Order Now Product Folder Support & Community Tools & Software Technical Documents Reference Design LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 LM73605/LM73606 3.5-V to 36-V, 5-A or 6-A Synchronous Step-Down Voltage Converter 1 Features 2 Applications • • • • • • 1 • • • • • • • • • • • • • • Synchronous Rectification Wettable Flanks QFN Package (WQFN) Low Quiescent Current – 0.8 µA in Shutdown (typical) – 15 µA in Active Mode with No Load (typical) Wide Voltage Conversion Range: – tON_MIN = 60 ns (typical) – tOFF_MIN = 70 ns (typical) Low MOSFET ON-Resistance: – RDS_ON_HS = 53 mΩ (typical) – RDS_ON_LS = 31 mΩ (typical) External Bias Input to Improve Efficiency Pin-Selectable Auto Mode or Forced PWM Operation Adjustable Frequency Range: 350 kHz to 2.2 MHz Synchronizable to External Clock Internal Compensation Power-Good Flag Precision Enable to Program System UVLO Flexible Soft-Start Features: – Start-up into Pre-Biased Load – Fixed or Adjustable Soft-Start Time – Output Voltage Tracking Cycle-by-Cycle Current Limiting Short-Circuit Protection With Hiccup Mode Thermal Shutdown Protection Create a Custom Design With the WEBENCH® Power Designer using LM73605 or LM73606 Industrial Distributed Power Applications Telecommunications Systems General-Purpose Wide VIN Regulation 3 Description The LM73605/LM73606 family of devices are easyto-use synchronous step-down DC-DC converters capable of driving up to 5 A (LM73605) or 6 A (LM73606) of load current from a supply voltage ranging from 3.5 V to 36 V. The LM73605/LM73606 provide exceptional efficiency and output accuracy in a very small solution size. Peak current-mode control is employed. Additional features such as adjustable switching frequency, synchronization to an external clock, FPWM option, power-good flag, precision enable, adjustable soft start, and tracking provide both flexible and easy-to-use solutions for a wide range of applications. Automatic frequency foldback at light load and optional external bias improve efficiency over the entire load range. The family requires few external components and has a pinout designed for simple PCB layout with optimal EMI and thermal performance. Protection features include thermal shutdown, input undervoltage lockout, cycleby-cycle current limiting, and hiccup short-circuit protection. The LM73605 and LM73606 devices are pin-to-pin compatible for easy current scaling. Device Information(1) PART NUMBER LM73605 LM73606 L PVIN CIN BODY SIZE (NOM) 6.00 mm × 4.00 mm (1) For all available packages, see the orderable addendum at the end of the data sheet. Simplified Schematic VIN PACKAGE WQFN (30) Wettable Flanks Efficiency vs Load Current VOUT = 5 V, fSW = 500 kHz, Auto Mode VOUT SW 100 CBOOT 95 COUT EN 90 CBOOT 85 PGOOD BIAS SS/TRK RT SYNC/ MODE FB RFBT Efficiency (%) PGND 80 75 70 65 60 RFBB VCC CVCC AGND VIN = 12 V VIN = 24 V 55 50 0.001 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 1 2 3 4 56 Eff_ Copyright © 2017, Texas Instruments Incorporated 1 An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications, intellectual property matters and other important disclaimers. PRODUCTION DATA. LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Table of Contents 1 2 3 4 5 6 7 Features .................................................................. Applications ........................................................... Description ............................................................. Revision History..................................................... Pin Configuration and Functions ......................... Specifications......................................................... 1 1 1 2 3 5 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 5 5 5 6 6 7 8 8 9 Absolute Maximum Ratings ...................................... ESD Ratings.............................................................. Recommended Operating Conditions....................... Thermal Information .................................................. Electrical Characteristics........................................... Timing Characteristics............................................... Switching Characteristics .......................................... System Characteristics ............................................. Typical Characteristics .............................................. 7.4 Device Functional Modes........................................ 23 8 Application and Implementation ........................ 25 8.1 Application Information............................................ 25 8.2 Typical Application ................................................. 25 9 Power Supply Recommendations...................... 39 10 Layout................................................................... 39 10.1 Layout Guidelines ................................................. 39 10.2 Layout Example .................................................... 42 11 Device and Documentation Support ................. 43 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 Detailed Description ............................................ 11 7.1 Overview ................................................................. 11 7.2 Functional Block Diagram ....................................... 11 7.3 Feature Description................................................. 12 Device Support...................................................... Related Documentation ....................................... Related Links ........................................................ Receiving Notification of Documentation Updates Community Resources.......................................... Trademarks ........................................................... Electrostatic Discharge Caution ............................ Glossary ................................................................ 43 43 43 43 44 44 44 44 12 Mechanical, Packaging, and Orderable Information ........................................................... 45 4 Revision History 2 DATE REVISION NOTES September 2017 * Initial release Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 5 Pin Configuration and Functions RNP Package 30-Pin Wettable Flanks QFN (WQFN) 6 mm × 4 mm × 0.8 mm Top View NC NC NC NC 30 29 28 27 SW 1 26 PGND SW 2 25 PGND SW 3 24 PGND SW 4 23 PGND SW 5 22 PVIN CBOOT 6 21 PVIN VCC 7 20 PVIN BIAS 8 19 AGND RT 9 18 EN SS/TRK 10 17 SYNC/ MODE FB 11 16 PGOOD DAP 12 13 14 15 NC NC NC NC Pin Functions PIN I/O (1) DESCRIPTION SW P Switching output of the regulator. Internally connected to source of the HS FET and drain of the LS FET. Connect to power inductor and boot-strap capacitor. 6 CBOOT P Boot-strap capacitor connection for HS FET driver. Connect a high-quality 470-nF capacitor from this pin to the SW pin. 7 VCC P Output of internal bias supply. Used as supply to internal control circuits and drivers. Connect a high-quality 2.2-µF capacitor from this pin to GND. TI does not recommend loading this pin by external circuitry. 8 BIAS P Optional BIAS LDO supply input. TI recommends tying to VOUT when 3.3 V ≤ VOUT ≤ 18 V, or tie to an external 3.3-V or 5-V rail if available, to improve efficiency. BIAS pin voltage must not be greater than VIN. Tie to ground when not in use. 9 RT A Switching frequency setting pin. Place a resistor from this pin to ground to set the switching frequency. If floating, the default switching frequency will be 500 kHz. Do not short to ground. NO. NAME 1, 2, 3, 4, 5 10 SS/TRK A Soft-start control pin. Leave this pin floating for a 5-ms internal soft-start ramp. An external capacitor can be connected from this pin to ground to extend the soft start time. A 2-µA current sourced from this pin charges the capacitor to provide the ramp. Connect to external ramp for tracking. Do not short to ground. 11 FB I Feedback input for output voltage regulation. Connect a resistor divider to set the output voltage. Never short this pin to ground during operation. 12–15, 27–30 NC — No internal connection. Connect to ground net and copper to improve heat sinking and board-level reliability. PGOOD O Open drain power-good flag output. Connect to suitable voltage supply through a current limiting resistor. High = VOUT regulation OK, Low = VOUT regulation fault. PGOOD = LOW when EN = low and VIN > 2 V. 16 (1) A = Analog, O = Output, I = Input, G = Ground, P = Power Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 3 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Pin Functions (continued) PIN NO. NAME I/O (1) DESCRIPTION 17 SYNC/MODE I Synchronization input and mode setting pin. Do not float. Tie to ground if not used. Tie to ground: auto mode, higher efficiency at light loads; Tie to logic high: forced PWM, constant switching frequency over load; Tie to external clock source: forced PWM, synchronize to the rising edge of the external clock. 18 EN I Enable input to regulator. Do not float. High = ON, Low = OFF. Can be tied to PVIN. Precision enable input allows adjustable input voltage UVLO using external resistor divider. 19 AGND G Analog ground. Ground reference for internal circuitry. All electrical parameters are measured with respect to this pin. Connect to system ground on PCB. 20–22 PVIN P Supply input to internal bias LDO and HS FET. Connect to input supply and input bypass capacitors CIN. CIN must be placed right next to this pin and PGND pins on PCB, and connected with short and wide traces. 23–26 PGND G Power ground, connected to the source of LS FET internally. Connect to system ground, DAP/EP, AGND, ground side of CIN and COUT on PCB. Path to CIN must be as short as possible DAP G Low impedance connection to AGND. Connect to system ground on PCB. Major heat dissipation path for the device. Must be used for heat sinking by soldering to ground copper on PCB. Thermal vias are preferred to improve heat dissipation to other layers. EP 4 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 6 Specifications 6.1 Absolute Maximum Ratings Over operating free-air temperature range of –40°C to +125°C (unless otherwise noted) (1) MIN MAX PVIN to PGND PARAMETER –0.3 42 EN to AGND –0.3 VIN + 0.3 FB, RT, SS/TRK to AGND –0.3 5 PGOOD to AGND –0.1 20 SYNC to AGND –0.3 5.5 BIAS to AGND –0.3 Lower of (VIN + 0.3) or 20 AGND to PGND –0.3 0.3 SW to PGND –0.3 VIN + 0.3 SW to PGND less than 10-ns transients –3.5 42 CBOOT to SW –0.3 5 VCC to AGND –0.3 5 Operating junction temperature, TJ –40 150 °C Storage temperature, Tstg –65 150 °C Input voltages Output voltages (1) UNIT V V Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. 6.2 ESD Ratings VALUE V(ESD) (1) (2) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1) ±2000 Charged-device model (CDM), per JEDEC specification JESD22C101 (2) ±750 UNIT V JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process. JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process. 6.3 Recommended Operating Conditions Over operating free-air temperature range of –40°C to +125°C (unless otherwise noted) (1) MIN MAX 3.5 36 EN 0 VIN FB 0 4.5 PGOOD 0 18 BIAS input not used 0 0.3 BIAS input used 0 Lower of (VIN + 0.3) or 18 AGND to PGND PVIN to PGND Input voltages Output voltage Output current Temperature (1) UNIT V –0.1 0.1 VOUT 1 95% of VIN V IOUT, LM73605 0 5 A IOUT, LM73606 Operating junction temperature, TJ 0 6 A –40 125 °C Recommended operating rating indicate conditions for which the device is intended to be functional, but do not ensure specific performance limits. For ensured specifications, see Electrical Characteristics Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 5 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com 6.4 Thermal Information LM73605/LM73606 THERMAL METRIC (1) RNP (WQFN) UNIT 30 PINS RθJA Junction-to-ambient thermal resistance 34.3 °C/W RθJC(top) Junction-to-case (top) thermal resistance 14.6 °C/W RθJB Junction-to-board thermal resistance 7.3 °C/W ψJT Junction-to-top characterization parameter 0.1 °C/W ψJB Junction-to-board characterization parameter 7.1 °C/W RθJC(bot) Junction-to-case (bottom) thermal resistance 1 °C/W (1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application report. 6.5 Electrical Characteristics Limits apply over the recommended operating junction temperature (TJ) range of –40°C to +125°C, unless otherwise stated. Minimum and maximum limits are specified through test, design or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated, the following conditions apply: VIN = 12 V, VOUT = 3.3 V, fSW = 500 kHz. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT SUPPLY VOLTAGE (PVIN PINS) VIN Operating input voltage range ISD Shutdown quiescent current; measured at VIN pin (1) VEN = 0 V TJ = 25℃ IQ_NONSW Operating quiescent current from VIN (non-switching) VEN = 2 V, VFB = 1.5 V, VBIAS = 3.3 V external 3.5 36 V 0.8 10 µA 0.6 12 µA 1.15 V ENABLE (EN PIN) VEN_VCC_H Enable input high level for VCC output VEN rising VEN_VCC_L Enable input low level for VCC output VEN falling 0.3 VEN_VOUT_H Enable input high level for VOUT VEN rising 1.14 VEN_VOUT_HYS Enable input hysteresis for VOUT VEN falling hysteresis ILKG_EN Enable input leakage current VEN = 2 V V 1.196 1.25 –100 1.4 V mV 200 nA INTERNAL LDO (VCC PIN, BIAS PIN) VCC Internal VCC voltage VCC_UVLO Internal VCC undervoltage lockout VBIAS_ON Input changeover IBIAS_NONSW Operating quiescent current from external VBIAS (nonswitching) PWM operation 3.27 V PFM operation 3.1 V VCC rising 2.96 3.14 VCC falling hysteresis –605 VBIAS rising 3.09 VBIAS falling hysteresis –63 VEN = 2 V, VFB = 1.5 V, VBIAS = 3.3 V external 3.27 V mV 3.25 V mV 21 50 µA 1.006 1.017 V 0.2 60 nA VOLTAGE REFERENCE (FB PIN) VFB Feedback voltage PWM mode ILKG_FB Input leakage current at FB pin VFB = 1 V (1) 6 0.987 Shutdown current includes leakage current of the switching transistors. