LM3100 SIMPLE SWITCHER® Synchronous 1MHz 1.5A Step-Down Voltage Regulator General Description Features The LM3100 Synchronously Rectified Buck Converter features all functions needed to implement a highly efficient, cost effective buck regulator capable of supplying 1.5A to loads with voltages as low as 0.8V. Dual 40V N-Channel synchronous MOSFET switches allow for low external component thus reducing complexity and minimizing board space. The LM3100 is designed to work exceptionally well with ceramic and other very low ESR output capacitors. The Constant ON-Time (COT) regulation scheme requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. Through the use of a unique design the regulator does not rely on output capacitor ESR for stability, as do most other COT regulators. The operating frequency remains nearly constant with line and load variations due to the inverse relationship between the input voltage and the on-time. The oprating frequency can be externally programmed up to 1MHz. Protection features include VCC under-voltage lockout, thermal shutdown and gate drive undervoltage lockout. The part is available in a thermally enhanced eTSSOP-20 package ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Input voltage range 4.5V - 36V 1.5A output current 0.8V, ±1.5% reference Integrated 40V, dual N-Channel buck synchronous switches Low component count and small solution size No loop compensation required Ultra-fast transient response Stable with ceramic and other low ESR capacitors Programmable switching frequency up to 1MHz Max. duty cycle limited during start-up Valley current limit Precision Internal Reference for adjustable output voltage down to 0.8V Thermal shutdown Thermally enhanced eTSSOP-20 package Typical Applications ■ ■ ■ ■ ■ ■ ■ ■ 5VDC, 12VDC, 24VDC, 12VAC, and 24VAC systems Embedded Systems Industrial Controls Automotive Telematics and Body Electronics Point of Load Regulators Storage Systems Broadband Infrastructure Direct Conversion from 2/3/4 Cell Lithium Batteries Systems Typical Application 20174702 SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation © 2007 National Semiconductor Corporation 201747 www.national.com LM3100 SIMPLE SWITCHER® Synchronous 1MHz 1.5A Step-Down Voltage Regulator August 2007 LM3100 Connection Diagram 20174703 20-lead Plastic eTSSOP (MXA20A) Ordering Information Order Number Package Type NSC Package Drawing LM3100MH Exposed Pad TSSOP-20 MXA0020 LM3100MHX www.national.com Supplied As 73 units per Anti-Static Tube 2500 Units on Tape and Reel 2 LM3100 Pin Descriptions Pin Name Description 1,9,10,12,19,20 N/C No Connection Application Information These pins must be left unconnected. 2, 3 SW Switching Node Internally connected to the buck switch source. Connect to output inductor. 4, 5 VIN Input supply voltage Supply pin to the device. Nominal input range is 4.5V to 36V. 6 BST Connection for bootstrap capacitor Connect a 0.033µF capacitor from SW pin to this pin. An internal diode charges the capacitor during the high-side switch off-time. 7 GND Analog Ground 8 SS Soft-start 11 TST Test mode enable pin Force the device into test mode. Must be connected to ground for normal operation. 13 FB Feedback Internally connected to the regulation and over-voltage comparators. The regulation setting is 0.8V at this pin. Connect to feedback divider. 14 EN Enable pin Connect a voltage higher than 1.26V to enable the regulator. 15 RON On-time Control An external resistor from VIN to this pin sets the highside switch on-time. 16 VCC Start-up regulator Output Nominally regulated to 6V. Connect a capacitor of not less than 680nF between VCC and GND for stable operation. 17, 18 PGND Power Ground Synchronous rectifier MOSFET source connection. Tie to power ground plane. DAP EP Exposed Pad Thermal connection pad, connect to GND. Ground for all internal circuitry other than the synchronous switches. An internal 8µA current source charges an external capacitor to provide the soft- start function. 