LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 LM25011, LM25011Q, LM25011A, LM25011AQ 42V 2A Constant On-Time Switching Regulator With Adjustable Current Limit Check for Samples: LM25011, LM25011-Q1 FEATURES 1 • 2 • • • • • • • • • LM25011Q is an Automotive Grade product that is AEC-Q100 grade 1 qualified (-40°C to +125°C operating junction temperature) LM25011A allows low dropout operation at high switching frequency Input operating voltage range: 6V to 42V Absolute maximum input rating: 45V Integrated 2A N-Channel Buck Switch Adjustable current limit allows for smaller inductor Adjustable output voltage from 2.51V Minimum ripple voltage at VOUT Power Good output Switching frequency adjustable to 2 MHz • • • • • COT topology features: – Switching frequency remains nearly constant with load current and input voltage variations – Ultra-fast transient response – No loop compensation required – Stable operation with ceramic output capacitors – Allows for smaller output capacitor and current sense resistor Adjustable Soft-Start timing Thermal shutdown Precision 2% feedback reference Package – MSOP-10EP DESCRIPTION The LM25011 Constant On-time Step-Down Switching Regulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying up to 2A of load current. This high voltage regulator contains an N-Channel Buck switch, a startup regulator, current limit detection, and internal ripple control. The constant on-time regulation principle requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. The operating frequency remains constant with line and load. The adjustable valley current limit detection results in a smooth transition from constant voltage to constant current mode when current limit is reached, without the use of current limit foldback. The PGD output indicates the output voltage has increased to within 5% of the expected regulation value. Additional features include: Low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal shutdown, gate drive pre-charge, gate drive undervoltage lock-out, and maximum duty cycle limit. The LM25011A has a shorter minimum off-time than the LM25011, which allows for higher frequency operation at low input voltages. 1 2 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 2009–2011, Texas Instruments Incorporated LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com TYPICAL APPLICATION, BASIC STEP-DOWN REGULATOR 6V to 42V Input VIN BST CBST LM25011 CIN RT L1 SW D1 RT VOUT CS RPGD VPGD Power Good RS PGD COUT RFB2 CSG SS CSS SGND FB RFB1 CONNECTION DIAGRAM Exposed Pad on Bottom Connect to Ground VIN 1 10 BST RT 2 9 SW PGD 3 8 CS SS 4 7 CSG SGND 5 6 FB Figure 1. Top View 10 Lead MSOP-EP 2 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 Pin Functions Pin Descriptions Pin No. Name Description 1 VIN Input supply voltage Application Information 2 RT On-time Control An external resistor from VIN to this pin sets the buck switch on-time, and the switching frequency. 3 PGD Power Good Logic output indicates when the voltage at the FB pin has increased to above 95% of the internal reference voltage. Hysteresis is provided. An external pull-up resistor to a voltage less than 7V is required. 4 SS Soft-Start 5 SGND Signal Ground 6 FB Feedback Internally connected to the regulation comparator. The regulation level is 2.51V. 7 CSG Current Sense Ground Ground connection for the current limit sensing circuit. Connect to ground and to the current sense resistor. 8 CS Current sense Connect to the current sense resistor and the anode of the free-wheeling diode. 9 SW Switching Node 10 BST Bootstrap capacitor connection of the buck switch gate driver. Operating input range is 6V to 42V. Transient capability is 45V. A low ESR capacitor must be placed as close as possible to the VIN and SGND pins. An internal current source charges an external capacitor to provide the softstart function. Ground for all internal circuitry other than the current limit sense circuit. Internally connected to the buck switch source. Connect to the external inductor, cathode of the free-wheeling diode, and bootstrap capacitor. Connect a 0.1 µF capacitor from SW to this pin. The capacitor is charged during the buck switch off-time via an internal diode. 