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Electrical Characteristics (continued) Limits apply over the recommended operating junction temperature (TJ) range of –40°C to +125°C, unless otherwise stated. Minimum and maximum limits are specified through test, design or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated, the following conditions apply: VIN = 12 V, VOUT = 3.3 V, fSW = 500 kHz. PARAMETER TEST CONDITIONS MIN TYP MAX 1.6 2.2 2.7 LM73605 6 7.3 8.35 LM73606 7.4 8.7 9.85 LM73605 4.79 5.5 6.1 LM73606 5.8 6.6 7.25 UNIT HIGH SIDE DRIVER (CBOOT PIN) VCBOOT_UVLO CBOOT - SW undervoltage lockout V CURRENT LIMITS AND HICCUP IHS_LIMIT Short-circuit, high-side current limit (2) ILS_LIMIT Low-side current limit INEG_LIMIT Negative current limit VHICCUP Hiccup threshold on FB pin IL_ZC Zero cross-current limit (2) LM73605 –5 LM73606 –6 0.36 0.4 A A A 0.44 V 0.06 A SOFT START (SS/TRK PIN) ISSC Soft-start charge current RSSD Soft-start discharge resistance 1.8 UVLO, TSD, OCP, or EN = 0 2 2.2 µA 1 kΩ POWER GOOD (PGOOD PIN) and OVERVOLTAGE PROTECTION VPGOOD_OV Power-good overvoltage threshold % of FB voltage 106% 110% 113% VPGOOD_UV Power-good undervoltage threshold % of FB voltage 86% 90% 93% VPGOOD_HYS Power-good hysteresis % of FB voltage VPGOOD_VALID Minimum input voltage for proper PGOOD function 50-µA pullup to PGOOD pin, VEN = 0 V, TJ = 25°C 1.3 2 RPGOOD Power-good ON-resistance VEN = 2.5V 40 100 VEN = 0 V 30 90 1.2% V Ω MOSFETS RDS_ON_HS (3) High-side MOSFET ONresistance IOUT = 1 A, VBIAS = VOUT = 3.3 V 53 90 mΩ RDS_ON_LS (3) Low-side MOSFET ONresistance IOUT = 1 A, VBIAS = VOUT = 3.3 V 31 55 mΩ THERMAL SHUTDOWN TSD (2) (3) (4) (4) Thermal shutdown threshold Shutdown threshold Recovery threshold 160 °C 135 °C This current limit was measured as the internal comparator trip point. Due to inherent delays in the current limit comparator and drivers, the peak current limit measured in closed loop with faster slew rate will be larger, and valley current limit will be lower. Measured at pins Ensured by design 6.6 Timing Characteristics MIN NOM MAX UNIT CURRENT LIMITS AND HICCUP NOC tOC (1) (1) Number of switching cycles before hiccup is tripped Overcurrent hiccup retry delay time 128 Cycles 46 ms Ensured by design Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 7 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Timing Characteristics (continued) MIN NOM 3.5 6.3 MAX UNIT SOFT START (SS/TRK PIN) tSS CSS = OPEN, from EN rising edge to PGOOD rising edge Internal soft-start time ms POWER GOOD (PGOOD PIN) and OVERVOLTAGE PROTECTION tPGOOD_RISE PGOOD rising edge deglitch delay 80 140 200 µs tPGOOD_FALL PGOOD falling edge deglitch delay 80 140 200 µs MAX UNIT 6.7 Switching Characteristics PARAMETER TEST CONDITIONS MIN TYP PWM LIMITS (SW PINS) tON-MIN Minimum switch on-time 60 82 ns tOFF-MIN Minimum switch off-time 70 120 ns tON-MAX Maximum switch on-time 3 6 9 µs HS timeout in dropout OSCILLATOR (RT and SYNC PINS) fOSC fADJ Internal oscillator frequency RT = Open 440 500 560 kHz Minimum adjustable frequency by RT or SYNC RT =115 kΩ, 0.1% 315 350 385 kHz Maximum adjustable frequency by RT or SYNC RT = 17.4 kΩ, 0.1% 1980 2200 2420 kHz VSYNC_HIGH Sync input high level threshold VSYNC_LOW Sync input low level threshold 2 VMODE_HIGH Mode input high level threshold for FPWM 0.42 V VMODE_LOW Mode input low level threshold for AUTO mode 0.4 V tSYNC_MIN Sync input minimum ON and OFFtime 80 ns 0.4 V V 6.8 System Characteristics The following specifications apply to the circuit found in typical schematic with appropriate modifications from typical bill of materials. These parameters are not tested in production and represent typical performance only. Unless otherwise stated the following conditions apply: TA = 25°C, VIN = 12 V, VOUT = 3.3 V, fSW = 500 kHz. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT VFB_PFM Output voltage offset at no load VIN = 3.8 V to 36 V, VSYNC = 0 V, auto mode in auto mode IOUT = 0 A 2% VDROP Minimum input to output voltage differential to maintain specified accuracy VOUT = 5 V, IOUT = 3 A, fSW = 2.2 MHz 0.4 V IQ_SW Operating quiescent current (switching) VEN = 3.3 V, IOUT = 0 A, RT = open, VBIAS = VOUT = 3.3 V , RFBT = 1 Meg 15 µA IPEAK_MIN Minimum inductor peak current LM73605: VSYNC = 0, IOUT = 10 mA 1 A LM73606: VSYNC = 0 V, IOUT = 10 mA 1.3 IBIAS_SW Operating quiescent current from external VBIAS (switching) fSW = 500 kHz, IOUT = 1 A 7 fSW = 2.2 MHz, IOUT = 1 A 25 DMAX Maximum switch duty cycle While in frequency foldback tDEAD Dead time between high-side and low-side MOSFETs 8 97.5% 4 Submit Documentation Feedback mA ns Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 6.9 Typical Characteristics Unless otherwise specified, VIN = 12 V, VOUT = 3.3 V and fSW = 500 kHz. Curves represent most likely parametric norm at specified condition. 75 70 HS Switch LS Switch Shutdown Current (nA) 65 RDS-ON (m:) 60 55 50 45 40 35 30 25 20 -40 -20 0 20 40 60 80 Temperature (°C) 100 120 140 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 -40 -20 0 CHAR Figure 1. High Side and Low Side Switches RDS-ON 20 40 60 Temperature (°C) 80 100 120 CHAR Plot Figure 2. Shutdown Quiescent Current 1.01 7.5 Temp = -40°C Temp = 25°C Temp = 125°C 1.009 1.008 7 1.007 Current Limits (A) Feedback Voltage (V) VIN = 3.5 V VIN = 12 V VIN = 36 V 1.006 1.005 1.004 1.003 1.002 6.5 HS LS 6 5.5 1.001 1 3 6 9 12 15 18 21 VIN (V) 24 27 30 33 5 -40 36 -20 0 CHAR Figure 3. Feedback Voltage 20 40 60 Temperature (°C) 80 100 120 CHAR Figure 4. LM73605 High Side and Low Side Current Limits 2500 9 2250 2000 Frequency (kHz) Current Limits (A) 8.4 7.8 HS LS 7.2 FREQ = 350 kHz FREQ = 1 MHz FREQ = 2.2 MHz 1750 1500 1250 1000 750 500 6.6 250 6 -40 -20 0 20 40 60 Temperature (°C) 80 100 0 -40 120 -20 CHAR Figure 5. LM73606 High Side and Low Side Current Limit 0 20 40 60 Temperature (°C) 80 100 120 CHAR Figure 6. Switching Frequency Set by RT Resistor Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 9 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Typical Characteristics (continued) 550 1.28 540 1.2 530 1.12 Enable Thresholds (V) Frequency with RT Pin Floating (kHz) Unless otherwise specified, VIN = 12 V, VOUT = 3.3 V and fSW = 500 kHz. Curves represent most likely parametric norm at specified condition. 520 510 500 490 480 VIN = 3.5 V VIN = 12 V VIN = 36 V 470 460 450 -40 -20 0 20 40 60 Temperature (°C) 80 100 1.04 VEN_VOUT Rising VEN_VOUT Falling VEN_VCC Rising VEN_VCC Falling 0.96 0.88 0.8 0.72 0.64 0.56 -40 120 -20 0 20 CHAR Figure 7. Switching Frequency with RT Pin Open Circuit 40 60 80 Temperature (°C) 100 120 140 CHAR Figure 8. Enable Thresholds 115 PGOOD Thresholds (%) 110 105 OV Tripping OV Recovery UV Recovery UV Tripping 100 95 90 85 -40 -20 0 20 40 60 Temperature (°C) 80 100 120 CHAR Figure 9. PGOOD Thresholds 10 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 7 Detailed Description 7.1 Overview The LM73605/6 is an easy-to-use synchronous step-down DC-DC converter that operates from a 3.5-V to 36-V supply voltage. It is capable of delivering up to 5-A (LM73605) or 6-A (LM73606) DC load current with exceptional efficiency and thermal performance in a very small solution size. The LM73605/6 employs fixed-frequency peak current-mode control with configurable auto or FPWM operation mode. Auto mode provides very high efficiency at light loads, and FPWM mode maintains constant switching frequency over entire load range. The device is internally compensated, which reduces design time and the number of external components. The switching frequency is programmable from 350 kHz to 2.2 MHz by an external resistor. The LM73605/6 can also synchronize to an external clock within the same frequency range. The wide switching frequency range allows the device to be optimized for a wide range of system requirements. It can be optimized for small solution size with higher frequency; or for high efficiency with lower switching frequency. The LM73605/6 has very low quiescent current, which is critical for battery operated systems. It allows for a wide range of voltage conversion ratios due to very small minimum ON-time (tON_MIN) and minimum OFF-time (tOFF_MIN). Automated frequency foldback is employed at very high or low duty cycles to further extend the operating range. The LM73605/6 also features a power-good (PGOOD) flag, precision enable, internal or adjustable soft start, pre-biased start-up, and output voltage tracking. Protection features include thermal shutdown, undervoltage lockout (UVLO), cycle-by-cycle current limiting, and short-circuit hiccup protection. It provides flexible and easyto-use solutions for a wide range of applications. The family requires very few external components and has a pin out designed for simple, optimum PCB layout for enhanced EMI and thermal performance. The LM73605/6 device is available in a 30-pin WQFN leadless package. 7.2 Functional Block Diagram ENABLE ISSC BIAS VCC LDO Internal SS CBOOT VCC SS/TRK HS I Sense ICMD + EA REF VBOOT VSW + ± RC FB FB OV/UV Detector ± + UVLO UVLO CC VSW PFM Detector CONTROL LOGIC SW PGood Hiccup Detector Slope Comp Oscillator TSD ± + PGOOD PVIN VBOOT Precision Enable CLK ILIMIT AGND LS I Sense FPWM RT SYNC/ MODE PGND Copyright © 2017, Texas Instruments Incorporated Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 11 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com 7.3 Feature Description 7.3.1 Synchronous Step-Down Regulator The LM73605/6 is a synchronous buck converter with both power MOSFETs integrated in the device. Figure 10 shows a simplified schematic for synchronous and non-synchronous buck converters. The synchronous buck integrates both high-side (HS) and low-side (LS) power MOSFETs. The non-synchronous buck integrates HS MOSFET and works with a discrete power diode as LS rectifier. VIN VIN Synchronous Buck L SW CIN VOUT COUT VIN VIN Non Synchronous Buck SW CIN L VOUT COUT PGND PGND Copyright © 2017, Texas Instruments Incorporated Figure 10. Simplified Synchronous vs Non-synchronous Buck Converters A synchronous converter with integrated HS and LS MOSFETs offers benefits such as less design effort, lower external components count, reduced total solution size, higher efficiency at heavier load, easier PCB design, and more control flexibility. The main advantage of a synchronous converter is that the voltage drop across the LS MOSFET is lower than the voltage drop across the power diode of a non-synchronous converter. Lower voltage drop translates into less power dissipation and higher efficiency. The LM73605/6 integrates HS and LS MOSFETs with very low on-time resistance to improve efficiency. It is especially beneficial when the output voltage is low. Because the LS MOSFET is integrated into the device, at light loads a synchronous converter has the flexibility to operate in either discontinuous or continuous conduction mode. An integrated LS MOSFET also allows the controller to obtain inductor current information when the LS switch is on. It allows the control loop to make more complex decisions based on HS and LS currents. It allows the LM73605/6 to have peak and valley cycle-by-cycle current limiting for more robust protection. 