input current occurs at the maximum output current. The maximum power dissipation in the input capacitor equals the square of the maximum RMS input current times the input capacitor’s total ESR. Ts 1 IIN_RMS f 2 t dt Ts 0 2 1 ΔI IOUT TON Fs 1 (6) 3 IOUT The voltage rating of the input capacitor needs to be greater than the maximum input voltage because of high frequency ringing at the phase node. The typical percentage is 25%. Output Capacitor Selection Selection of the output capacitor requires meeting voltage overshoot requirements during load removal, and meeting steady state output ripple voltage requirements. The output capacitor is the most expensive converter component and increases the overall system cost. The output capacitor decoupling in the converter typically includes the low frequency capacitor, such as Specialty Polymer Aluminum, and mid frequency ceramic capacitors. The first purpose of output capacitors is to provide current when the load demand exceeds the inductor current, as shown in Figure 25. Equation 7 shows the charge requirement for a certain load step. The advantage provided by the IR3863 at a load step is the reduced delay compared to a fixed frequency control method. If the load increases right after the PWM signal goes low, the longest delay will be equal to the minimum lower gate on-time as shown in the Electrical Specification table. The IR3863 also reduces the inductor current slew time, the time it takes for the inductor current to reach equality with the output current, by increasing the switching frequency up to 1/(TON + Min Off Time). This results in reduced recovery time. Load Current I STEP Output Charge Inductor Slew Rate t Δt Figure 25. Charge Requirement during Load Step Q C V 0.5 ISTEP t (7a) 1 L ISTEP 2 COUT (7b) VDROP 2 VIN VOUT 1 8/8/2012 Rev3.2 14 IR3863MPBF COMPONENT SELECTION The output voltage drop, VDROP, initially depends on the characteristic of the output capacitor. VDROP is the sum of the equivalent series inductance (ESL) of the output capacitor times the rate of change of the output current and the ESR times the change of the output current. VESR is usually much greater than VESL. The IR3863 requires a total ESR such that the ripple voltage at the FB pin is greater than 7mV. The second purpose of the output capacitor is to minimize the overshoot of the output voltage when the load decreases as shown in Figure 26. By using the law of energy before and after the load removal, equation 8 shows the output capacitance requirement for a load step down. COUT L ISTEP 2 (8) VOS 2 VOUT 2 VOS VOUT VL VDROP VESR IOUT Boot Capacitor Selection The boot capacitor starts the cycle fully charged to a voltage of VB(0). Cg equals 0.6nF in IR3863. Choose a sufficiently small ΔV such that VB(0)-ΔV exceeds the maximum gate threshold voltage to turn on the upper MOSFET. V (0) CBOOT Cg B 1 (9) ΔV Choose a boot capacitor value larger than the calculated CBOOT in equation 9. Equation 9 is based on charge balance at CCM operation. Usually the boot capacitor will be discharged to a much lower voltage when the circuit is operating in DCM mode at light load, due to much longer lower MOSFET off time and the bias current drawn by the IC. Boot capacitance needs to be increased if insufficient turn-on of the upper MOSFET is observed at light load, typically larger than 0.1µF is needed. The voltage rating of this part needs to be larger than VB(0) plus the desired derating voltage. Its ESR and ESL needs to be low in order to allow it to deliver the large current and di/dt’s which drive MOSFETs most efficiently. In support of these requirements a ceramic capacitor should be chosen. ISTEP Figure 26. Typical Output Voltage Response Waveform 8/8/2012 Rev3.2 15 IR3863MPBF However, if space allows, PIMB104T-3R3MS39 (11mm x 10mm) with 10.8mΩ DCR will provide higher efficiency. Ripple current needs to be recalculated using the chosen inductor. DESIGN EXAMPLE Design Criteria: Input Voltage, VIN, = 7V to 16V Output Voltage, VOUT = 1.5V