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. ABSOLUTE MAXIMUM RATINGS (1) VIN to SGND (TJ = 25°C) 45V BST to SGND 52V SW to SGND (Steady State) -1.5V to 45V BST to SW -0.3V to 7V CS to CSG -0.3V to 0.3V CSG to SGND -0.3V to 0.3V PGD to SGND -0.3V to 7V SS to SGND -0.3V to 3V RT to SGND -0.3V to 1V FB to SGND -0.3V to 7V ESD Rating, Human Body Model (2) 2kV Storage Temperature Range -65°C to +150°C For soldering specs, see www.national.com/ms/MS/MS=SOLDERING.pdf Junction Temperature (1) (2) 150°C 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. The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. OPERATING RATINGS (1) VIN Voltage 6.0V to 42V Junction Temperature (1) –40°C to +125°C 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. Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 3 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com 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 = 12V, RT = 50 kΩ. Symbol Parameter Conditions Min Input operating current Non-switching, FB = 3V VIN under-voltage lock-out threshold VIN Increasing Typ Max Unit 1200 1600 µA 5.3 5.9 V Input (VIN Pin) IIN UVLOVIN 4.6 VIN under-voltage lock-out threshold hysteresis 200 mV Switch Characteristics RDS(ON) Buck Switch RDS(ON) ITEST = 200 mA UVLOGD Gate Drive UVLO BST-SW 2.4 UVLOGD Hysteresis 0.3 0.6 3.4 4.4 Ω V 350 mV 1.4 V Pre-charge switch on-time 120 ns VSS Pull-up voltage 2.51 V ISS Internal current source Pre-charge switch voltage ITEST = 10 mA into SW pin Soft-Start Pin VSS-SH Shutdown Threshold 10 µA 70 140 mV -146 -130 Current Limit VILIM Threshold voltage at CS -115 mV CS bias current FB = 3V -120 µA CSG bias current FB = 3V -35 µA tON - 1 On-time VIN = 12V, RT = 50 kΩ tON - 2 On-time VIN = 32V, RT = 50 kΩ 75 ns tON - 3 On-time (current limit) LM25011 VIN = 12V, RT = 50 kΩ 100 ns tON - 3 On-time (current limit) LM25011A VIN = 12V, RT = 50 kΩ 200 ns tON - 4 On-time VIN = 12V, RT = 301 kΩ 1020 tON - 5 On-time VIN = 9V, RT = 30.9 kΩ 130 171 215 ns tON - 6 On-time VIN = 12V, RT = 30.9 kΩ 105 137 170 ns tON - 7 On-time VIN = 16V, RT = 30.9 kΩ 79 109 142 ns Minimum Off-time (LM25011) 90 150 208 ns Minimum Off-time (LM25011A) 52 75 93 2.46 2.51 2.56 On Timer, RT Pin 150 200 250 ns ns Off Timer tOFF Regulation Comparator (FB Pin) VREF FB regulation threshold SS pin = steady state FB bias current FB = 3V V 100 nA 95 % Power Good (PGD pin) Threshold at FB, with respect to VREF FB increasing 91 Threshold hysteresis 4 3.3 PGDVOL Low state voltage IPGD = 1mA, FB = 0V 125 PGDLKG Off state leakage VPGD = 7V, FB = 3V 0.1 Submit Documentation Feedback % 180 mV µA Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 ELECTRICAL CHARACTERISTICS (continued) 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 = 12V, RT = 50 kΩ. Symbol Parameter Conditions Min Thermal shutdown Junction temperature increasing Typ Max Unit Thermal Shutdown TSD 155 °C Thermal shutdown hysteresis 20 °C θJA Junction to Ambient, 0 LFPM Air Flow (1) 48 °C/W θJC Junction to Case (1) 10 °C/W Thermal Resistance (1) JEDEC test board description can be found in JESD 51-5 and JESD 51-7. Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 5 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com TYPICAL PERFORMANCE CHARACTERISTICS 6 Efficiency (Circuit of Figure 7) Efficiency at 2 MHz On-Time vs VIN and RT Voltage at the RT Pin Shutdown Current into VIN Operating Current into VIN Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 TYPICAL PERFORMANCE CHARACTERISTICS (continued) PGD Low Voltage vs. Sink Current Reference Voltage vs. Temperature Current Limit Threshold vs. Temperature Operating Current vs. Temperature VIN UVLO vs. Temperature SS Pin ShutdownThreshold vs. Temperature Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 7 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com TYPICAL PERFORMANCE CHARACTERISTICS (continued) On-Time vs. Temperature Minimum Off-Time vs. Temperature 190 MINIMUM OFF-TIME (ns) 170 LM25011 150 130 110 90 LM25011A 70 50 -40 -20 0 20 40 60 80 100 120 JUNCTION TEMPERATURE (°C) BLOCK DIAGRAM 6V to 42V LM25011(A) VIN 5V REGULATOR Input CIN CBYP UVLO CL RT THERMAL SHUTDOWN OFF TIMER ON TIMER RT FINISH START START FINISH BST 2.5V Gate Drive 10 PA SD UVLO VIN CBST SS LOGIC CSS LEVEL SHIFT L1 + FB CL REGULATION COMPARATOR VOUT SW FCIC CONTROL CURRENT LIMIT COMPARATOR D1 + COUT Pre - Chg - RFB2 CS RPGD Power Good 0.8V PGD SGND + CURRENT LIMIT THRESHOLD + 125 mV RS CSG 2.375V RFB2 Figure 2. Block Diagram 8 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 UVLO VIN SW Pin Inductor Current SS Pin VOUT PGD t1 Figure 3. Startup Sequence FUNCTIONAL DESCRIPTION The LM25011 Constant On-time Step-down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying up to 2.0A to the load. This high voltage regulator contains an N-Channel buck switch, is easy to implement, and is available