7.3.2 Auto Mode and FPWM Mode The LM73605/6 has configurable auto mode or FPWM options. In auto mode, the device operates in diode emulation mode (DEM) at light loads. In DEM, inductor current stops flowing when it reaches 0 A. This is also referred to as discontinuous conduction mode (DCM). This is the same behavior as the non-synchronous regulator, with higher efficiency. At heavier load, when the inductor current valley is above 0 A, the device operates in continuous conduction mode (CCM), where the switching frequency is fixed and set by RT. In auto mode, the peak inductor current has a minimum limit, IPEAK_MIN, in the LM73605/6. When peak current reaches IPEAK_MIN, the switching frequency reduces to regulate the required load current. Switching frequency lowers when load reduces. This is when the device operates in pulse frequency modulation (PFM). PFM further improves efficiency by reducing switching losses. Light load efficiency is especially important for battery operated systems. In forced PWM (FPWM) mode, the device operates in CCM regardless of load with the frequency set by RT or synchronization input. Inductor current can go negative at light loads. At light loads, the efficiency is lower than auto mode, due to higher conduction losses and switching losses. In FPWM, the device has fixed switching frequency over the entire load range, which is beneficial to noise sensitive applications. Figure 11 shows the inductor current waveforms in each mode with heavy load, light load, and very light load. The difference between the two modes is at lighter loads where inductor current valley reaches zero. 12 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Feature Description (continued) Auto Mode IL FPWM Mode IL CCM CCM Heavy Loads t Light Loads IL t IL DCM CCM t IL t IL PFM CCM Very Light Loads t t Figure 11. Inductor Current Waveforms at Auto Mode and FPWM Mode with Different Loads In CCM, the inductor current peak-to-peak ripple can be estimated by Equation 1: ILripple = (VIN VOUT ) V u OUT fSW u L VIN (1) The average or DC value of the inductor current equals the load current, or output current IOUT, in steady state. Peak inductor current can be calculated by Equation 2 IPEAK = IOUT + ILripple / 2 (2) Valley inductor current can be calculated by Equation 3 IVALLEY = IOUT – ILripple / 2 (3) In auto mode, the CCM to DCM boundary condition is when IVALLEY = 0 A. When ILripple >= IPEAK-MIN, the load current at the DCM boundary condition can be found by Equation 4. When the peak-to-peak ripple current is smaller than ILripple >= IPEAK-MIN, the PFM boundary will be reached first. IOUT-DCM = ILripple / 2 when • ILripple ≥ IPEAK-MIN (4) In auto mode, the PFM operation boundary condition is when IPEAK = IPEAK-MIN. Frequency foldback occurs when peak current drops to IPEAK_MIN, no matter in CCM or DCM operation. When current ripple is small, ILripple < IPEAK_MIN, the peak current will reach IPEAK_MIN when still in CCM. The output current at CCM PFM boundary can be found by Equation 5 IOUT-CCM-PFM = IPEAK-MIN – ILripple / 2 when • ILripple < IPEAK-MIN (5) The current ripple increases with reduced frequency if load reduces. When valley current reaches zero, the frequency continues to fold back with constant peak current and discontinuous current. In FPWM mode, there is no IPEAK-MIN limit. The peak current is defined by Equation 2 at light loads and heavy loads. See Frequency Synchronization and Mode Setting for mode setting options in LM73605/6. Mode setting only affects operation at light loads. There is no difference if load current is above the DCM and PFM boundary conditions discussed above. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 13 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Feature Description (continued) 7.3.3 Fixed-Frequency Peak Current-Mode Control The LM73605/6 synchronous switched mode voltage regulator employs fixed frequency peak current mode control with advanced features. The fixed switching frequency is controlled by an internal clock. To get accurate DC load regulation, a voltage feedback loop is implemented to generate peak current command. The HS switch is turned on at the rising edge of the clock. As shown in Figure 12, during the HS switch on-time tON, the SW pin voltage VSW swings up to approximately VIN, and the inductor current IL increases with a linear slope. The HS switch is turned off when the inductor current reaches the peak current command. During the HS switch off-time tOFF, the LS switch is turned on. Inductor current discharges through the LS switch, which forces the VSW to swing below ground by the voltage drop across the LS switch. The LS switch is turned off at the next clock cycle, before the HS switch is turned on. The regulation loop adjusts the peak current command to maintain a constant output voltage. VSW SW Voltage D = tON / TSW VIN tON tOFF 0 -VD t TSW Inductor Current IL IL-PEAK IOUT ILripple IL-VALLEY t 0 Figure 12. SW Voltage and Inductor Current Waveforms in CCM Duty cycle D is defined by the on-time of the HS switch over the switching period: D = tON / TSW where • TSW = 1 / fSW is the switching period (6) In an ideal buck converter, where losses are ignored, D is proportional to the output voltage and inverse proportional to the input voltage: D = VOUT ⁄ VIN. When the LM73605/6 is set to operate in auto mode, the LS switch is turned off when its current reaches zero ampere before the next clock cycle comes. Both HS switch and LS switch are off before the HS switch is turned on at the next clock cycle. 7.3.4 Adjustable Output Voltage The voltage regulation loop in the LM73605/6 regulates the FB pin voltage to be the same as the internal reference voltage. The output voltage of the LM73605/6 is set by a resistor divider to program the ratio from VOUT to VFB. The resistor divider is connected from the output to ground with the mid-point connecting to the FB pin. VOUT RFBT FB RFBB Figure 13. Output Voltage Setting by Resistor Divider 14 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Feature Description (continued) The internal voltage reference and feedback loop produce precise voltage regulation over temperature. TI recommends using divider resistors with 1% tolerance or better, and with temperature coefficient of 100 ppm or lower. Typically, RFBT = 10 kΩ to 100 kΩ is recommended. Larger RFBT and RFBB values reduce the quiescent current going through the divider, which help maintain high efficiency at very light load. But larger divider values also make the feedback path more susceptible to noise. If efficiency at very light load is critical in a certain application, RFBT up to 1 MΩ can be used. RFBB can be calculated by Equation 7: VFB RFBB RFBT VOUT VFB (7) The minimum programmable VOUT equals VFB, with RFBB open. The maximum VOUT is limited by the maximum duty cycle at a given frequency: DMAX = 1 – (tOFF_MIN / TSW) where • • tOFF_MIN is the minimum off time of the HS switch TSW = 1 / fSW is the switching period (8) Ideally, without frequency foldback, VOUT_MAX = VIN_MIN × DMAX. Power losses in the circuit reduces the maximum output voltage. The LM73605/6 folds back switching frequency under tOFF_MIN condition to further extend VOUT_MAX. The device maintains output regulation with lower input voltage. The minimum fold-back frequency is limited by the maximum HS on-time, tON_MAX. Maximum output voltage with frequency foldback can be estimated by: VOUT _ MAX VIN_MIN u t ON _ MAX t ON _ MAX tOFF_MIN IOUT u (RDS_ON_HS DCR) (9) The voltage drops on the HS MOSFET and inductor DCR have been taken into account in Equation 9. The switching losses were not included. If the resistor divider is not connected properly, the output voltage cannot be regulated because the feedback loop cannot obtain correct output voltage information. If the FB pin is shorted to ground or disconnected, the output voltage is driven close to VIN. The load connected to the output could be damaged under this condition. Do not short FB to ground or leave it open circuit during operation. The FB pin is a noise sensitive node. It is important to place the resistor divider as close as possible to the FB pin, and route the feedback node with a short and thin trace. The trace connecting VOUT to RFBT can be long, but it must be routed away from the noisy area of the PCB. For more layout recommendations, see Layout. 7.3.5 Enable and UVLO The LM73605/6 regulates output voltage when the VCC voltage is higher than the undervoltage lock out (UVLO) level, VCC_UVLO, and the EN voltage is higher than VEN_VOUT_H. The internal LDO output voltage VCC is turned on when the EN voltage is higher than VEN_VCC_H. The precision enable circuitry is also turned on when VCC is above UVLO. Normal operation of the LM73605/6 with regulated output voltage is enabled when the EN voltage is greater than VEN_VOUT_H. When the EN voltage is less than VEN_VCC_L, the device is in shutdown mode. The internal dividers make sure VEN_VOUT_H is always higher than VEN_VCC_H. The EN pin cannot be left floating. The simplest way to enable the operation of the LM73605/6 is to connect the EN pin to PVIN, which allows self-start-up of the LM73605/6 when VIN rises. Use of a pullup resistor between PVIN and EN pins helps reduce noise coupling from PVIN pin to the EN pin. Many applications benefit from employing an enable divider to establish a customized system UVLO. This can be used either for sequencing, system timing requirement, or to reduce the occurrence of deep discharge of a battery power source. Figure 14 shows how to use a resistor divider to set a system UVLO level. An external logic output can also be used to drive the EN pin for system sequencing. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 15 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Feature Description (continued) VIN RENT ENABLE RENB Figure 14. System UVLO With a selected RENT, the RENB can be calculated by: RENB = VEN _ VOUT _ H VIN _ ON _ H VEN_VOUT_H RENT where • VIN_ON_H is the desired supply voltage threshold to turn on this device (10) Note that the divider adds to supply quiescent current by VIN / (RENT + RENB). Small RENT and RENB values add more quiescent current loss. However, large divider values make the node more sensitive to noise. RENT in the hundreds of kΩ range is a good starting point. 7.3.6 Internal LDO, VCC_UVLO, and BIAS Input The LM73605/6 integrates an internal LDO, generating VCC voltage for control circuitry and MOSFET drivers. The VCC pin must have a 1-µF to 4.7-µF bypass capacitor placed as close as possible to the pin and properly grounded. Do not load the VCC pin or short it to ground during operation. Shorting VCC pin to ground during operation may damage the device. The UVLO on VCC voltage, VCC_UVLO, turns off the regulation when VCC voltage is too low. It prevents the LM73605/6 from operating until the VCC voltage is enough for the internal circuitry. Hysteresis on VCC_UVLO prevents the part from turning off during power up if VIN droops due to input current demands. The LDO generates VCC voltage from one of the two inputs: the supply voltage VIN, or the BIAS input. When BIAS is tied to ground, the LDO input is VIN. When BIAS is tied to a voltage higher than 3.3 V, the LDO input is VBIAS. BIAS voltage must be lower than both VIN and 18 V. The BIAS input is designed to reduce the LDO power loss. The LDO power loss is: PLOSS_LDO = ILDO × (VIN_LDO – VOUT_LDO) (11) The higher the difference between the input and output voltages of the LDO, the more loss occurs to supply the same LDO output current. The BIAS input provides an option to supply the LDO with a lower voltage than VIN, to reduce the difference of the input and output voltages of the LDO and reduce power loss. For example, if the LDO current is 10 mA at a certain frequency with VIN = 24 V and VOUT = 5 V. The LDO loss with BIAS tied to ground is equal to 10 mA × (24 V – 3.27 V) = 207.3 mW, while the loss with BIAS tied to VOUT is equal to 10 mA × (5 – 3.27) = 17.3 mW. The efficiency improvement is more significant at light and mid loads because the LDO loss is a higher percentage in the total loss. The improvements is more significant with higher switching frequency because the LDO current is higher at higher switching frequency. The improvement is more significant when VIN » VOUT because the voltage difference is higher. Figure 15 and Figure 16 show efficiency improvement with bias tied to VOUT in a VOUT = 5 V and fSW = 2200 kHz application, in auto mode and FPWM mode, respectively. 16 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 0.001 VOUT = 5 V 100 VIN = 12 V BIAS = VOUT VIN = 12 V BIAS = GND VIN = 24 V BIAS = VOUT VIN = 24 V BIAS = GND 80 Efficiency (%) Efficiency (%) Feature Description (continued) VIN = 12 V BIAS = VOUT VIN = 12 V BIAS = GND VIN = 24 V BIAS = VOUT VIN = 24 V BIAS = GND 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 fSW = 2200 kHz 1 60 40 20 2 3 4 56 0 0.001 0.01 0.02 0.05 0.1 0.2 0.5 Load Current (A) EFF_ Auto Mode Figure 15. LM73606 Efficiency Comparison with Bias = VOUT to Bias = GND in Auto Mode VOUT = 5 V fSW = 2200 kHz 1 2 3 4 5 7 10 EFF_ FPWM Mode Figure 16. LM73606 Efficiency Comparison with Bias = VOUT to Bias = GND in FPWM Mode TI recommends tying the BIAS pin to VOUT when VOUT is equal to or greater than 3.3 V and no greater than 18 V. Tie the BIAS pin to ground when not in use. A ceramic capacitor, CBIAS, can be used from the BIAS pin to ground for bypassing. If VOUT has high frequency noise or spikes during transients or fault conditions, a resistor (1 to 10 Ω) connected between VOUT to BIAS can be used together with CBIAS for filtering. VCC (V) The VCC voltage is typically 3.27 V. When the LM73605/6 is operating in PFM mode with frequency foldback, VCC voltage is reduced to 3.1 V (typical) to further decrease the quiescent current and improve efficiency at very light loads. Figure 17 shows an example of VCC voltage change with mode change. 3.5 3.4 3.3 3.2 3.1 3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 0.001 VOUT = 5 V Auto Mode FPWM Mode 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 1 2 3 45 fSW = 500 kHz VCC_ VIN = 12 V Figure 17. VCC Voltage vs Load Current VCC voltage has an internal undervoltage lockout threshold, VCC_UVLO. When VCC voltage is higher than VCC_UVLO rising threshold, the device is active and in normal operation if VEN > VEN_VOUT_H. If VCC voltage droops below VCC_UVLO falling threshold, the VOUT is shut down. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 17 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Feature Description (continued) 7.3.7 Soft Start and Voltage Tracking The LM73605/6 features controlled output voltage ramp during start-up. The soft-start feature reduces inrush current during start-up and improves system performance and reliability. If the SS/TRK pin is floating, the LM73605/6 starts up following the fixed internal soft-start ramp. If longer soft-start time is desired, an external capacitor can be added from SS/TRK pin to ground. There is a 2µA (typical) internal current source, ISSC, to charge the external capacitor. For a desired soft-start time tSS, capacitance of CSS can be found by Equation 12. CSS = ISSC × tSS where • • • CSS = soft-start capacitor value (F) ISSC = soft-start charging current (A) tSS = desired soft-start time (s) (12) The FB voltage always follows the lower potential of the internal voltage ramp or the voltage on the SS/TRK pin. Thus, the soft-start time can only be extended longer than the internal soft-start time by connecting CSS. Use CSS to extend soft-start time when there are a large amount of output capacitors, or the output voltage is high, or the output is heavily loaded during start-up. LM73605/6 is operating in diode emulation mode during start-up regardless of mode setting. The device is capable of starting up into pre-biased output conditions. During start-up, the device sets the minimum inductor current to zero to avoid back charging the input capacitors. LM73605/6 can track an external voltage ramp applied to the SS/TRK pin, if the ramp is slower than the internal soft-start ramp. The external ramp final voltage after start-up must be greater than 1.5 V to avoid noise interfering with the reference voltage. Figure 18 shows how to use resistor divider to set VOUT to follow an external ramp. EXT RAMP RTRT SS/TRK RTRB Figure 18. Soft Start Tracking External Ramp VOUT tracking also provides the option of ramping up faster than the internal start-up ramp. The FB voltage always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 19 shows the case when VOUT ramps slower than the internal ramp, while Figure 20 shows when VOUT ramps faster than the internal ramp. If the tracking ramp is delayed after the internal ramp is completed, VFB follows the tracking ramp even if it is faster than the internal ramp. Faster start-up time may result in large inductor current during start-up. Use with special care. Enable Internal SS Ramp Ext Tracking Signal to SS pin VOUT Figure 19. Tracking with Longer Start-up Time than the Internal Ramp 18 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Feature Description (continued) Enable Internal SS Ramp Ext Tracking Signal to SS pin VOUT Figure 20. Tracking with Shorter Start-up Time than the Internal Ramp The SS/TRK pin is discharged to ground by an internal pulldown resistor RSSD when the output voltage is shutting down, such as in the event of UVLO, thermal shutdown, hiccup, or VEN = 0. If a large CSS is used, and the time when VEN = 0 V is very short, the CSS may not be fully discharged before the next soft start. Under this condition, the FB voltage follows the internal ramp slew rate until the voltage on CSS is reached, then follow the slew rate defined by CSS. 7.3.8 Adjustable Switching Frequency The internal oscillator frequency is controlled by the impedance on the RT pin. If the RT pin is open circuit, the LM73605/6 operates at its default switching frequency, 500 kHz. The RT pin is not designed to be connected directly to ground. To program the switching frequency by RT resistor, Equation 13, or Figure 21, or Table 1 can be used to find the resistance value. RT (k:) = 1 fSW (kHz) u 2.675 u 10 -5 0.0007 (13) 120 110 100 90 RT (k:) 80 70 60 50 40 30 20 10 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Frequency (kHz) RT_F Figure 21. RT Resistance vs Switching Frequency Table 1. Typical Frequency Setting Resistance SWITCHING FREQUENCY fSW (kHz) RT RESISTANCE (kΩ) 350 115 400 100 500 78.7 (or open) 750 52.3 1000 39.2 1500 26.1 2000 19.1 2200 17.4 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 19 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com The choice of switching frequency is usually a compromise between conversion efficiency and the size of the solution. Lower switching frequency has lower switching losses (including gate charge losses, switch transition losses, etc.). It usually results in higher overall efficiency. However, higher switching frequency allows the use of smaller power inductor and output capacitors, hence a more compact design. Lower inductance also helps transient response (higher large signal slew rate of inductor current), and has lower DCR. The optimal switching frequency is usually a trade-off in a given application and thus needs to be determined on a case-by-case basis. Factors that need to be taken into account include input voltage range, output voltage, most frequent load current level(s), external component choices, solution size/cost requirements, efficiency and thermal management requirements. The choice of switching frequency may also be limited whether an operating condition triggers tON_MIN or tOFF_MIN. Minimum on-time, tON_MIN, is the smallest time that the HS switch can be on. Minimum OFF-time, tOFF_MIN, is the smallest duration that the HS switch can be off. In CCM operation, tON_MIN and tOFF_MIN limits the voltage conversion range given a selected switching frequency, FSW. The minimum duty cycle allowed is: DMIN = tON_MIN × fSW (14) The maximum duty cycle allowed is: DMAX = 1 – tOFF_MIN × fSW (15) Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size and efficiency. The maximum operational supply voltage can be found by: VIN-MAX = VOUT / (fSW × tON_MIN) (16) At lower supply voltage, the switching frequency decreases once tOFF_MIN is tripped. The minimum VIN without frequency foldback can be approximated by: VIN-MIN = VOUT / (1 – fSW × tOFF_MIN) (17) With a desired VOUT, the range of allowed VIN is narrower with higher switching frequency. LM73605/6 has an advanced frequency fold-back algorithm under both tON_MIN and tOFF_MIN conditions. With frequency foldback, stable output voltage regulation is extended to wider range of supply voltages. At very high VIN conditions, where tON_MIN limitation is met, the switching frequency reduces to allow higher VIN while maintaining VOUT regulation. Note that the peak to peak inductor current ripple will increase with higher VIN and lower frequency. TI does not recommend designing the circuit to operate with tON_MIN under typical conditions. At very low VIN conditions, where tOFF_MIN limitation is met, the switching frequency decreases until tON_MAX condition is met. Such frequency fold-back mechanism allows the LM73605/6 to have very low dropout voltage regardless of frequency setting. 7.3.9 Frequency Synchronization and Mode Setting The LM73605/6 switching action can synchronize to an external clock from 350 kHz to 2.2 MHz. TI recommends connecting the external clock to the SYNC/MODE pin with an appropriate termination resistor. Ground the SYNC/MODE pin if not used. SYNC/ MODE EXT CLOCK RSYNC Figure 22. Frequency Synchronization Recommendations for the external clock include a high level no lower than 2 V, low level no higher than 0.4 V, duty cycle between 10% and 90%, and both positive and negative pulse width no shorter than 80 ns. When the external clock fails at logic high or low, the LM73605/6 switches at the frequency programmed by the RT resistor after a time-out period. TI recommends connecting a resistor to the RT pin such that the internal oscillator frequency is the same as the external clock frequency. This allows the regulator to continue operating at approximately the same switching frequency if the external clock fails with the same control loop behavior. 20 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 The SYNC/MODE pin is also used as an operation mode control input. • To set the operation in auto mode, connect SYNC/MODE pin to ground, or a logic signal lower than 0.3 V. • To set the operation in FPWM mode, connect SYNC/MODE pin to a bias voltage or logic signal greater than 0.6 V. • When the LM73605/6 is synchronized to an external clock, the operation mode is FPWM. Table 2 summarizes the operation mode and features according to the SYNC/MODE input signal. For more details, see Active Mode and Auto Mode and FPWM Mode. Table 2. SYNC/MODE Pin Settings and Operation Modes SYNC/MODE INPUT SWITCHING FREQUENCY OPERATING MODE LIGHT LOAD BEHAVIOR • Logic low Set by RT resistor Logic high Set by RT resistor External clock Set by external clock Auto mode FPWM mode • No negative inductor current, device operates in discontinuous conduction mode (DCM) when current valley reaches 0 A. Minimum peak inductor current is limited at IPEAK-MIN; device operates in pulse frequency modulation (PFM) mode when peak current reaches IPEAK-MIN. Switching frequency reduces in PFM mode. • • • Fixed frequency continuous conduction mode (CCM) regardless of load Inductor current have negative portion at light loads No IPEAK-MIN • 7.3.10 Internal Compensation and CFF The LM73605/6 is internally compensated. The internal compensation is designed such that the loop response is stable over a wide operating frequency and output voltage range. The internal R-C values are 500 kΩ and 30 pF respectively. When large resistance value (MΩ) is used for RFBT, the pole formed by an internal parasitic capacitor and RFBT can be low enough to reduce the phase margin. If only low ESR output capacitors (ceramic types) are used for COUT, the control loop could have low phase margin. To provide a phase boost an external feed-forward capacitor (CFF) can be added in parallel with RFBT. Choose the CFF capacitor to provide most phase boost at the estimated crossover frequency fX: fX = K VOUT u COUT where • • K = 20.27 with LM73605 K = 24.16 with LM73606 (18) Select COUT so that the fX is no higher than 1/6 of the switching frequency. Typically, fX / fSW = 1/10 to 1/8 provides a good combination of stability and performance. Place the external feed-forward capacitor in parallel with the top resistor divider RFBT when additional phase boost is needed. VOUT RFBT CFF FB RFBB Figure 23. Feed-Forward Capacitor for Loop Compensation The feed-forward capacitor CFF in parallel with RFBT places an additional zero before the crossover frequency of the control loop to boost phase margin. The zero frequency can be found by Equation 19: fZ-CFF = 1 / (2π × RFBT × CFF) (19) An additional pole is also introduced with CFF at the frequency of: fP-CFF = 1 / (2π × CFF × (RFBT // RFBB)) (20) Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 21 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Select the CFF so that the bandwidth of the control loop without the CFF is centered between fZ-CFF and fP-CFF. The zero at fZ-CFF adds phase boost at the crossover frequency and improves transient response. The pole at fP-CFF helps maintaining proper gain margin at frequency beyond the crossover. The need of CFF depends on RFBT and COUT. Typically, choose RFBT ≤ 100 kΩ. CFF may not be required, because the internal parasitic pole is at higher frequency. If COUT has larger ESR, and ESR zero fZ-ESR = 1 / (2π × ESR × COUT) is low enough to provide phase boost around the crossover frequency, do not use CFF. Equation 21 was tested for ceramic output capacitors: CFF = 1 u 2 u S u fx RFBT 1 u (RFBT // RFBB ) (21) The CFF creates a time constant with RFBT that couples in the attenuated output voltage ripple to the FB node. If the CFF value is too large, it can couple too much ripple to the FB and affect VOUT regulation. It could also couple too much transient voltage deviation and falsely trigger PGOOD flag. 7.3.11 Bootstrap Capacitor and VBOOT-UVLO The driver of the HS switch requires a bias voltage higher than the VIN voltage. The capacitor, CBOOT in Simplified Schematic, connected between CBOOT and SW pins works as a charge pump to boost voltage on the CBOOT pin to (VSW + VCC). A boot diode is integrated on the die to minimize external component count. TI recommends a high-quality 0.47-µF, 6.3-V or higher voltage ceramic capacitor for CBOOT. The VBOOT_UVLO threshold is designed to maintain proper HS switch operation. If the CBOOT is not charged above this voltage with respect to SW, the device initiates a charging sequence using the LS switch before turning on the HS switch. 