3 www.national.com LM3100 ESD Rating (Note 2) Human Body Model Storage Temperature Range Junction Temperature (TJ) Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN, RON to GND SW to GND SW to GND (Transient) VIN to SW BST to SW All Other Inputs to GND -0.3V to 40V -0.3V to 40V -2V (< 100ns) -0.3V to 40V -0.3V to 7V -0.3V to 7V Operating Ratings ±2kV -65°C to +150°C 150°C (Note 1) Supply Voltage Range (VIN) Junction Temperature Range (TJ) 4.5V to 36V −40°C to + 125°C Thermal Resistance (θJC) (Note 3) 6.5°C/W Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed 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 = 18V, VOUT = 3.3V. Symbol Parameter Conditions Min Typ Max Units 5.0 6.0 7.2 V mV Start-Up Regulator, VCC VCC VIN - VCC IVCCL VCC output voltage CCC = 680nF, no load VIN - VCC dropout voltage ICC = 2mA 50 140 ICC = 20mA 350 570 VCC current limit (Note 4) VCC = 0V 40 65 VCC under-voltage lockout threshold (UVLO) VIN increasing 3.6 3.75 VCC-UVLO-HYS VCC UVLO hysteresis VIN decreasing tVCC-UVLO-D VCC UVLO filter delay VCC-UVLO IIN IIN-SD IIN operating current No switching, VFB = 1V IIN operating current, Device shutdown VEN = 0V mA 3.85 V 130 mV 3 µs 0.7 1 mA 17 30 µA Switching Characteristics RDS-UP-ON Main MOSFET Rds(on) 0.18 0.35 Ω RDS- DN-ON Syn. MOSFET Rds(on) 0.11 0.2 Ω 3.3 4 V 8 9.8 µA VG-UVLO Gate drive voltage UVLO VBST - VSW increasing SS pin source current VSS = 0.5V Soft-start ISS 6 Current Limit ICL Syn. MOSFET current limit threshold 1.9 A VIN = 10V, RON = 100 kΩ 1.38 µs VIN = 30V, RON = 100 kΩ 0.47 ON/OFF Timer tON tON-MIN tOFF ON timer pulse width ON timer minimum pulse width 200 ns OFF timer pulse width 260 ns Enable Input VEN VEN-HYS EN Pin input threshold VEN rising Enable threshold hysteresis VEN falling 1.236 1.26 1.285 90 V mV Regulation and Over-Voltage Comparator VFB VFB-OV In-regulation feedback voltage VSS ≥ 0.8V TJ = −40°C to + 125°C 0.784 VSS ≥ 0.8V TJ = 0°C to + 125°C 0.788 Feedback over-voltage threshold 0.894 IFB www.national.com 4 0.8 0.816 V 0.812 0.920 0.940 V 5 100 nA Parameter Conditions Min Typ Max Units Thermal Shutdown TSD Thermal shutdown temperature TJ rising 165 °C TSD-HYS Thermal shutdown temperature hysteresis TJ falling 20 °C Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: θJC measurements are performed in general accordance with Mil-Std 883B, Method 1012.1 and utilizes the copper heat sink technique. Copper Heat Sink @ 60°C. Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading. 5 www.national.com LM3100 Symbol LM3100 Typical Performance Characteristics All curves taken at VIN = 18V with configuration in typical application circuit for VOUT = 3.3V shown in this datasheet. TA = 25°C, unless otherwise specified. Quiescent Current, IIN vs VIN VCC vs ICC 20174718 20174719 VCC vs VIN TON vs VIN 20174720 www.national.com 20174721 6 LM3100 Switching Frequency, FSW vs VIN VFB vs Temperature 20174722 20174723 RDS(ON) vs Temperature Efficiency vs Load Current (VOUT = 3.3V) 20174724 20174725 VOUT Regulation vs Load Current (VOUT = 3.3V) Efficiency vs Load Current (VOUT = 0.8V) 20174727 20174726 7 www.national.com LM3100 VOUT Regulation vs Load Current (VOUT = 0.8V) Power Up (VOUT = 3.3V, 1.5A Loaded) 20174729 20174728 Enable Transient (VOUT = 3.3V, 1.5A Loaded) Shutdown Transient (VOUT = 3.3V, 1.5A Loaded) 20174730 20174731 Continuous Mode Operation (VOUT = 3.3V, 1.5A Loaded) Discontinuous Mode Operation (VOUT = 3.3V, 0.15A Loaded) 20174732 www.national.com 20174733 8 Load Transient (VOUT = 3.3V, 0.15A - 1.5A Load, Current slew-rate: 2.5A/µs) 20174735 20174734 9 www.national.com LM3100 CCM to DCM Transition (VOUT = 3.3V, 0.15A - 1.5A Load) LM3100 Simplified Functional Block Diagram 20174701 www.national.com 10 VOUT = 0.8V x (RFB1 + RFB2)/RFB2 The LM3100 Step Down Switching Regulator features all functions needed to implement a cost effective, efficient buck power converter capable of supplying 1.5A to a load. This voltage regulator contains Dual 40V N-Channel buck synchronous switches and is available in a thermally enhanced eTSSOP-20 package. The Constant ON-Time (COT) regulation scheme requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. It will work correctly even with an all ceramic output capacitor network and does not rely on the output capacitor’s ESR for stability. The operating frequency remains constant with line and load variations due to the inverse relationship between the input voltage and the on-time. The valley current limit detection circuit, internally set at 1.9A, inhibits the high-side switch until the inductor current level subsides. Please refer to the functional block diagram with a typical application circuit. The LM3100 can be applied in numerous applications and can operate efficiently from inputs as high as 36V. Protection features include: Thermal shutdown, VCC under-voltage lockout, gate drive under-voltage