Switching Frequency, Fs = 300kHz Inductor Ripple Current, 2ΔI = 1.5A Maximum Output Current, IOUT = 6A Over Current Trip, IOC = 9A Overshoot Allowance for 3A Load Step Down, VOS = VOUT + 50mV Undershoot Allowance for 3A Load Step Up, VDROP = 50mV Find RFF : RFF 1.5V 250 k 1V 20 pF 300kHz Pick a standard value 255 kΩ, 1% resistor. Find RSET : RSET 20.1m 9 A 19A 9.5k 2ΔI 1.5V 16V - 1.5V 1.37 A 16V 3.3H 300kHz Choose an input capacitor: 2 1.5V 1 1.37 A / 2 IIN_RMS 6 A 1 1.8 A 16V 3 6A A Panasonic 10µF (ECJ3YB1E106M) accommodates 6 Arms of ripple current at 300kHz. Due to the chemistry of multilayer ceramic capacitors, the capacitance varies over temperature and operating voltage, both AC and DC. One 10µF capacitor is recommended. In a practical solution, one 1µF capacitor is required along with 10µF. The purpose of the 1µF capacitor is to suppress the switching noise and deliver high frequency current. Pick a 9.53kΩ, 1% standard resistor. Choose an output capacitor: Find a resistive voltage divider for VOUT = 1.5V: To meet the undershoot and overshoot specification, equation 7b and 8 will be used to calculate the minimum output capacitance. As a result, 130µF will be needed for 3A load removal. To meet the stability requirement, choose an output capacitors with ESR larger than 16mΩ. Combine those two requirements, one can choose a set of output capacitors from manufactures such as SP-Cap (Specialty Polymer Capacitor) from Panasonic or POSCAP from Sanyo. A 150µF (4TPE150M) from Sanyo with 25mΩ ESR will meet both requirements. VFB R2 VOUT 0.5V R2 R1 R2 = 1.40kΩ, R1 = 2.80 kΩ, both 1% standard resistors. Choose the soft start capacitor: Once the soft start time has chosen, such as 1000us to reach to the reference voltage, a 22nF for CSS is used to meet 1000µs. Choose an inductor to meet the design specification: VOUT VIN VOUT VIN 2ΔI Fs 1.5V 16V - 1.5V 16V 1.5 A 300kHz 3.0H L Choose an inductor with low DCR and AC power loss to increase the overall system efficiency based on allowed physical size requirement. For instance, PCMB065T-3R3MS from CYNTEC is a 7mm x 6.6mm inductor with 19.9mΩ DCR. If an all ceramic output capacitor solution is desired, the external slope injection circuit composed of R6, C13, and C14 is required as explained in the Stability Consideration Section. In this design example, we can choose C14 = 1nF and C13 = 100nF. To calculate the value of R6 with PCMB065T-3R3MS as our inductor: R6 L DCR C13 3.3H 19.9m 100nF 1.66k Pick a standard value for R6 = 1.65kΩ. 8/8/2012 Rev3.2 16 IR3863MPBF STABILITY CONSIDERATIONS LAYOUT RECOMMENDATIONS Constant-on-time control is a fast , ripple based control scheme. Unstable operation can occur if certain conditions are not met. The system instability is usually caused by: Bypass Capacitor: As VCC bypass capacitor, a 1µF high quality ceramic capacitor should be placed on the same side as the IR3863 and connected to VCC and PGND pins directly. A 1µF ceramic capacitor should be connected from 3VCBP to AGND to avoid noise coupling into controller circuits. For single-ground designs, a resistor (R12) in the range of 5 to 10Ω in series with the 1µF capacitor as shown in Figure 7 is recommended. • Switching noise coupled to FB input: This causes the PWM comparator to trigger prematurely after the 400ns minimum on-time for lower MOSFET. It will result in double or multiple pulses every switching cycle instead of the expected single pulse. Double pulsing can causes higher output voltage ripple, but in most application it will not affect operation. This can usually be prevented by careful layout of the ground plane and the FB sensing trace. • Steady state ripple on FB pin being too small: The PWM comparator in IR3863 requires minimum 7mVp-p ripple voltage to operate stably. Not enough ripple will result in similar double pulsing issue described above. Solving this may require using output capacitors with higher ESR. • ESR loop instability: The stability criteria of constant on-time is: ESR*Cout>Ton/2. If ESR is too