in a 10-pin MSOP power enhanced package. The regulator’s operation is based on a constant on-time control principle with the on-time inversely proportional to the input voltage. This feature results in the operating frequency remaining relatively constant with load and input voltage variations. The constant on-time feedback control principle requires no loop compensation resulting in very fast load transient response. The adjustable valley current limit detection results in a smooth transition from constant voltage to constant current when current limit is reached. To aid in controlling excessive switch current due to a possible saturating inductor the on-time is reduced by approximately 40% when current limit is detected. The Power Good output (PGD pin) indicates when the output voltage is within 5% of the expected regulation voltage. The LM25011 can be implemented to efficiently step-down higher voltages in non-isolated applications. Additional features include: Low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal shutdown, gate drive pre-charge, gate drive under-voltage lock-out, and maximum duty cycle limit. Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 9 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com CONTROL CIRCUIT OVERVIEW The LM25011 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (2.51V). If the FB voltage is below the reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one-shot period. When in regulation, the LM25011 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is always greater than zero, and the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The approximate operating frequency is calculated as follows: VOUT FS = -11 (4.1 x 10 x (RT + 0.5k)) + (VIN x 15 ns) (1) The buck switch duty cycle is approximately equal to: DC = tON VOUT = tON x FS = tON + tOFF VIN (2) When the load current is less than one half the inductor’s ripple current amplitude the circuit operates in discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is maintained at light loads since the switching losses are reduced with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: FS = VOUT2 x L1 x 1.19 x 1021 2 RL x R T (3) where RL = the load resistance, and L1 is the circuit’s inductor. The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Block Diagram). The regulated output voltage is calculated as follows: VOUT = 2.51V x (RFB1 + RFB2) / RFB1 (4) Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM (Emulated Ripple Mode) control circuit generates the required internal ripple voltage from the ripple waveform at the CS pin. The LM25011A, which is designed for higher frequency operation, requires additional ripple voltage, which must be generated externally and provided to the FB pin. This is described in the Applications Information section. ON-TIME TIMER The on-time for the LM25011/LM25011A is determined by the RT resistor and the input voltage (VIN), calculated from: tON = 4.1 x 10 -11 x (RT + 500:) (VIN) + 15 ns (5) The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific continuous conduction mode switching frequency (FS), the RT resistor is determined from the following: VOUT - (VIN x FS x 15 ns) - 500: RT = -11 FS x 4.1 x 10 (6) 10 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 The on-time must be chosen greater than 90 ns for proper operation. Equation 1, Equation 5, and Equation 6 are valid only during normal operation; that is, the circuit is not in current limit. When the LM25011 operates in current limit, the on-time is reduced by approximately 40% (this feature is not present in LM25011A). This feature reduces the peak inductor current which may be excessively high if the load current and the input voltage are simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be less than 2 mA. For high frequency applications, the maximum switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period (90 ns). At minimum input voltage the maximum switching frequency is limited by the minimum off-time one-shot period, which, if reached, prevents achievement of the proper duty cycle. CURRENT LIMIT Current limit detection occurs during the off-time by monitoring the voltage across the external current sense resistor RS. Referring to the Block Diagram, during the off-time the recirculating current flows through the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the sense resistor exceeds the threshold (VILIM) the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage across RS reduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by