lockout. (3) Start-up Regulator (VCC) The start-up regulator is integrated within LM3100. The input pin (VIN) can be connected directly to line voltage up to 36V, with transient capability of 40V. The VCC output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator sources current into the external capacitor at VCC (CVCC). CVCC must be at least 680nF for stability. When the voltage on the VCC pin reaches the under-voltage lockout threshold of 3.75V, the buck switch is enabled and the Softstart pin is released to allow the soft-start capacitor (CSS) to charge. The minimum input voltage is determined by the dropout voltage of VCC regulator, and the VCC UVLO falling threshold (≊3.7 V). If VIN is less than ≊4.0V, the VCC UVLO activates to shut off the output. Regulation Comparator The feedback voltage at FB pin is compared to the internal reference voltage of 0.8V. In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 0.8V. The buck switch stays on for the on-time, causing the FB voltage to rise above 0.8V. After the on-time period, the buck switch stays off until the FB voltage falls below 0.8V again. Bias current at the FB pin is nominally 100 nA. Hysteretic Control Circuit Overview The LM3100 buck DC-DC regulator employs a control scheme in which the high-side switch on-time varies inversely with the line voltage (VIN). Control is based on a comparator and the one-shot on-timer, with the output voltage feedback (FB) compared with an internal reference of 0.8V. If the FB level is below the reference the buck switch is turned on for a fixed time determined by the input voltage and a programming resistor (RON). Following the on-time, the switch remains off for a minimum of 260ns. If FB is below the reference at that time the switch turns on again for another on-time period. The switching will continue until regulation is achieved. The regulator will operate in discontinuous conduction mode at light load currents, and continuous conduction mode with heavy load current. In discontinuous conduction mode (DCM), current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the internal reference. Until then the inductor current remains zero and the load is supplied entirely by the output capacitor. In this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. Conversion efficiency is maintained since the switching losses are reduced with the reduction in load and switching frequency. The discontinuous operating frequency can be calculated approximately as follows: Over-Voltage Comparator The voltage at FB pin is compared to an internal 0.92V reference. If the feedback voltage rises above 0.92V the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load, changes suddenly. Once the OVP is activated, the buck switch remains off until the voltage at FB pin falls below 0.92V. The low side switch will stay on to discharge the inductor energy until the inductor current decays to zero. The low side switch will be turned off. ON-Time Timer, Shutdown The ON-Time of LM3100 main switch is determined by the RON resistor and the input voltage (VIN), and is calculated from: (4) The inverse relationship of tON and VIN results in a nearly constant switching frequency as VIN is varied. RON should be selected for a minimum on-time (at maximum VIN) greater than 200 ns for proper current limit operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT, calculated from equation 5: (1) In continuous conduction mode (CCM), current always flows through the inductor and never reaches zero during the offtime. In this mode, the operating frequency remains relatively constant with load and line variations. The CCM operating frequency can be calculated approximately as follows: (5) The LM3100 can be remotely shut down by taking the EN pin below 1.1V. Refer to Figure 1. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin allows normal operation to resume. For normal operation, the voltage at the EN pin is set between 1.5V and 3.0V, depending on VIN and the exter- (2) The output voltage is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as follows: 11 www.national.com LM3100 Functional Description LM3100 nal pull-up resistor. For all cases, this voltage must be limited not to exceed 7V. Current Limit Current limit detection occurs during the off-time by monitoring the re-circulating current through the low-side synchronous switch. Referring to Functional Block Diagram, when the buck switch is turned off, inductor current flows through the load, into PGND, and through the internal lowside synchronous switch. If that current exceeds 1.9A the current limit comparator toggles, forcing a delay to the start of the next on-time period. The next cycle starts when the recirculating current falls back below 1.9A and the voltage at FB is below 0.8V. The inductor current is monitored during the low-side switch on-time. As long as the overload condition persists and the inductor current exceeds 1.9A, the high-side switch will remain inhibited. The operating frequency is lower during an over-current due to longer than normal off-times. Figure 2 illustrates an inductor current waveform, the average inductor current is equal to the output current, IOUT in steady state. When an overload occurs, the inductor current will increase until it exceeds the current limit threshold, 1.9A. Then the control keeps the high-side switch off until the inductor current ramps down below 1.9A. Within each on-time period, the current ramps up an amount equal to: 20174704 FIGURE 1. Shutdown Implementation (6) During this time the LM3100 is in a constant current mode, with an average load current (IOCL) equal to 1.9A +ΔI/2. 20174705 FIGURE 2. Inductor Current - Current Limit Operation www.national.com 12 LM3100 N-Channel Buck Switch and Driver The LM3100 integrates an N-Channel buck (high-side) switch and associated floating high voltage gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 33 nF capacitor (CBST) connected between BST and SW pins provides voltage to the high-side driver during the buck switch on-time. During each off-time, the SW pin falls to approximately -1V and CBST charges from the VCC supply through the internal diode. The minimum off-time of 260ns ensures adequate time each cycle to recharge the bootstrap capacitor. RFB1 and RFB2 should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio. For VOUT = 0.8V, the FB pin can be connected to the output directly. However, the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected. This minimum load is about 10 µA and can be implemented by adding a pre-load resistor to the output. RON: The minimum value for RON is calculated from: Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal 8µA current source charges up the external capacitor at the SS pin. The ramping voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if any of the following cases happen: (i) VCC falls below the under-voltage lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the converter can be disabled by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin return to pull high and the output voltage returns to normal. The shut-down configuration is shown in Figure 3 . The equation 2 in Control Overview section can be used to select RON if a specific frequency is desired as long as the above limitation is met. L: The main parameter affected by the inductor is the output current ripple amplitude (IOR). The maximum allowable (IOR must be determined at both the minimum and maximum nominal load currents. At minimum load current, the lower peak must not reach 0A. At maximum load current, the upper peak must not exceed the current limit threshold (1.9A). The allowable ripple current is calculated from the following equations: IOR(MAX1) = 2 x IO(min) or IOR(MAX2) = 2 x (1.9A - IO(max)) The lesser of the two ripple amplitudes calculated above is then used in the following equation: 20174706 (7) where VIN is the maximum input voltage and Fs is determined from equation 1. This provides a value for L. The next larger standard value should be used. L should be rated for the IPK current level shown in Figure 2. FIGURE 3. Alternate Shutdown Implementation Thermal Protection The LM3100 should be operated so the junction temperature does not exceed the maximum limit. An internal Thermal Shutdown circuit, which activates (typically) at 165°C, takes the controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the SS pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation resumes. Inductor Selector for VOUT = 3.3V Applications Information EXTERNAL COMPONENTS The following guidelines can be used to select the external components. RFB1 and RFB2 : The ratio of these resistors is calculated from: 20174736 13 www.national.com LM3100 Inductor Selector for VOUT = 0.8V CBST supplies a surge current to charge the buck switch gate at turn-on. A low ESR also helps ensure a complete