small that this criteria is violated then sub-harmonic oscillation will occur. This is similar to the instability problem of peak-current-mode control with D>0.5. Increasing ESR is the most effective way to stabilize the system, but the tradeoff is the larger output voltage ripple. • System with all ceramic output capacitors: For applications with all ceramic output capacitors, the ESR is usually too small to meet the stability criteria. In these applications, external slope compensation is necessary to make the loop stable. The ramp injection circuit, composed of R6, C13, and C14, shown in Figure 7 is required. The inductor current ripple sensed by R6 and C13 is AC coupled to the FB pin through C14. C14 is usually chosen between 1 to 10nF, and C13 between 10 to 100nF. R6 should then be chosen such that L/DCR = C13*R6. Boot Circuit: CBOOT should be placed near the BOOT and PHASE pins to reduce the impedance when the upper MOSFET turns on. Power Stage: Figure 27 shows the current paths and their directions for the on and off periods. The on time path has low average DC current and high AC current. Therefore, it is recommended to place the input ceramic capacitor, upper, and lower MOSFET in a tight loop as shown in Figure 27. The purpose of the tight loop from the input ceramic capacitor is to suppress the high frequency (10MHz range) switching noise and reduce Electromagnetic Interference (EMI). If this path has high inductance, the circuit will cause voltage spikes and ringing, and increase the switching loss. The off time path has low AC and high average DC current. Therefore, it should be laid out with a tight loop and wide trace at both ends of the inductor. Lowering the loop resistance reduces the power loss. The typical resistance value of 1-ounce copper thickness is 0.5mΩ per square inch. Q1 Q2 Figure 27. Current Path of Power Stage 8/8/2012 Rev3.2 17 IR3863MPBF PCB Metal and Components Placement Lead lands (the 13 IC pins) width should be equal to nominal part lead width. The minimum lead to lead spacing should be ≥ 0.2mm to minimize shorting. Lead land length should be equal to maximum part lead length + 0.3 mm outboard extension. The outboard extension ensures a large toe fillet that can be easily inspected. Pad lands (the 4 big pads) length and width should be equal to maximum part pad length and width. However, the minimum metal to metal spacing should be no less than 0.17mm for 2 oz. Copper, or no less than 0.1mm for 1 oz. Copper, or no less than 0.23mm for 3 oz. Copper. 8/8/2012 Rev3.2 18 IR3863MPBF Solder Resist It is recommended that the lead lands are Non Solder Mask Defined (NSMD). The solder resist should be pulled away from the metal lead lands by a minimum of 0.025mm to ensure NSMD pads. The land pad should be Solder Mask Defined (SMD), with a minimum overlap of the solder resist onto the copper of 0.05mm to accommodate solder resist misalignment. Ensure that the solder resist in between the lead lands and the pad land is ≥ 0.15mm due to the high aspect ratio of the solder resist strip separating the lead lands from the pad land. 8/8/2012 Rev3.2 19 IR3863MPBF Stencil Design The Stencil apertures for the lead lands should be approximately 80% of the area of the lead lads. Reducing the amount of solder deposited will minimize the occurrences of lead shorts. If too much solder is deposited on the center pad the part will float and the lead lands will open. The maximum length and width of the land pad stencil aperture should be equal to the solder resist opening minus an annular 0.2mm pull back in order to decrease the risk of shorting the center land to the lead lands when the part is pushed into the solder paste. 8/8/2012 Rev3.2 20 IR3863MPBF IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105 TAC Fax: (310) 252-7903 This product has been designed and qualified for the Industrial Market (Note 2) Visit us at www.irf.com for sales contact information Data and specifications subject to change without notice. 03/12 8/8/2012 Rev3.2 21