approximately 40% (only in LM25011) if the voltage at the FB pin is below its threshold when the voltage across RS reduces to its threshold (VOUT is low due to current limiting). Figure 4 illustrates the inductor current waveform during normal operation and in current limit. During the first “Normal Operation” the load current is I01, the average of the inductor current waveform. As the load resistance is reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold. During the “Current Limited” portion of Figure 4, each on-time is reduced by approximately 40%, resulting in lower ripple amplitude for the inductor’s current. During this time the LM25011 is in a constant current mode with an average load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2), allowing VOUT and the on-time to return to their normal values. Note that in the second period of “Normal Operation”, even though the inductor’s peak current exceeds the current limit threshold during part of each cycle, the circuit is not in current limit since the inductor current falls below the current limit threshold during each off time. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 2.0A. IPK IOCL Current Limit Threshold IO2 'I Inductor Current IO1 0V Voltage at the CS Pin Voltage at the FB Pin 2.51V Normal Operation Load Current Increases Current Limited Load Current Decreases Normal Operation Figure 4. Normal and Current Limit Operation Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 11 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com RIPPLE REQUIREMENTS The LM25011 requires about 25mVp-p of ripple voltage at the CS pin. Higher switching frequency may require more ripple. That ripple voltage is generated by the decreasing recirculating current (the inductor’s ripple current) through RS during the off-time. See Figure 5. Inductor Current 'I 0V Voltage at CS VRIPPLE tOFF tON Figure 5. CS Pin Waveform The ripple voltage is equal to: VRIPPLE = ΔI x RS (7) where ΔI is the inductor current ripple amplitude, and RS is the current sense resistor at the CS pin. More ripple can be achieved by decreasing the inductor value. The LM25011A, with its shorter minimum off-time, typically will require more ripple than the LM25011. An external circuit to increase the effective ripple voltage may be needed. Different methods of generating this ripple are explained in the “Application Information” section. N-CHANNEL BUCK SWITCH AND DRIVER The LM25011 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor (CBST) and an internal high voltage diode. A 0.1 µF capacitor connected between BST and SW provides the supply voltage for the driver during the ontime. During each off-time, the SW pin is at approximately -1V, and CBST is recharged from the internal 5V regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor. SOFT-START The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turn-on, when VIN reaches its under-voltage lock-out threshold an internal 10 µA current source charges the external capacitor at the SS pin to 2.51V (t1 in Figure 3). The ramping voltage at SS ramps the non-inverting input of the regulation comparator, and the output voltage, in a controlled manner. For proper operation, the soft-start capacitor should be no smaller than 1000 pF. The LM25011 can be employed as a tracking regulator by applying the controlling voltage to the SS pin. The regulator’s output voltage tracks the applied voltage, gained up by the ratio of the feedback resistors. The applied voltage at the SS pin must be within the range of 0.5V to 2.6V. The absolute maximum rating for the SS pin is 3.0V. If the tracking function causes the voltage at the FB pin to go below the thresholds for the PGD pin, the PGD pin will switch low (see POWER GOOD OUTPUT (PGD)). An internal switch grounds the SS pin if the input voltage at VIN is below its under-voltage lock-out threshold or if the Thermal Shutdown activates. If the tracking function (described above) is used, the tracking voltage applied to the SS pin must be current limited to a maximum of 1 mA. SHUTDOWN FUNCTION The SS pin can be used to shutdown the LM25011 by grounding the SS pin as shown in Figure 6. Releasing the pin allows normal operation to resume. 