7.3.12 Power-Good and Overvoltage Protection The LM73605/6 has a built-in power-good (PGOOD) flag to indicate whether the output voltage is at an appropriate level or not. The PGOOD flag can be used for start-up sequencing of multiple rails. The PGOOD pin is an open-drain output that requires a pullup resistor to an appropriate logic voltage (any voltage below 15 V). The pin can sink 5 mA of current and maintain its specified logic low level. A typical pullup resistor value is 10 kΩ to 100 kΩ. When the FB voltage is higher than VPGOOD-OV or lower than VPGOOD-UV threshold, the PGOOD internal switch is turned on, and the PGOOD pin voltage is pulled low. When the FB is within the range, the PGOOD switch is turned off, and the pin is pulled up to the voltage connected to the pullup resistor. The PGOOD function also have a deglitch timer for about 140 µs for each transition. If it is desired to pull up PGOOD pin to a voltage higher than 15 V, a resistor divider can be used to divide the voltage down. VPU RPGT PGOOD RPGB Figure 24. Divider for PGOOD Pullup Voltage With a given pullup voltage VPU, select a desired voltage on the PGOOD pin, VPG. With a selected RPGT, the RPGB can be found by: RPGB = VPG RPGT VPU VPG (22) When the device is disabled, the output voltage is low, and the PGOOD flag indicates logic low as long as VIN > 2 V. 22 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 7.3.13 Overcurrent and Short-Circuit Protection The LM73605/6 is protected from overcurrent conditions with cycle-by-cycle current limiting on both HS and LS MOSFETs. The HS switch is turned off when HS current goes beyond the peak current limit, IHS-LIMIT. The LS switch can only be turned off when LS current is below LS current limit, ILS-LIMIT. If the LS switch current is higher than ILS-LIMIT at the end of a switching cycle, the switching cycle is extended until the LS current reduces below the limit. Current limiting on both HS and LS switches provides tighter control of the maximum DC inductor current, or output current. They also help prevent runaway current at extreme conditions. With LM73605/6, the maximum output current is always limited at IDC_LIMIT = (IHS_LIMIT + ILS_LIMIT) / 2 (23) The LM73605/6 employs hiccup current protection at extreme overload conditions, including short-circuit condition. Hiccup is only activated when VOUT droops below 40% (typical) of the regulation voltage and stays below for 128 consecutive switching cycles. Under overcurrent conditions when VOUT has not fallen below 40% of regulation, the LM73605/6 continues operation with cycle-by-cycle HS and LS current limiting. Hiccup is disabled during soft-start. When hiccup is triggered, the device turns off VOUT regulation and re-tries soft start after a re-try delay time, TOC = 46 ms (typical). The long wait time allows the device, and the load, to cool down under such fault conditions. If the fault condition still exists when re-try, hiccup shuts down the device and repeats the wait and re-try cycle. If the fault condition has been removed, the device starts up normally. If tracking was used for initial sequencing, the device restarts using the internal soft-start ramp. Hiccup mode helps to reduce the device power dissipation and die temperature under severe overcurrent conditions and short circuits. It improves system reliability and prolongs the life span of the device. In FPWM mode, negative current protection is implemented to protect the switches from extreme negative currents. When LS switch current reaches INEG-LIMIT, LS switch turns off, and HS switch turns on to conduct the negative current. HS switch is turned off once its current reaches 0 A. 7.3.14 Thermal Shutdown Thermal shutdown protection prevents the device from extreme junction temperature. The device is turned off when the junction temperature exceeds 160°C (typical). After thermal shutdown occurs, hysteresis prevents the device from switching until the junction temperature drops to approximately 135°C. When the junction temperature falls below 135°C, the LM73605/6 restarts. 7.4 Device Functional Modes 7.4.1 Shutdown Mode The EN pin provides electrical on/off control of the device. When the EN pin voltage is below VEN_VCC_L, the device is in shutdown mode. The LDO output voltage VCC = 0 V and the output voltage VOUT = 0 V. In shutdown mode the quiescent current drops to a very low value. 7.4.2 Standby Mode The internal LDO has a lower EN threshold than that required to start the regulator. When the EN pin voltage is above VEN_VCC_H, the internal LDO regulates the VCC voltage. The precision enable circuitry is turned on once VCC is above VCC_UVLO. The device is in standby mode if EN voltage is below VEN_VOUT_H. The internal MOSFETs remains in tri-state unless the voltage on EN pin goes beyond VEN_VOUT_H threshold. The LM73605/6 also employs UVLO protection. If the VCC voltage is below the VCC_UVLO level, the output of the regulator is turned off. 7.4.3 Active Mode The LM73605/6 is in active mode when the EN voltage is above VEN_VOUT_H, and VCC is above VCC_UVLO. The simplest way to enable the operation of the LM73605/6 is to pull up the EN pin to PVIN which allows self-start-up when the input voltage ramps up. In active mode, depending on the load current and mode setting, the LM73605/6 is in one of four modes: 1. CCM with fixed switching frequency when load current is above half of the peak-to-peak inductor current ripple; Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 23 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Device Functional Modes (continued) 2. DCM with fixed switching frequency when load current is lower than half of the peak-to-peak inductor current ripple in CCM operation; 3. PFM when switching frequency is decreased at very light load; 4. Under overcurrent or overtemperature conditions, the device operates in one of the fault protection modes. Please refer to Table 2 for mode-setting details. 7.4.3.1 CCM Mode In continuous conduction mode (CCM) operation, inductor current has a continuous triangular waveform. The HS switch is on at the beginning of a switching cycle and the LS switch is turned off the end of each switching cycle. In auto mode, the LM73605/6 operates in CCM when the load current is higher than ½ of the peak-to-peak inductor current (ILripple). In FPWM mode, the LM73605/6 operates in CCM regardless of load. In CCM operation, the switching frequency is typically constant, unless tON_MIN, tOFF_MIN, or IPEAK-MIN conditions are met. The constant switching frequency is determined by RT setting, or the external synchronization clock frequency. The duty cycle is also constant in CCM: D = VOUT / VIN if loss is ignored, regardless of load. The peak-to-peak inductor ripple is constant with the same VIN and VOUT, regardless of load. With very high or very low supply voltages, when the tON_MIN or tOFF_MIN condition is met, the frequency reduces to maintain VOUT regulation with even higher or lower VIN, respectively. When the IPEAK-MIN condition is met in auto mode, switching frequency will fold back to provide higher efficiency. IPEAK-MIN is disabled in FPWM mode. 7.4.3.2 DCM Mode Discontinuous conduction mode (DCM) operation only happens in auto mode, when the load current is lower than half of the CCM inductor current ripple, and peak current is higher than IPEAK-MIN. There is no DCM in FPWM mode. DCM is also known as diode emulation mode. The LS FET is turned off when the inductor current ramps to 0 A. DCM has the same switching frequency as CCM, which is set by the RT pin. Duty cycle and peak current reduces with lighter load in DCM. DCM is more efficient than FPWM under the same condition, because of lower switching losses and lower conduction losses. When the peak current reduces to IPEAK-MIN at lighter load, the LM73605/6 operates in PFM mode. 7.4.3.3 PFM Mode Pulse-frequency-modulation (PFM) mode is activated when peak current is lower than IPEAK-MIN, only in auto mode. Peak current is kept constant and VOUT is regulated by frequency. Efficiency is greatly improved by lowered switching losses, especially at very light loads. In PFM operation, a small DC positive offset appears on VOUT. The lower the frequency is folded back in PFM, the more the DC offset is on VOUT. See VOUT regulation curves in Application Curves. If the DC offset on VOUT is not acceptable, a dummy load at VOUT, or lower RFBT and RFBB resistance values can be used to reduce the offset. Alternatively the device can be run in FPWM mode where the switching frequency is constant, and no offset is added to affect the VOUT accuracy unless tON_MIN is reached. 7.4.3.4 Fault Protection Mode The LM73605/6 has hiccup current protection at extreme overload and short circuit conditions. Hiccup is activated when VOUT droops below 40% (typical) of the regulation voltage and stays for 128 consecutive switching cycles. Hiccup is disabled during soft start. In hiccup, the device turns off VOUT and re-tries soft start after 46-ms wait time. Cycle repeats until overcurrent fault condition has been removed. Hiccup mode helps to reduce the device power dissipation and die temperature under severe overcurrent conditions and short circuits. It improves system reliability and prolongs the life span of the device. Under overcurrent conditions when VOUT droops below regulation but above 40% of regulated voltage, the LM73605/6 stays in cycle-by-cycle HS and LS current limiting protection mode. Thermal shutdown prevents the device from extreme junction temperature by turning off the device when the junction temperature exceeds 160°C (typical). After thermal shutdown occurs, hysteresis prevents the device from switching until the junction temperature drops to approximately 135°C. When the junction temperature falls below 135°C, the LM73605/6 restarts. 24 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 8 Application and Implementation NOTE Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. 8.1 Application Information The LM73605/6 is a step-down DC-DC voltage regulator. It is designed to operate with a wide supply voltage range (3.5 V to 36 V), wide switching frequency range (350 kHz to 2.2 MHz), and wide output voltage range: up to 95% VIN. It is a synchronous converter with both HS and LS MOSFETs integrated, and capable of delivering a maximum output current of 5 A (LM73605) or 6 A (LM73606). The following design procedure can be used to select component values for the LM73605/6. Alternately, the WEBENCH® software may be used to generate a complete design. The WEBENCH® software uses an iterative design procedure and accesses a comprehensive database of components when generating a design (see Custom Design With WEBENCH® Tools). This section presents a simplified discussion of the design process. 8.2 Typical Application The LM73605/6 only requires a few external components to perform high efficiency power conversion, as shown in Figure 25. L VIN PVIN CIN VOUT SW CBOOT COUT EN CBOOT PGND PGOOD BIAS SS/TRK RT SYNC/ MODE FB RFBB VCC CVCC RFBT AGND Copyright © 2017, Texas Instruments Incorporated Figure 25. LM73605/6 Basic Schematic The LM73605/6 also integrates many practical features to meet a wide range of system design requirements and optimization, such as UVLO, programmable soft-start time, start-up tracking, programmable switching frequency, clock synchronization and a power-good flag. Note that for ease of use, the feature pins do not require an additional component when not in use. They can be either left floating or shorted to ground as defined in . A comprehensive schematic with all features utilized is shown in Figure 26. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 25 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Typical Application (continued) VIN L CIN PVIN RENT COUT PGND CBOOT CBOOT EN RENB VOUT SW BIAS SS/TRK PGOOD CSS RFBT FB RT RFBB RT AGND SYNC/ MODE VCC CVCC RSYNC Copyright © 2017, Texas Instruments Incorporated Figure 26. LM73605/6 Comprehensive Schematic with All Features Utilized The external components must fulfill not only the needs of the power conversion, but also the stability criteria of the control loop. The LM73605/6 is optimized to work with a range of external components. can be used to for quick component selection. Table 3. Typical Component Selection (1) 26 fSW (kHz) VOUT (V) L (µH) COUT (µF) (1) RFBT (kΩ) RFBB (kΩ) 350 1 2.2 500 100 OPEN 115 500 1 1.5 400 100 OPEN 78.7 or open 1000 1 0.68 200 100 OPEN 39.2 2200 1 0.47 100 100 OPEN 17.4 350 3.3 4.7 200 100 43.5 115 500 3.3 3.3 150 100 43.5 78.7 or open 1000 3.3 1.8 88 100 43.5 39.2 2200 3.3 1.2 44 100 43.5 17.4 350 5 6.8 120 100 25 115 500 5 4.7 88 100 25 78.7 or open 1000 5 3.3 66 100 25 39.2 2200 5 2.2 44 100 25 17.4 350 12 15 66 100 9.1 115 500 12 10 44 100 9.1 78.7 or open 1000 12 6.8 22 100 9.1 39.2 350 24 22 40 100 4.3 115 500 24 15 30 100 4.3 78.7 or open RT (kΩ) All the COUT values are after derating. Add more when using ceramics. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 8.2.1 Design Requirements Detailed design procedure is described based on a design example. For this design example, use the parameters listed in Table 4. Table 4. Design Example Parameters DESIGN PARAMETER VALUE Typical input voltage 12 V Output voltage 5V Output current 5A Operating frequency 500 kHz Soft-start time 11 ms 8.2.2 Detailed Design Procedure 8.2.2.1 Custom Design With WEBENCH® Tools To 1. 2. 3. create a custom design with the WEBENCH® Power Designer, click the LM73605 or LM73606 device. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial. Compare the generated design with other possible solutions from Texas Instruments. The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time pricing and component availability. In most cases, these actions are available: • Run electrical simulations to see important waveforms and circuit performance • Run thermal simulations to understand board thermal performance • Export customized schematic and layout into popular CAD formats • Print PDF reports for the design, and share the design with colleagues Get more information about WEBENCH tools at www.ti.com/WEBENCH. 