recharge during each off-time. CSS: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the regulation comparator, and the output voltage, to reach their final value. The time is determined from the following: CFB: If output voltage is higher than 1.6V, this feedback capacitor is needed for Discontinuous Conduction Mode to improve the output ripple performance, the recommended value for CFB is 10 nF. PC BOARD LAYOUT The LM3100 regulation, over-voltage, and current limit comparators are very fast, and will respond to short duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be as neat and compact as possible, and all external components must be as close as possible to their associated pins. Refer to the functional block diagram, the loop formed by CIN, the high and low-side switches internal to the IC, and the PGND pin should be as small as possible. The PGND connection to Cin should be as short and direct as possible. There should be several vias connecting the Cin ground terminal to the ground plane placed as close to the capacitor as possible. The boost capacitor should be connected as close to the SW and BST pins as possible. The feedback divider resistors and the CFB capacitor should be located close to the FB pin. A long trace run from the top of the divider to the output is generally acceptable since this is a low impedance node. Ground the bottom of the divider directly to the GND (pin 7). The output capacitor, COUT, should be connected close to the load and tied directly into the ground plane. The inductor should connect close to the SW pin with as short a trace as possible to help reduce the potential for EMI (electro-magnetic interference) generation. If it is expected that the internal dissipation of the LM3100 will produce excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of the IC package can be soldered to a ground plane and that plane should extend out from beneath the IC to help dissipate the heat. The exposed pad is internally connected to the IC substrate. Additionally the use of thick copper traces, where possible, can help conduct heat away from the IC. Using numerous vias to connect the die attach pad to an internal ground plane is a good practice. Judicious positioning of the PC board within the end product, along with the use of any available air flow (forced or natural convection) can help reduce the junction temperature. 20174737 CVCC: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false triggering of the VCC UVLO at the buck switch on/off transitions. For this reason, CVCC should be no smaller than 680 nF for stability, and should be a good quality, low ESR, ceramic capacitor. CO and CO3: CO should generally be no smaller than 10 µF. Experimentation is usually necessary to determine the minimum value for CO, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for CO than a fixed load. CO3 is a small value ceramic capacitor to further suppress high frequency noise at VOUT. A 47nF is recommended, located close to the LM3100. CIN and CIN3: CIN’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, assume the voltage source feeding VIN has an output impedance greater than zero. If the source’s dynamic impedance is high (effectively a current source), CIN supplies the average input current, but not the ripple current. At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average current during the ontime is the load current. For a worst case calculation, CIN must supply this average load current during the maximum on-time. CIN is calculated from: (8) where IOUT is the load current, tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN. CIN3’s purpose is to help avoid transients and ringing due to long lead inductance at VIN. A low ESR, 0.1µF ceramic chip capacitor is recommended, located close to the LM3100. CBST: The recommended value for CBST is 33 nF. A high quality ceramic capacitor with low ESR is recommended as www.national.com 14 LM3100 20174716 Typical Application Schematic for VOUT = 3.3V 20174717 Typical Application Schematic for VOUT = 0.8V 15 www.national.com LM3100 Physical Dimensions inches (millimeters) unless otherwise noted 20-Lead Plastic eTSSOP Package NS Package Number MXA20A www.national.com 16 LM3100 Notes 17 www.national.com LM3100 SIMPLE SWITCHER® Synchronous 1MHz 1.5A Step-Down Voltage Regulator Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. 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