12 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 SS STOP RUN LM25011 CSS Figure 6. Shutdown Implementation POWER GOOD OUTPUT (PGD) The Power Good output (PGD) indicates when the voltage at the FB pin is close to the internal 2.51V reference voltage. The rising threshold at the FB pin for the PGD output to switch high is 95% of the internal reference. The falling threshold for the PGD output to switch low is approximately 3.3% below the rising threshold. The PGD pin is internally connected to the drain of an N-channel MOSFET switch. An external pull-up resistor (RPGD), connected to an appropriate voltage not exceeding 7V, is required at PGD to indicate the LM25011’s status to other circuitry. When PGD is low, the pin’s voltage is determined by the current into the pin. See the graph “PGD Low Voltage vs. Sink Current”. Upon powering up the LM25011, the PGD pin is high until the voltage at VIN reaches 2V, at which time PGD switches low. As VIN is increased PGD stays low until the output voltage takes the voltage at the FB pin above 95% of the internal reference voltage, at which time PGD switches high. As VIN is decreased (during shutdown) PGD remains high until either the voltage at the FB pin falls below approximately 92% of the internal reference, or when VIN falls below its lower UVLO threshold, whichever occurs first. PGD then switches low, and remains low until VIN falls below 2V, at which time PGD switches high. If the LM25011 is used as a tracking regulator (see SOFT-START), the PGD output is high as long as the voltage at the FB pin is above the thresholds mentioned above. THERMAL SHUTDOWN The LM25011 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 155°C, taking the controller to a low power reset state by disabling the buck switch and taking the SS pin to ground. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 135°C (typical hysteresis = 20°C) normal operation resumes. APPLICATIONS INFORMATION EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with a design example using the LM25011. Referring to the Block Diagram, the circuit is to be configured for the following specifications: • VOUT = 5V • VIN = 8V to 36V • Minimum load current for continuous conduction mode (IOUT(min) = 300 mA • Maximum load current (IOUT(max) = 1.5 A • Switching frequency (FS) = 1.0 MHz • Soft-start time = 5 ms RFB2 and RFB1: These resistors set the output voltage, and their ratio is calculated from: RFB2/RFB1 = (VOUT/2.51V) - 1 (8) For this example, RFB2/RFB1 = 0.992. 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 this example, 4.99 kΩ is chosen for both resistors, providing a 5.02V output. RT: This resistor sets the on-time, and (by default) the switching frequency. First check that the desired frequency does not require an on-time or off-time shorter than the minimum allowed values (90 ns and 150, respectively). The minimum on-time occurs at the maximum input voltage. For this example: Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 13 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 VOUT tON(min) = VIN(max) x FS = www.ti.com 5V = 139 ns 36V x 1 MHz (9) The minimum off-time occurs at the minimum input voltage. For this example: tOFF(min) = VIN(min) - VOUT VIN(min) x FS = 8V - 5V = 375 ns 8V x 1 MHz (10) Both the on-time and off-time are acceptable since they are significantly greater than the minimum value for each. The RT resistor is calculated from Equation 6 using the minimum input voltage: 5 - (8V x 1MHz x 15 ns) - 500: = 118.5 k: RT = -11 1MHz x 4.1 x 10 (11) A standard value 118 kΩ resistor is selected. The minimum on-time calculates to 152 ns at VIN = 36V, and the maximum on-time calculates to 672 ns at Vin = 8V L1: The parameters controlled by the inductor are the inductor current ripple amplitude (IOR), and the ripple voltage amplitude across the current sense resistor RS. The minimum load current is used to determine the maximum allowable ripple in order to maintain continuous conduction mode (the lower peak does not reach 0 mA). This is not a requirement of the LM25011, but serves as a guideline for selecting L1. For this example, the maximum ripple current should be less than: IOR(max) = 2 x IOUT(min) = 600 mA p-p (12) For applications where the minimum load current is zero, a good starting point for allowable ripple is 20% of the maximum load current. In this case substitute 20% of IOUT(max) for IOUT(min) in Equation 12. The ripple amplitude calculated in Equation 12 is then used in Equation 13: L1(min) = tON(min) x (VIN(max) - VOUT) = 7.85 PH IOR(max) (13) A standard value 10 µH inductor is chosen. Using this inductor value, the maximum ripple current amplitude, which occurs at maximum VIN, calculates to 472 mAp-p, and the peak current is 1736 mA at maximum load current. Ensure the selected inductor is rated for this peak current. The