8.2.2.2 Output Voltage Setpoint The output voltage of the LM73605/6 device is externally adjustable using a resistor divider network. The divider network is comprised of top feedback resistor RFBT and bottom feedback resistor RFBB. Use Equation 24 to determine the output voltage of the converter. · § R VOUT = VFB u ¨ 1 + FBT ¸ R FBB ¹ © (24) Typically, RFBT = 10 kΩ to 100 kΩ is recommended. Larger RFBT and RFBB values reduce the quiescent current going through the divider, which help maintain high efficiency at very light loads. But larger divider values also make the feedback path more susceptible to noise. If efficiency at very light loads is critical in a certain application, RFBT up to 1 MΩ can be used. VFB RFBB RFBT VOUT VFB (25) RFBT = 100 kΩ is selected here. RFBB = 24.99 kΩ can be calculated to get 5-V output voltage. 8.2.2.3 Switching Frequency The default switching frequency of the LM73605/6 device is set at 500 kHz. For this design, the RT pin can be floating, and the LM73605/6 switches at 500 kHz in CCM mode. An RT resistor of 78.7 kΩ, calculated using Equation 13, Figure 21, or Table 1, can be connected from RT pin to ground to obtain 500-kHz operation frequency as well. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 27 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com The LM73605/6 switching action can synchronize to an external clock from 350 kHz to 2.2 MHz. TI recommends connecting an external clock to the SYNC/MODE pin with a 50-Ω to 100-Ω termination resistor. The SYNC/MODE pin must be grounded if not used. RT pin is floating and SYNC/MODE pin is tied to ground in this design. 8.2.2.4 Input Capacitors The LM73605/6 device requires high-frequency ceramic input decoupling capacitors. Depending on the application, a bulk input capacitor can also be added. The typical recommended ceramic decoupling capacitors include one small, 0.1 µF to 1µF, and one large, 10 µF to 22 µF, capacitors. High-quality ceramic type X5R or X7R is recommended. The voltage rating must be greater than the maximum input voltage. As a general rule, to compensate the derating TI recommends a voltage rating of twice the maximum input voltage. It is very important in buck regulator to place the small decoupling capacitor right next to the PVIN and PGND pins. This capacitor is used to bypass the high frequency switching noise by providing a return path of the noise. It prevents the noise from spreading to wider area of the board. The large bypass ceramic capacitor must also be as close as possible to the PVIN and PGND pins. Additionally, some bulk capacitance may be required, especially if the LM73605/6 circuit is not located within approximately 2 inches from the input voltage source. This capacitor is used to provide damping to the voltage spike due to the lead inductance of the cable. The optimum value for this capacitor is four times the ceramic input capacitance with ESR close to the characteristic impedance of the LC filter formed by your input inductance and your ceramic input capacitors. It is not critical that the electrolytic filter be at the optimum value for damping, but it must be rated to handle the maximum input voltage including ripple voltage. For this design, two 10-µF, X7R dielectric capacitors rated for 50 V are used for the input decoupling capacitance, and a capacitor with a value of 0.47 µF for high-frequency filtering. NOTE DC bias effect: High capacitance ceramic capacitors have a DC bias derating effect, which will have a strong influence on the final effective capacitance. Therefore, the right capacitor value has to be chosen carefully. Package size and voltage rating in combination with dielectric material are responsible for differences between the rated capacitor value and the effective capacitance. 8.2.2.5 Inductor Selection The first criterion for selecting an output inductor is the inductance. In most buck converters, this value is based on the desired peak-to-peak ripple current in the inductor, ILripple. An inductance that gives a ripple current of 10% to 30% of the maximum output current (5 A or 6 A) is a good starting point. The inductance can be calculated from Equation 26: L VIN VOUT u D ¦SW u ILripple where • • • ILripple = (0.1 to 0.3) × IL_MAX IL_MAX = 5 A for LM73605 and 6 A for LM73606 D = VOUT / VIN (26) Selected ILripple is between 10% to 30% of the rated current of the device. As with switching frequency, the selection of the inductor is a tradeoff between size, cost, and performance. Higher inductance gives lower ripple current and hence lower output voltage ripple. With peak current mode control, the current ripple is the input signal to the control loop. A certain amount of ripple current is needed to maintain the signal to noise ratio of the control loop. Within the same series (same size/height), a larger inductance will have a higher series resistance (ESR). With similar ESR, size and/or height will be greater. Larger inductance also has slower current slew rate during large load transients. 28 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Lower inductance usually results in a smaller, less expensive component; however, the current ripple will be higher, thus more output capacitor is needed to maintain the same amount of output voltage ripple. The RMS current is higher with the same load current due to larger ripple. The switching loss is higher because the switch current, which is the peak current, is higher when the HS switch turns off and LS switch turns on. Core loss of the inductor is also larger with higher ripple. Core loss needs to be considered, especially with higher switching frequencies. Check the ripple current over VINMIN to VINMAX range to make sure current ripple is reasonable over entire supply voltage range. For applications with large VOUT and typical VOUT/VIN > 50%, sub-harmonic oscillation can be a concern in peak current-mode-controlled buck converters. Select inductance so that L ≥ VOUT / (N × FSW) where • • N = 3 with LM73605 N = 3.6 with LM73606 (27) The second criterion is inductor saturation current rating. Because the maximum inductor current is limited by the high-side switch current limit, it is advised to select an inductor with a saturation current higher than the ILIMIT-HS. TI recommends selection of soft saturation inductors. A power inductor could be the major source of radiated noise. When EMI is a concern in the application, select a shielded inductor, if possible. For this design, 20% ripple of 5 A yields 5.8-µH inductance. A 4.7-µH inductor is selected, which gives 25% ripple current. 8.2.2.6 Output Capacitor Selection The output capacitor is responsible for filtering the inductor current, and supplying load current during transients. Capacitor selection depends on application conditions as well as ripple and transient requirements. Best performance is achieved by using ceramic capacitors or combinations of ceramic and other types of capacitors. For high output voltage conditions, such as 12 V and above, finding ceramic capacitors that are rated for an appropriate voltage becomes challenging. In such cases choose a low-ESR SP-CAP™™ or POSCAP™-type capacitor. It is a good idea to use a low-value ceramic capacitor in parallel with other capacitors, to bypass high frequency noise between ground and VOUT. For a given input and output requirement, Equation 28 gives an approximation for a minimum output capacitor required. COUT ! (fSW ª§ r 2 · 1 u «¨ u (1 Dc) ¸ ¨ ¸ u r u 'VOUT / IOUT ) «¬© 12 ¹ º Dc u (1 r) » »¼ where • • r = Ripple ratio of the inductor ripple current (ILripple / 5 A or 6 A) ΔVOUT = Target output voltage undershoot, for example, 5% to 10% of VOUT D’ = 1 – duty cycle • • fSW = switching frequency IOUT = load current (28) Along with this for the same requirement, calculate the maximum ESR with Equation 29. ESR Dc 1 u ( 0.5) fSW u COUT r (29) The output capacitor is also the dominating factor in the loop response of a peak-current mode controlled buck converter. A simplified estimation of the control loop crossover frequency can be found by Equation 18. Select COUT so that the fX is no higher than 1/6 of the switching frequency. Typically, fX / fSW = 1/10 to 1/8 provides a good combination of stability and performance. For this design, one 0.47-µF, 50-V X7R and four 22-µF, 16-V, X7R ceramic capacitors are used in parallel. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 29 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com 8.2.2.7 Feed-Forward Capacitor The LM73605/6 is internally compensated. Typically, select RFBT ≤ 100 kΩ, then CFF is not needed. When very low quiescent current is needed, RFBT = 1 MΩ may be used. If COUT is mainly ceramic type low ESR capacitors, an external feed-forward capacitor CFF may be needed to improve the phase margin. Add CFF in parallel with RFBT. CFF is chosen such that the phase boost is maximized at the estimated crossover frequency fX. Equation 21 was tested. With this design, because RFBT = 100 kΩ is selected, no CFF is needed. 8.2.2.8 Bootstrap Capacitors Every LM73605/6 design requires a bootstrap capacitor, CBOOT. The recommended bootstrap capacitor is 0.47 µF and rated at 6.3 V or greater. The bootstrap capacitor is located between the SW pin and the CBOOT pin. The bootstrap capacitor must be a high-quality ceramic type with X7R or X5R grade dielectric for temperature stability. 8.2.2.9 VCC Capacitor The VCC pin is the output of an internal LDO for LM73605/6. The input for this LDO comes from either VIN or BIAS pin voltage. The recommended CVCC capacitor is 2.2 µF and rated at 6.3 V or greater. It must be a highquality ceramic type with X7R or X5R grade to insure stability. Never short VCC pin to ground during operation. 8.2.2.10 BIAS Because VOUT = 5 V in this design, the BIAS pin is tied to VOUT to reduce LDO power loss. The output voltage is supplying the LDO current instead of the input voltage. The power saving is ILDO × (VIN – VOUT). The power saving is more significant when VIN >> VOUT and with higher frequency operation. To prevent VOUT noise and transients from coupling to BIAS, a series resistor, 1 Ω to 10 Ω, may be added between VOUT and BIAS. A bypass capacitor with a value of 1 μF or higher can be added close to the BIAS pin to filter noise. 8.2.2.11 Soft Start The SS/TRK pin can be floating to start up following the internal soft-start ramp. In order to extend the soft-start time, an external soft-start capacitor can be used. Use Equation 12 in order to calculate the soft-start capacitor value. With a desired soft-start time tSS = 11 ms, a soft-start charging current of ISSC = 2 µA (typical), and VFB = 1.006 V (typical), Equation 12 yields a soft-start capacitor value of 22 nF. 8.2.2.12 Undervoltage Lockout Setpoint The system undervoltage lockout (UVLO) is adjusted using the external voltage divider network of RENT and RENB. With one selected RENT value, RENB can be found by Equation 10. Note that the divider adds to supply quiescent current by VIN / (RENT + RENB). Small RENT and RENB values add more quiescent current loss. However, large divider values make the node more sensitive to noise. In this design, EN pin is tied to PVIN pin with a 100-kΩ resistor. 8.2.2.13 PGOOD For this design, a 100-kΩ resistor is used to pull up PGOOD to VOUT. 30 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 100 100 95 95 90 90 85 85 Efficiency (%) Efficiency (%) 8.2.3 Application Curves 80 75 70 65 80 75 70 65 VIN = 5 V VIN = 8V VIN = 12 V 60 55 50 0.001 VIN = 5 V VIN = 8V VIN = 12 V 60 55 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 3.3 V 0.5 1 2 3 45 0 0.5 fSW = 500 kHz Auto Mode 2 2.5 3 Load Current (A) 3.5 4 4.5 5 EFF_ fSW = 500 kHz FPWM Mode Figure 28. LM73605 Efficiency 100 100 95 95 90 90 85 85 Efficiency (%) Efficiency (%) 1.5 VOUT = 3.3 V Figure 27. LM73605 Efficiency 80 75 70 65 80 75 70 65 VIN = 5 V VIN = 8V VIN = 12 V 60 55 50 0.001 VIN = 5 V VIN = 8V VIN = 12 V 60 55 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 3.3 V 0.5 1 2 3 45 0 0.5 1 EFF_ fSW = 2200 kHz Auto Mode VOUT = 3.3 V Figure 29. LM73605 Efficiency 1.5 2 2.5 3 Load Current (A) 3.5 4 4.5 5 EFF_ fSW = 2200 kHz FPWM Mode Figure 30. LM73605 Efficiency 100 100 95 95 90 90 85 85 Efficiency (%) Efficiency (%) 1 EFF_ 80 75 70 65 80 75 70 65 VIN = 5 V VIN = 8V VIN = 12 V 60 55 50 0.001 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 3.3 V 0.5 1 VIN = 5 V VIN = 8V VIN = 12 V 60 55 2 3 45 50 0.001 0.01 0.02 0.05 0.1 0.2 Load Current (A) EFF_ fSW = 350 kHz Auto Mode VOUT = 3.3 V Figure 31. LM73605 Efficiency 0.5 1 fSW = 1000 kHz 2 3 45 EFF_ Auto Mode Figure 32. LM73605 Efficiency Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 31 LM73605, LM73606 www.ti.com 100 100 95 95 90 90 85 85 Efficiency (%) Efficiency (%) SNVSAH5 – SEPTEMBER 2017 80 75 70 VIN = 7 V VIN = 12 V VIN = 24 V VIN = 36 V 65 60 80 75 70 VIN = 7 V VIN = 12 V VIN = 24 V VIN = 36 V 65 60 55 55 50 0.001 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 5 V 0.5 1 2 3 4 56 0 0.6 1.2 fSW = 500 kHz Auto Mode VOUT = 5 V 100 100 95 95 90 90 85 85 80 75 70 VIN = 7 V VIN = 12 V VIN = 24 V 60 4.8 5.4 6 EFF_ fSW = 500 kHz FPWM Mode 80 75 70 VIN = 7 V VIN = 12 V VIN = 24 V 60 55 50 0.001 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 5 V 0.5 1 2 3 4 56 0 0.6 1.2 fSW = 1000 kHz Auto Mode VOUT = 5 V 95 95 90 90 85 85 Efficiency (%) 100 80 75 70 VIN = 7 V VIN = 12 V VIN = 24 V 60 2.4 3 3.6 Load Current (A) 4.2 5.4 6 EFF_ fSW = 1000 kHz FPWM Mode 80 75 70 65 VIN = 7 V VIN = 12 V VIN = 24 V 60 55 55 50 0.001 4.8 Figure 36. LM73606 Efficiency 100 65 1.8 EFF_ Figure 35. LM73606 Efficiency Efficiency (%) 4.2 65 55 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 5 V 0.5 1 2 3 4 56 0 0.6 1.2 EFF_ fSW = 2200 kHz Auto Mode VOUT = 5 V Figure 37. LM73606 Efficiency 32 2.4 3 3.6 Load Current (A) Figure 34. LM73606 Efficiency Efficiency (%) Efficiency (%) Figure 33. LM73606 Efficiency 65 1.8 EFF_ Submit Documentation Feedback 1.8 2.4 3 3.6 Load Current (A) 4.2 4.8 fSW = 2200 kHz 5.4 6 EFF_ FPWM Mode Figure 38. LM73606 Efficiency Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 100 100 95 95 90 90 85 85 Efficiency (%) Efficiency (%) www.ti.com 80 75 70 65 80 75 70 65 VIN = 14 V VIN = 24V VIN = 36 V 60 55 50 0.001 VIN = 14 V VIN = 24V VIN = 36 V 60 55 50 0.01 0.02 0.05 0.1 0.2 Load Current (A) VOUT = 12 V 0.5 1 2 3 45 0 0.5 1 fSW = 500 kHz Auto Mode 5.16 5.08 5.12 5.06 5.08 5.04 Output Voltage (V) Output Voltage (V) 5.1 5.04 5 4.96 VIN = 7 V VIN = 12 V VIN = 24 V VIN = 36 V 4.84 4.8 0.001 VOUT = 5 V 5 EFF_ fSW = 500 kHz FPWM Mode 5 4.96 VIN = 7 V VIN = 12 V VIN = 24 V VIN = 36 V 4.94 4.92 4.9 0.01 0.02 0.05 0.1 0.2 0.5 Load Current (A) 1 2 3 4 5 7 10 0 fSW = 500 kHz Auto Mode 1.8 2.4 3 3.6 Load Current (A) 4.2 4.8 5.4 6 REG_ fSW = 500 kHz FPWM Mode Figure 42. LM73606 Load and Line Regulation 5.05 5.04 5.12 5.03 5.08 5.02 5.04 5 4.96 4.92 VIN = 7 V VIN = 12 V VIN = 24 V 4.84 1.2 VOUT = 5 V 5.16 4.88 0.6 REG_ Output Voltage (V) Output Voltage (V) 4.5 4.98 5.2 VOUT = 5 V 4 5.02 Figure 41. LM73606 Load and Line Regulation 4.8 0.001 3.5 Figure 40. LM73605 Efficiency 5.2 4.88 2 2.5 3 Load Current (A) VOUT = 12 V Figure 39. LM73605 Efficiency 4.92 1.5 EFF_ 5.01 5 4.99 4.98 VIN = 7 V VIN = 12 V VIN = 24 V 4.97 4.96 4.95 0.01 0.02 0.05 0.1 0.2 Load Current (A) fSW = 2200 kHz 0.5 1 2 3 45 0 0.5 1 REG_ Auto Mode Figure 43. LM73605 Load and Line Regulation VOUT = 5 V 1.5 2 2.5 3 Load Current (A) fSW = 2200 kHz 3.5 4 4.5 5 REG_ FPWM Mode Figure 44. LM73605 Load and Line Regulation Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 33 LM73605, LM73606 www.ti.com 3.4 3.32 3.38 3.315 3.36 3.31 3.34 3.305 Output Voltage (V) Output Voltage (V) SNVSAH5 – SEPTEMBER 2017 3.32 3.3 3.28 3.26 VIN = 5 V VIN = 8V VIN = 12 V 3.24 3.22 3.2 0.001 VOUT = 3.3 V 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 1 3.3 3.295 3.29 3.28 3.275 3.27 0.001 2 3 45 REG_ fSW = 2200 kHz Auto Mode VOUT = 3.3 V 3.4 3.4 3.38 3.38 3.36 3.36 3.34 3.34 3.32 3.3 3.28 3.26 VIN = 5 V VIN = 8V VIN = 12 V 3.24 3.22 VOUT = 3.3 V 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 1 FPWM Mode 3.3 3.26 VIN = 5 V VIN = 8V VIN = 12 V 3.22 3.2 0.001 2 3 45 Auto Mode VOUT = 3.3 V 0.01 0.02 0.05 0.1 0.2 Load Current (A) 0.5 1 2 3 45 REG_ fSW = 1000 kHz Auto Mode Figure 48. LM73605 Load and Line Regulation 12.1 12.08 12.06 VIN = 14 V VIN = 24V VIN = 36 V 0.01 0.02 0.05 0.1 0.2 Load Current (A) fSW = 500 kHz 12.04 12.02 12 11.98 11.96 VIN = 14 V VIN = 24V VIN = 36 V 11.94 11.92 0.5 1 2 3 45 11.9 0.001 REG_ Auto Mode Figure 49. LM73605 Load and Line Regulation 34 2 3 45 REG_ fSW = 2200 kHz 3.24 Output Voltage (V) Output Voltage (V) VOUT = 12 V 1 3.28 Figure 47. LM73605 Load and Line Regulation 12.4 12.36 12.32 12.28 12.24 12.2 12.16 12.12 12.08 12.04 12 11.96 11.92 11.88 11.84 11.8 0.001 0.5 3.32 REG_ fSW = 500 kHz 0.01 0.02 0.05 0.1 0.2 Load Current (A) Figure 46. LM73605 Load and Line Regulation Output Voltage (V) Output Voltage (V) Figure 45. LM73605 Load and Line Regulation 3.2 0.001 VIN = 5 V VIN = 8V VIN = 12 V 3.285 VOUT = 12 V 0.01 0.02 0.05 0.1 0.2 Load Current (A) fSW = 500 kHz 0.5 1 2 3 45 REG_ FPWM Mode Figure 50. LM73605 Load and Line Regulation Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4 3.8 3.6 3.4 3.2 Output Voltage (V) Output Voltage (V) www.ti.com Load = 1.5mA Load = 1A Load = 3A Load = 5A 4 4.4 4.8 VOUT = 5 V 5.2 5.6 VIN (V) 6 6.4 6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4 3.8 3.6 3.4 3.2 6.8 Load = 1.5mA Load = 1A Load = 3A Load = 5A 4 4.4 fSW = 2200 kHz Auto Mode VOUT = 5 V 6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4 3.8 3.6 3.4 3.2 Load = 1.5mA Load = 1A Load = 3A Load = 5A 4.4 4.8 5.2 VOUT = 5 V 5.6 VIN (V) 6 6.4 6.8 Auto Mode Output Voltage (V) Output Voltage (V) 3.2 3.1 3 2.9 Load = 1.5mA Load = 1A Load = 3A Load = 5A 3.7 VOUT = 3.3 V 3.9 4.1 4.3 VIN (V) 4.5 4.7 4.8 4.9 13 12.8 12.6 12.4 12.2 12 11.8 11.6 11.4 11.2 11 10.8 10.6 10.4 10.2 10 11 Auto Mode Figure 55. LM73605 Dropout Curve DO_5 FPWM Mode 5.2 5.6 VIN (V) 6 6.4 6.8 DO_5 fSW = 1000 kHz Auto Mode Load = 1.5mA Load = 1A Load = 3A Load = 5A 11.4 DO_3 fSW = 500 kHz 6.8 Figure 54. LM73605 Dropout Curve 3.3 3.5 4.4 VOUT = 5 V 3.4 2.5 3.3 6.4 Load = 1.5mA Load = 1A Load = 3A Load = 5A 4 3.5 2.6 6 6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4 3.8 3.6 3.4 3.2 Figure 53. LM73605 Dropout Curve 2.7 5.6 VIN (V) fSW = 2200 kHz DO_5 fSW = 500 kHz 2.8 5.2 Figure 52. LM73605 Dropout Curve Output Voltage (V) Output Voltage (V) Figure 51. LM73605 Dropout Curve 4 4.8 DO_5 VOUT = 12 V 11.8 12.2 12.6 VIN (V) 13 13.4 fSW = 500 kHz 13.8 DO_1 Auto Mode Figure 56. LM73605 Dropout Curve Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 35 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com IINDUCTOR IINDUCTOR (1 A/DIV) (1 A/DIV) VOUT Ripple (20 mV/DIV) VOUT Ripple (20 mV/DIV) VSW (5 V/DIV) VSW (5 V/DIV) Time (500 µs/DIV) VIN = 12 V IOUT = 1 mA VOUT = 3.3 V Auto Mode Time (2 µs/DIV) fSW = 500 kHz Figure 57. LM73606 Switching Waveform and VOUT Ripple VIN = 12 V IOUT = 1 mA VOUT = 3.3 V FPWM Mode fSW = 500 kHz Figure 58. LM73606 Switching Waveform and VOUT Ripple IINDUCTOR IINDUCTOR (1 A/DIV) (1 A/DIV) VOUT Ripple (20 mV/DIV) VOUT Ripple (20 mV/DIV) VSW (5 V/DIV) VSW (5 V/DIV) Time (5 µs/DIV) VIN = 12 V IOUT = 100 mA VOUT = 3.3 V Auto Mode Time (5 µs/DIV) fSW = 500 kHz Figure 59. LM73606 Switching Waveform and VOUT Ripple VIN = 12 V IOUT = 100 mA IINDUCTOR (2 A/DIV) (1 A/DIV) VOUT Ripple (20 mV/DIV) VOUT Ripple (20 mV/DIV) VSW (5 V/DIV) VSW (5 V/DIV) Time (2 µs/DIV) VOUT = 3.3 V Auto Mode Time (5 µs/DIV) fSW = 500 kHz Figure 61. LM73606 Switching Waveform and VOUT Ripple 36 fSW = 500 kHz Figure 60. LM73606 Switching Waveform and VOUT Ripple IINDUCTOR VIN = 12 V IOUT = 6 A VOUT = 3.3 V FPWM Mode VIN = 3.66 V IOUT = 3 A VOUT = 3.3 V Auto Mode fSW set at 500 kHz Figure 62. LM73606 Switching Waveform at Dropout Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 IINDUCTOR (2 A/DIV) VOUT (1 V/DIV) VOUT (1 V/DIV) IINDUCTOR (2 A/DIV) VSW (5 V/DIV) VSW (5 V/DIV) Time (5 µs/DIV) VIN = 12 V IOUT = 7.5 A VOUT set at 3.3 V VOUT droops to 2 V Time (50 ms/DIV) fSW set at 500 kHz Figure 63. LM73606 Overcurrent Behavior VIN = 12 V Enable (5 V/DIV) VOUT (2 V/DIV) VOUT (2 V/DIV) IINDUCTOR (2 A/DIV) IINDUCTOR (2 A/DIV) PGOOD PGOOD (10 V/DIV) (10 V/DIV) Time (2 ms/DIV) VOUT = 3.3 V FPWM Mode fSW = 500 kHz Figure 64. LM73606 Short-Circuit Hiccup Protection and Recovery Enable (5 V/DIV) VIN = 12 V IOUT= 200 mA VOUT = 3.3 V Time (2 ms/DIV) fSW = 500 kHz Figure 65. LM73606 Soft Start With 200-mA Load in FPWM Mode VIN = 12 V IOUT= 200 mA fSW = 500 kHz Figure 66. LM73606 Soft Start With 200-mA Load in Auto Mode Enable (5 V/DIV) Enable (5 V/DIV) VOUT (2 V/DIV) VOUT (2 V/DIV) IINDUCTOR (2 A/DIV) IINDUCTOR (2 A/DIV) PGOOD PGOOD (5 V/DIV) VOUT = 3.3 V Auto Mode (5 V/DIV) Time (2 ms/DIV) VIN = 12 V IOUT= 5 A VOUT = 3.3 V Auto Mode Time (2 ms/DIV) fSW = 500 kHz Figure 67. LM73606 Soft Start With 5-A Load VIN = 12 V VPRE-BIAS= 1.5 V VOUT = 3.3 V Auto Mode fSW = 500 kHz Figure 68. LM73606 Soft Start With Pre-Biased Output Voltage Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 37 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com IOUT IOUT (5 A/DIV) (5 A/DIV) IINDUCTOR IINDUCTOR (5 A/DIV) (5 A/DIV) VOUT (200 mV/ DIV AC) VOUT (200 mV/ DIV AC) Time (200 µs/DIV) VIN = 12 V VOUT = 3.3 V IOUT = 10 mA to 6 A to 10 mA Time (200 µs/DIV) fSW = 500 kHz Auto Mode VIN = 12 V VOUT = 3.3 V IOUT = 10 mA to 6 A to 10 mA Figure 69. LM73606 Load Transients Figure 70. LM73606 Load Transients IOUT IOUT (5 A/DIV) (5 A/DIV) IINDUCTOR IINDUCTOR (5 A/DIV) (5 A/DIV) VOUT (500 mV/ DIV AC) VOUT (500 mV/ DIV AC) Time (200 µs/DIV) VIN = 12 V VOUT = 5 V IOUT = 10 mA to 5 A to 10 mA Time (200 µs/DIV) fSW = 2200 kHz Auto Mode VIN = 12 V VOUT = 5 V IOUT = 10 mA to 5 A to 10 mA Figure 71. LM73605 Load Transients VIN VIN (20 V/DIV) VOUT (200 mV/ DIV) VOUT (200 mV/ DIV) IINDUCTOR IINDUCTOR (2 A/DIV) (2 A/DIV) Time (200 µs/DIV) VOUT = 3.3 V Time (200 µs/DIV) fSW = 500 kHz Auto Mode Figure 73. LM73606 Line Transients 38 fSW = 2200 kHz FPWM Mode Figure 72. LM73605 Load Transients (20 V/DIV) IOUT = 100 mA VIN = 10 V to 35 V to 10 V fSW = 500 kHz FPWM Mode IOUT = 2 A VIN = 10 V to 35 V to 10 V VOUT = 3.3 V fSW = 500 kHz Auto Mode Figure 74. LM73606 Line Transients Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 9 Power Supply Recommendations The LM73605/6 is designed to operate from an input voltage supply range from 3.5 V to 36 V. This input supply must be able to withstand the maximum input current and maintain a voltage above 3.5 V at the PVIN pin. The resistance of the input supply rail must be low enough that an input current transient does not cause a high enough drop at the LM73605/6 supply voltage that can cause a false UVLO fault triggering and system reset. If the input supply is located more than a few inches from the LM73605/6, additional bulk capacitance may be required in addition to the ceramic bypass capacitors. A 47-μF or 100-μF electrolytic capacitor is a typical choice. 10 Layout 10.1 Layout Guidelines The performance of any switching converter depends heavily upon the layout of the PCB. Use the following guidelines to design a PCB layout with optimum power conversion performance, EMI performance, and thermal performance. 1. Place ceramic high frequency bypass capacitors as close as possible to the PVIN and PGND pins, which are right next to each other on the package. Place the small value ceramic capacitor closest to the pins. This is very important for EMI performance. 2. Use short and wide traces, or localized IC layer planes, for high current paths, such as VIN, VOUT, SW and GND connections. Short and wide copper traces reduce power loss and noise due to low parasitic resistances and inductances. Wide copper traces also help reduce die temperature, because they also provide wide heat dissipation paths. Use thick copper (2 oz) on high current layer(s) if possible. 3. Confine pulsing current paths (VIN, SW, and ground return for VIN) on the device layer as much as possible to prevent switching noises from contaminating other layers. 4. CBOOT capacitor also contains pulsing current. Place CBOOT close to the pin and route to SW with short trace. The pinout of the device makes it easy to optimize the CBOOT placement and routing. 5. Use a solid ground plane at the layer right underneath the device as a noise shielding and heat dissipation path. 6. Place the VCC bypass capacitor close to the VCC pin. Tie the ground pad of the capacitor to the ground plane using a via right next to it. 7. Use via next to AGND pin to the ground plane. 8. Minimize trace length to the FB pin. Both feedback resistors must be located right next to the FB pin. Tie the ground side of RFBB to the ground plane with a via right next to it. Place CFF directly in parallel with RFBT if used. 9. If VOUT accuracy at the load is important, make sure the VOUT sense point is made close to the load. Route VOUT sense to RFBT through a path away from noisy nodes and preferably on a layer on the other side of the ground plane. If BIAS is connected to VOUT, do not use the same trace to route VOUT to BIAS and to RFBT. BIAS current contains pulsing driver current and it changes with operating mode. Use separated traces for BIAS and VOUT sense to optimize VOUT regulation accuracy. 10. Provide adequate device heat sinking. Use an array of heat-sinking vias to connect the exposed pad to the ground plane and the bottom PCB layer. Connect the DAP and NC pins on the short sides of the device to the GND net, so that IC layer ground copper can provide an optimal dog-bone shape heat sink. Heat generated on the die can flow directly from device junction to the DAP then to the copper and spread to the wider copper outside of the device. Try to keep copper area solid on the top and bottom layer around thermal vias on the DAP to optimize heat dissipation. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 39 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com Layout Guidelines (continued) 10.1.1 Layout For EMI Reduction To optimize EMI performance, place the components in the high di/dt current path, as shown in Figure 75, as close as possible to each other. When the components are close to each other, the area of the loop enclosed by these components, and the parasitic inductance of this loop, are minimized. The noises generated by the pulsing current and parasitic inductances are then minimized. BUCK CONVERTER VIN VIN SW L CIN VOUT COUT PGND High di/dt current PGND Figure 75. Pulsing Current Path of Buck Converter In a buck converter, the high di/dt current path is composed of the HS and LS MOSFETs and the input capacitors. Because the two MOSFETs are integrated inside the device, they are closer to each other than in discrete solutions. PVIN and PGND pins are the connections from the MOSFETs to the input capacitors. The first step of the layout must be placing the input capacitors, especially the small value ceramic bypass one, as close as possible to PVIN and PGND pins. The LM73605/6 pin out is optimized for low EMI layout. Multiple pins are used for PVIN and PGND to minimized bond wire resistances and inductances. The PVIN and PGND pins are right next to each other to simplify optimal layout. The CBOOT pin is placed next to SW pin for easy and compact CBOOT capacitor layout. 10.1.2 Ground Plane The ground plane of a PCB provides the best return path for the pulsing current on the device layer. Make sure the ground plane is solid, especially the part right underneath the pulsing current paths. Solid copper under a pulsing current path provide a mirrored return path for the high frequency components and minimize voltage spikes generated by the pulsing current. It shields the layers on the other side of the plane from switching noises. Route signal traces on the other side of the ground plane as much as possible. Use multiple vias in parallel to connect the grounds on the device layer to the ground plane. 10.1.3 Optimize Thermal Performance The key to thermal optimization on PCB design is to provide heat transferring paths from the device to the outer large copper area. Use thick copper (2 oz) on high current layer(s) if possible. Use thermal vias under the DAP to transfer heat to other layers. Connect NC pins to the GND net, so that GND copper can run underneath the device to create dog-bone shape heat sink. Try to leave copper solid on IC side as much as possible above and below the device. Place components and route traces away from major heat transferring paths if possible, to avoid blocking heat dissipation path. Try to leave copper solid, free of components and traces, around the thermal vias on the other side of the board as well. Solid copper behaves as heat sink to spread the heat to a larger area and provide more contact area to the air. When calculating power dissipation, use the maximum input voltage and the average output current for the application. Many common operating conditions are provided in Application Curves. Less common applications can be derived through interpolation. In all designs, the junction temperature must be kept below the rated maximum of 125°C. 40 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 Layout Guidelines (continued) The thermal characteristics of the LM73605/6 are specified using the parameter RθJA, which characterize thermal resistance from the junction of the silicon to the ambient in a specific system. Although the value of RθJA is dependant on many variables, it still can be used to approximate the operating junction temperature of the device. To obtain an estimate of the device junction temperature, one may use Equation 30: TJ = PIC_LOSS × RθJA + TA where • • • • • TJ = Junction temperature in °C PIC_LOSS = VIN × IIN × (1 − Efficiency) − 1.1 × IOUT × DCR DCR = Inductor DC parasitic resistance in Ω RθJA = Junction-to-ambient thermal resistance of the device in °C/W TA = Ambient temperature in °C. (30) The maximum operating junction temperature of the LM73605/6 is 125°C. RθJA is highly related to PCB size and layout, as well as environmental factors such as heat sinking and air flow. Figure 76 shows measured results of RθJA with different copper area on 2-layer boards and 4-layer boards, with 1-W and 2-W power dissipation on the LM73605/6. 30 1W @0 fpm - 2layer 1W @0 fpm - 4layer 2W @0 fpm - 2layer 2W @0 fpm - 4layer 28 R,JA (°C/W) 26 24 22 20 18 16 14 12 10 20 30mm 30 × 30mm 40mm 40 × 40mm 50mm 50 × 50mm 60 70mm 70 ×70mm 80 Copper Area Figure 76. Measured RθJA vs PCB Copper Area on 2-Layer Boards and 4-Layer Boards Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 41 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com 10.2 Layout Example A layout example is shown in Figure 77. A four-layer board is used with 2-oz copper on the top and bottom layers and 1-oz copper on the inner two layers. Figure 77 shows the relative scale of the LM73605/6 device with 0805 and 1210 input and output capacitors, 7-mm × 7-mm inductor and 0603 case size for all other passive components. The trace width of the signal connections are not to scale. The components are placed on the top layer and the high current paths are routed on the top layer as well. The remaining space on the top layer can be filled with GND polygon. Thermal vias are used under the DAP and around the device. The GND copper was extended to the outside of the device, which serves as copper heat sink. The mid-layer 1 is right underneath the top layer. It is a solid ground plane, which serves as noise shielding and heat dissipation path. The VOUT sense trace is routed on the 3rd layer, which is mid-layer 2. Ground plane provided noise shielding for the sense trace. The VOUT to BIAS connection is routed by a separate trace. The bottom layer is also a solid ground copper in this example. Solid copper provides best heat sinking for the device. If components and traces need to be on the bottom layer, please leave the area around thermal vias as solid as possible. Try not to cut heat dissipation path by a trace. The board can be used for various frequencies and output voltages, with component variation. For more details, see the LM73605/LM73606 EVM User's Guide. Figure 77. LM73605/6 Layout Example 42 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 11 Device and Documentation Support 11.1 Device Support 11.1.1 Third-Party Products Disclaimer TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE. 11.1.2 Custom Design With WEBENCH® Tools Click here to create a custom design using the LM73605 or LM73606 device with the WEBENCH® Power Designer. 1. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements. 2. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial. 3. Compare the generated design with other possible solutions from Texas Instruments. The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time pricing and component availability. In most cases, these actions are available: • Run electrical simulations to see important waveforms and circuit performance • Run thermal simulations to understand board thermal performance • Export customized schematic and layout into popular CAD formats • Print PDF reports for the design, and share the design with colleagues Get more information about WEBENCH tools at www.ti.com/WEBENCH. 11.2 Related Documentation For related documentation see the following: AN-2020 Thermal Design By Insight, Not Hindsight 11.3 Related Links The table below lists quick access links. Categories include technical documents, support and community resources, tools and software, and quick access to order now. Table 5. Related Links PARTS PRODUCT FOLDER ORDER NOW TECHNICAL DOCUMENTS TOOLS & SOFTWARE SUPPORT & COMMUNITY LM73605 Click here Click here Click here Click here Click here LM73606 Click here Click here Click here Click here Click here 11.4 Receiving Notification of Documentation Updates To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper right corner, click on Alert me to register and receive a weekly digest of any product information that has changed. For change details, review the revision history included in any revised document. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 43 LM73605, LM73606 SNVSAH5 – SEPTEMBER 2017 www.ti.com 11.5 Community Resources The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of Use. TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help solve problems with fellow engineers. Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and contact information for technical support. 11.6 Trademarks E2E is a trademark of Texas Instruments. WEBENCH is a registered trademark of Texas Instruments. SP-CAP is a trademark of Panasonic. POSCAP is a trademark of Sanyo Electric Co., Ltd.. 11.7 Electrostatic Discharge Caution These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. 11.8 Glossary SLYZ022 — TI Glossary. This glossary lists and explains terms, acronyms, and definitions. 44 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 LM73605, LM73606 www.ti.com SNVSAH5 – SEPTEMBER 2017 12 Mechanical, Packaging, and Orderable Information The following pages include mechanical, packaging, and orderable information. This information is the most current data available for the designated devices. This data is subject to change without notice and revision of this document. For browser-based versions of this data sheet, refer to the left-hand navigation. Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Product Folder Links: LM73605 LM73606 45 PACKAGE OPTION ADDENDUM www.ti.com 28-Sep-2017 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan Lead/Ball Finish MSL Peak Temp (2) (6) (3) Op Temp (°C) Device Marking (4/5) LM73605RNPR ACTIVE WQFN RNP 30 3000 Green (RoHS & no Sb/Br) CU SN Level-2-260C-1 YEAR -40 to 125 LM73605R NP LM73605RNPT ACTIVE WQFN RNP 30 250 Green (RoHS & no Sb/Br) CU SN Level-2-260C-1 YEAR -40 to 125 LM73605R NP LM73606RNPR ACTIVE WQFN RNP 30 3000 Green (RoHS & no Sb/Br) CU SN Level-2-260C-1 YEAR -40 to 125 LM73606R NP LM73606RNPT ACTIVE WQFN RNP 30 250 Green (RoHS & no Sb/Br) CU SN Level-2-260C-1 YEAR -40 to 125 LM73606R NP (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based flame retardants must also meet the <=1000ppm threshold requirement. (3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. (4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device. (5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation of the previous line and the two combined represent the entire Device Marking for that device. (6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish value exceeds the maximum column width. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and Addendum-Page 1 Samples PACKAGE OPTION ADDENDUM www.ti.com 28-Sep-2017 continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release. 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Addendum-Page 2 PACKAGE MATERIALS INFORMATION www.ti.com 28-Sep-2017 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing LM73605RNPR WQFN RNP 30 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant 3000 330.0 16.4 4.25 6.25 0.95 8.0 16.0 Q1 LM73605RNPT WQFN RNP 30 250 180.0 16.4 4.25 6.25 0.95 8.0 16.0 Q1 LM73606RNPR WQFN RNP 30 3000 330.0 16.4 4.25 6.25 0.95 8.0 16.0 Q1 LM73606RNPT WQFN RNP 30 250 180.0 16.4 4.25 6.25 0.95 8.0 16.0 Q1 Pack Materials-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 28-Sep-2017 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM73605RNPR WQFN RNP 30 3000 370.0 355.0 55.0 LM73605RNPT WQFN RNP 30 250 195.0 200.0 45.0 LM73606RNPR WQFN RNP 30 3000 370.0 355.0 55.0 LM73606RNPT WQFN RNP 30 250 195.0 200.0 45.0 Pack Materials-Page 2 PACKAGE OUTLINE RNP0030A WQFN - 0.8 mm max height SCALE 2.700 PLASTIC QUAD FLATPACK - NO LEAD 4.1 3.9 A B PIN 1 INDEX AREA 6.1 5.9 0.1 MIN (0.05) SECTION A-A SECTION A-A SCALE 25.000 TYPICAL 0.8 MAX C SEATING PLANE 0.05 0.00 0.08 2.2 0.1 2X 1.5 26X 0.5 (0.2) TYP EXPOSED THERMAL PAD 15 12 11 16 SYMM A A 4.6 0.1 2X 5 1 26 PIN 1 ID (OPTIONAL) 0.5 8X 0.3 30 SYMM 27 30X 0.65 22X 0.45 0.3 0.2 0.1 0.05 C A B 4222145/B 09/2017 NOTES: 1. All linear dimensions are in millimeters. Any dimensions in parenthesis are for reference only. Dimensioning and tolerancing per ASME Y14.5M. 2. This drawing is subject to change without notice. 3. The package thermal pad must be soldered to the printed circuit board for thermal and mechanical performance. www.ti.com EXAMPLE BOARD LAYOUT RNP0030A WQFN - 0.8 mm max height PLASTIC QUAD FLATPACK - NO LEAD (2.2) SYMM 30 8X (0.6) 27 22X (0.75) 1 26 30X (0.25) 2X (2.05) (0.5) TYP (5.8) SYMM (4.6) 6X (1.16) (R0.05) TYP 16 11 ( 0.2) TYP VIA 15 12 6X (0.85) (3.65) LAND PATTERN EXAMPLE SCALE:15X 0.07 MAX ALL AROUND 0.07 MIN ALL AROUND SOLDER MASK OPENING METAL SOLDER MASK OPENING METAL UNDER SOLDER MASK NON SOLDER MASK DEFINED (PREFERRED) SOLDER MASK DEFINED SOLDER MASK DETAILS 4222145/B 09/2017 NOTES: (continued) 4. This package is designed to be soldered to a thermal pad on the board. For more information, see Texas Instruments literature number SLUA271 (www.ti.com/lit/slua271). www.ti.com EXAMPLE STENCIL DESIGN RNP0030A WQFN - 0.8 mm max height PLASTIC QUAD FLATPACK - NO LEAD SYMM (0.59) TYP 27 30 8X (0.6) 22X (0.75) 1 26 30X (0.25) 26X (0.5) (1.16) TYP (0.58) TYP SYMM (5.8) METAL TYP 8X (0.96) 16 11 (R0.05) TYP 12 15 8X (0.98) (3.65) SOLDER PASTE EXAMPLE BASED ON 0.125 mm THICK STENCIL EXPOSED PAD 74.4% PRINTED SOLDER COVERAGE BY AREA SCALE:20X 4222145/B 09/2017 NOTES: (continued) 5. Laser cutting apertures with trapezoidal walls and rounded corners may offer better paste release. IPC-7525 may have alternate design recommendations. www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated (TI) reserves the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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