minimum ripple current, which occurs at minimum VIN, calculates to 200 mAp-p. RS: The minimum current limit threshold is calculated at maximum load current, using the minimum ripple current calculated above. The current limit threshold is the lower peak of the inductor current waveform when in current limit (see Figure 4). ILIM = 1.5A – (0.2 A/2) = 1.4A (14) Current limit detection occurs when the voltage across the sense resistor (RS) reaches the current limit threshold. To allow for tolerances, the sense resistor value is calculated using the minimum threshold specification: RS = 115 mV/1.4A = 82 mΩ (15) The next smaller standard value, 80 mΩ, is selected. The next step is to ensure that sufficient ripple voltage occurs across RS with this value sense resistor. As mentioned in the Ripple Requirements section, a minimum of 15 mVp-p voltage ripple is required across the RS sense resistor during the off-time to ensure the regulation circuit operates properly. The ripple voltage is the product of the inductor ripple current amplitude and the sense resistor value. In this case, the minimum ripple voltage calculates to: VRIPPLE = ΔI x RS = 200 mA x 0.080Ω = 16 mV (16) If the ripple voltage had calculated to less than 15 mVp-p the inductor value would have to be reduced to increase the ripple current amplitude. This would have required a recalculation of ILIM and RS in the above equations. Since the minimum requirement is satisfied in this case no change is necessary. The nominal current limit threshold calculates to 1.63A. The minimum and maximum thresholds calculate to 1.44A and 1.83A respectively, using the minimum and maximum limits for the current limit threshold specification. The load current is equal to the threshold current plus one half the ripple current. Under normal load conditions, the maximum power dissipation in RS occurs at maximum load current, and at maximum input voltage where the on-time duty cycle is minimum. In this design example, the minimum on-time duty cycle is: 14 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 Duty Cycle = D = VOUT 5V = 13.9% = 36V VIN (17) At maximum load current, the power dissipation in RS is equal to: P(RS) = (1.5A)2 x 0.080Ω x (1 – 0.139) = 155 mW (18) When in current limit the maximum power dissipation in RS calculates to P(RS) = (1.83A + 0.472A/4)2 x 0.080Ω = 304 mW (19) Duty cycle is not included in this power calculation since the on-time duty cycle is typically <5% when in current limit. COUT: The output capacitor should typically be no smaller than 3.3 µF, although that is dependent on the frequency and the desired output characteristics. COUT should be a low ESR good quality ceramic capacitor. Experimentation is usually necessary to determine the minimum value for COUT, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for COUT than a nonvarying load. CIN and CBYP: The purpose of CIN is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, since it is assumed the voltage source feeding VIN has some amount of source impedance. When the buck switch turns on, the current into VIN suddenly increases to the lower peak of the inductor’s ripple current, then ramps up to the upper peak, then drops to zero at turn-off. The average current during the on-time is the average load current. For a worst case calculation, CIN must supply this average load current during the maximum on-time, without letting the voltage at the VIN pin drop below a minimum operating level of 5.5V. For this exercise 0.5V is chosen as the maximum allowed input ripple voltage. Using the maximum load current, the minimum value for CIN is calculated from: CIN = IOUT(max) x tON(max) 1.5A x 672 ns = 2.02 PF = 0.5V 'V (20) where tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN. The purpose of CBYP is to minimize transients and ringing due to long lead inductance leading to the VIN pin. A low ESR 0.1 µF ceramic chip capacitor is recommended, and CBYP must be located close to the VIN and SGND pins. CBST: The recommended value for CBST is 0.1 µF. A high quality ceramic capacitor with low ESR is recommended as CBST supplies a surge current to charge the buck switch gate at each 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 output voltage to reach its final value (t1 in Figure 3). For a soft-start time of 5 ms, the capacitor value is determined from the following: 5 ms x 10 PA = 0.02 PF CSS = 2.51V (21) D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed transitions at the SW pin may affect the regulator’s operation due to the diode’s reverse recovery transients. The diode must be rated for the maximum input voltage, the maximum load current, and the peak current which occurs when the current limit and maximum ripple current are reached simultaneously. The diode’s average power dissipation is calculated from: PD1 = VF x IOUT x (1 - D) (22) where VF is the diode’s forward voltage drop, and D is the on-time duty cycle. FINAL CIRCUIT The final circuit is shown in Figure 7, and its performance is shown in Figure 8 and Figure 9. The current limit measured approximately 1.62A at Vin = 8V, and 1.69A at Vin = 36V. Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 15 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com 8V to 36V Input RT 118 k: CIN 4.7 PF CBYP 0.1 PF BST VIN CBST 0.1 PF L1 10 PH LM25011 SW RT VOUT D1 5V VPGD CS RPGD 10 k: Power Good COUT 10 PF RS 80 m: PGD RFB2 4.99 k: CSG SS CSS 0.022 PF SGND FB RFB1 4.99 k: Figure 7. Example Circuit Figure 8. Efficiency (Circuit of Figure 7) Figure 9. Frequency vs VIN (Circuit of Figure 7) OUTPUT RIPPLE CONTROL (LM25011A) The LM25011A most likely will require more ripple voltage than is generated across the RS resistor. Additional ripple can be supplied to the FB pin, in phase with the switching waveform at the SW pin, for proper operation. The required ripple can be supplied from ripple generated at VOUT, through the feedback resistors, as described in Option A below. Options B and C provide for lower output ripple with one or two additional components. The amount of additional ripple voltage needed at the FB pin is typically in the range of 30-150mV. Higher switching frequencies or higher inductor values (less ripple current) require more ripple voltage injected on FB. Insufficient ripple voltage will result in frequency jitter. For a particular application, add only as much ripple as needed to stabilize the switching frequency over the required input voltage. Option A) Lowest Cost Configuration: In this configuration R1 is installed in series with the output capacitor (COUT) as shown in Figure 10. The inductor’s ripple current passes through R1, generating a ripple voltage at VOUT. The minimum value for R1 is: R1 = VRIPPLE x (RFB2 + RFB1) 'I x RFB1 (23) where ΔI is the minimum ripple current amplitude, which occurs at minimum Vin. 16 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 BST C BST LM25011A L1 SW VOUT D1 CS RFB2 R1 RS C OUT CSG SGND R FB1 FB Figure 10. Option A – Lowest Cost Ripple Configuration Option B) Intermediate Ripple Configuration: This configuration generates less ripple at VOUT than Option A above by the addition of one capacitor (Cff), as shown in Figure 11. BST C BST LM25011A L1 SW VOUT D1 CS R1 Cff RS RFB2 C OUT CSG SGND FB R FB1 Figure 11. Option B – Intermediate Ripple Configuration Since the output ripple is passed by Cff to the FB pin with little or no attenuation, R1’s value can be chosen so the minimum ripple at VOUT is approximately 150 mVp-p. The minimum value for R1 is calculated from: VRIPPLE R1 = 'I (24) where ΔI is the minimum ripple current amplitude, which occurs at minimum Vin. The minimum value for Cff is calculated from: 3 x tON(max) Cff > RFB1//RFB2 (25) where tON(max) is the maximum on-time (at minimum VIN), and RFB1//RFB2 is the parallel equivalent of the feedback resistors. Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 Submit Documentation Feedback 17 LM25011, LM25011-Q1 SNVS617F – MAY 2009 – REVISED JANUARY 2011 www.ti.com Option C) Minimum Ripple Configuration: BST C BST LM25011A L1 SW VOUT D1 Rr CS Cr COUT Cac RS CSG SGND FB RFB1 Figure 12. Option C: Minimum Output Ripple Configuration In some applications, the ripple induced by series resistor R1 may not be acceptable. An external ripple circuit, as shown in Figure 12, can be used to provide the required ripple to the FB pin. 1. The time constant τ=Rr*Cr should be greater than 8-10 times the switching period to generate a triangular ramp at FB pin. 2. The smallest ripple at feedback ΔVFB = (VIN(min)-VOUT)*TON(max)/τ. 3. The ramp capacitor Cr should much smaller than the ac coupling capacitor Cac. Usually Cac=100nF, Cr=1nF, and Rr is chosen to satisfy conditions 1 and 2 above. PC BOARD LAYOUT The LM25011 regulation and current limit comparators are very fast, and 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 of the components must be as close as possible to their associated pins. The two major current loops conduct currents which switch very fast, and therefore those loops must be as small as possible to minimize conducted and radiated EMI. The first loop is formed by CIN, through the VIN to SW pins, L1, COUT, and back to CIN. The second current loop is formed by RS, D1, L1, COUT and back to RS. The ground connection from CSG to the ground end of CIN should be as short and direct as possible. The power dissipation within the LM25011 can be approximated by determining the circuit’s total conversion loss (PIN - POUT), and then subtracting the power losses in the free-wheeling diode, the sense resistor, and the inductor. The power loss in the diode is approximately: PD1 = IOUT x VF x (1-D) (26) where Iout is the load current, VF is the diode’s forward voltage drop, and D is the on-time duty cycle. The power loss in the sense resistor is: PRS = (IOUT)2 x RS x (1 – D) (27) The power loss in the inductor is approximately: PL1 = IOUT2 x RL x 1.1 (28) where RL is the inductor’s DC resistance, and the 1.1 factor is an approximation for the AC losses. If it is expected that the internal dissipation of the LM25011 will produce excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help to dissipate heat. Additionally the use of wide PC board traces, where possible, can help conduct heat away from the IC pins. 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. 18 Submit Documentation Feedback Copyright © 2009–2011, Texas Instruments Incorporated Product Folder Links: LM25011 LM25011-Q1 PACKAGE OPTION ADDENDUM www.ti.com 12-Nov-2012 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Qty Drawing Eco Plan Lead/Ball Finish (2) MSL Peak Temp Samples (3) (Requires Login) LM25011AQ1MY/NOPB ACTIVE MSOPPowerPAD DGQ 10 1000 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR LM25011AQ1MYX/NOPB ACTIVE MSOPPowerPAD DGQ 10 3500 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR LM25011MY/NOPB ACTIVE MSOPPowerPAD DGQ 10 1000 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR LM25011MYX/NOPB ACTIVE MSOPPowerPAD DGQ 10 3500 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR LM25011Q1MY/NOPB ACTIVE MSOPPowerPAD DGQ 10 1000 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR LM25011Q1MYX/NOPB ACTIVE MSOPPowerPAD DGQ 10 3500 Green (RoHS & no Sb/Br) CU SN Level-3-260C-168 HR (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) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. TBD: The Pb-Free/Green conversion plan has not been defined. Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above. Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material) (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. 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 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. Addendum-Page 1 PACKAGE OPTION ADDENDUM www.ti.com 12-Nov-2012 In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis. OTHER QUALIFIED VERSIONS OF LM25011, LM25011-Q1 : • Catalog: LM25011 • Automotive: LM25011-Q1 NOTE: Qualified Version Definitions: • Catalog - TI's standard catalog product • Automotive - Q100 devices qualified for high-reliability automotive applications targeting zero defects Addendum-Page 2 PACKAGE MATERIALS INFORMATION www.ti.com 15-Nov-2012 TAPE AND REEL INFORMATION *All dimensions are nominal Device LM25011AQ1MY/NOPB Package Package Pins Type Drawing SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant MSOPPower PAD DGQ 10 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM25011AQ1MYX/NOPB MSOPPower PAD DGQ 10 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM25011MY/NOPB MSOPPower PAD DGQ 10 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM25011MYX/NOPB MSOPPower PAD DGQ 10 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM25011Q1MY/NOPB MSOPPower PAD DGQ 10 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM25011Q1MYX/NOPB MSOPPower PAD DGQ 10 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 Pack Materials-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 15-Nov-2012 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM25011AQ1MY/NOPB MSOP-PowerPAD DGQ 10 1000 203.0 190.0 41.0 LM25011AQ1MYX/NOPB MSOP-PowerPAD DGQ 10 3500 358.0 343.0 63.0 LM25011MY/NOPB MSOP-PowerPAD DGQ 10 1000 203.0 190.0 41.0 LM25011MYX/NOPB MSOP-PowerPAD DGQ 10 3500 358.0 343.0 63.0 LM25011Q1MY/NOPB MSOP-PowerPAD DGQ 10 1000 203.0 190.0 41.0 LM25011Q1MYX/NOPB MSOP-PowerPAD DGQ 10 3500 358.0 343.0 63.0 Pack Materials-Page 2 MECHANICAL DATA DGQ0010A MUC10A (Rev A) BOTTOM VIEW www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve 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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