LM25011

LM25011 NSC | Alldatasheet

Document overview

• Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
• PDF pages: 16

Technical content

Features

■ Input operating voltage range: 6V to 42V ■ Absolute maximum input rating: 45V ■ Integrated 2A N-Channel Buck Switch ■ Adjustable current limit ■ Adjustable output voltage from 2.51V ■ Minimum ripple voltage at VOUT ■ Power Good output ■ Switching frequency adjustable to 2 MHz ■ 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 ■ Adjustable Soft-Start timing ■ Thermal shutdown ■ Precision 2% feedback reference Package ■ MSOP-10EP Typical Application, Basic Step-Down Regulator 30094601 © 2009 National Semiconductor Corporation 300946 www.national.com LM25011 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit

10 Lead MSOP-EP

Ordering Information

Order Number Package Type NSC Package Drawing Supplied As LM25011MY MSOP-10EP MUC10A 1000 Units on Tape and Reel LM25011MYX MSOP-10EP MUC10A 3500 Units on Tape and Reel Pin Descriptions Pin No. Name Description Application Information 1 VIN Input supply voltage 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.

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 An internal current source charges an external capacitor to provide the soft-

start function. 5 SGND Signal Ground Ground for all internal circuitry other than the current limit sense circuit. 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 Internally connected to the buck switch source. Connect to the external inductor, cathode of the free-wheeling diode, and bootstrap capacitor.

10 BST Bootstrap capacitor connection of the

buck switch gate driver. 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. www.national.com 2 LM25011

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 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 (Note 2) Human Body Model 2kV Lead Temperature (soldering 4 sec) 260°C Storage Temperature Range -65°C to +150°C Junction Temperature 150°C Operating Ratings (Note 1) VIN Voltage 6.0V to 42V Junction Temperature –40°C to +125°C Electrical Charateristics 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 Typ Max Units Input (VIN Pin) IIN Input operating current Non-switching, FB = 3V 1200 1600 µA UVLOVIN VIN under-voltage lock-out threshold VIN Increasing 4.6 5.3 5.9 V VIN under-voltage lock-out threshold hysteresis 200 mV Switch Characteristics RDS(ON) Buck Switch RDS(ON) ITEST = 200 mA 0.3 0.6 Ω UVLOGD Gate Drive UVLO BST-SW 2.4 3.4 4.4 V UVLOGD Hysteresis 350 mV Pre-charge switch voltage ITEST = 10 mA into SW pin 1.4 V Pre-charge switch on-time 120 ns Soft-Start Pin VSS Pull-up voltage 2.51 V ISS Internal current source 10 µA VSS-SH Shutdown Threshold 70 140 mV Current Limit VILIM Threshold voltage at CS -146 -130 -115 mV CS bias current FB = 3V -120 µA CSG bias current FB = 3V -35 µA On Timer, RT Pin tON - 1 On-time VIN = 12V, RT = 50 kΩ 150 200 250 ns tON - 2 On-time VIN = 32V, RT = 50 kΩ 75 ns tON - 3 On-time (current limit) VIN = 12V, RT = 50 kΩ 100 ns tON - 4 On-time VIN = 12V, RT = 301 kΩ 1020 ns Off Timer tOFF Minimum Off-time 90 150 208 ns Regulation Comparator (FB Pin) VREF FB regulation threshold SS pin = steady state 2.46 2.51 2.56 V FB bias current FB = 3V 100 nA 3 www.national.com LM25011

Symbol Parameter Conditions Min Typ Max Units Power Good (PGD pin) Threshold at FB, with respect to VREF FB increasing 91 95 % Threshold hysteresis 3.3 % PGDVOL Low state voltage IPGD = 1mA, FB = 0V 125 180 mV PGDLKG Off state leakage VPGD = 7V, FB = 3V 0.1 µA Thermal Shutdown TSD Thermal shutdown Junction temperature increasing 155 °C Thermal shutdown hysteresis 20 °C Thermal Resistance θJA Junction to Ambient, 0 LFPM Air Flow (note 3) 48 °C/W θJC Junction to Case, (note 3) 10 °C/W 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 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: JEDEC test board description can be found in JESD 51-5 and JESD 51-7. Note 4: Current flow out of a pin is indicated as a negative number. www.national.com 4 LM25011

Typical Performance Characteristics Efficiency (Circuit of Figure 5) 30094603 Efficiency at 2 MHz 30094604 On-Time vs VIN and RT 30094605 Voltage at the RT Pin 30094606 Shutdown Current into VIN 30094607 Operating Current into VIN 30094608 5 www.national.com LM25011

PGD Low Voltage vs. Sink Current 30094609 Reference Voltage vs. Temperature 30094610 Current Limit Threshold vs. Temperature 30094611 Operating Current vs. Temperature 30094612 VIN UVLO vs. Temperature 30094613 SS Pin ShutdownThreshold vs. Temperature 30094614 www.national.com 6 LM25011

On-Time vs. Temperature 30094615 Minimum Off-Time vs. Temperature 30094616 7 www.national.com LM25011

www.national.com 8 LM25011

FIGURE 1. Startup Sequence within 5% of the expected regulation voltage. the minimum off-time forced by the off-time one-shot timer. on for another on-time one-shot period.

plitude. The approximate operating frequency is calculated as follows: (1) The buck switch duty cycle is approximately equal to: (2) When the load current is less than one half the inductor’s rip- ple 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 re- duced. Conversion efficiency is maintained at light loads since the switching losses are reduced with the reduction in load and frequency. The approximate discontinuous operat- ing frequency can be calculated as follows: (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 volt- age 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 inter- nally by the LM25011’s ERM (Emulated Ripple Mode) control block. The ERM circuit generates the required internal ripple voltage from the ripple waveform at the CS pin during each off-time. This feature eliminates the need for ripple at V OUT, allowing output ripple to be kept to a minimum. Output ripple is therefore a function of the inductor’s ripple current and the characteristics of the output capacitor. On-Time Timer The on-time for the LM25011 is determined by the RT resistor and the input voltage (VIN), calculated from: (5) The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific continuous con- duction mode switching frequency (F S), the R T resistor is determined from the following: (6) The on-time must be chosen greater than 90 ns for proper operation. Equations 1, 5 and 6 are valid only during normal operation - i.e., the circuit is not in current limit. When the LM25011 operates in current limit, the on-time is reduced by ≊40%. This feature reduces the peak inductor current which may be excessively high if the load current and the input volt- age 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 max- imum 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 min- imum 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, pre- vents achievement of the proper duty cycle. Current Limit Current limit detection occurs during the off-time by monitor- ing 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 in- ductor. 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 de- tected, the on-time is reduced by ≊40% if the voltage at the FB pin is below its threshold when the voltage across RS re- duces to its threshold (VOUT is low due to current limiting). Figure 2 illustrates the inductor current waveform during nor- mal 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 in- ductor current increases until the lower peak of the inductor ripple current exceeds the threshold. During the “Current Lim- ited” portion of Figure 2, each on-time is reduced by ≊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 thresh- old plus half the ripple amplitude (IOCL), and the output voltage is below the normal regulated value. Normal operation re- sumes when the load current is reduced (to I O2), 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 dur- ing each off time. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 2.0A. www.national.com 10 LM25011

FIGURE 4. Shutdown Implemetation the FB pin is close to the internal 2.51V reference voltage. voltage at VIN reaches 2V, at which time PGD switches low. low until VIN falls below 2V, at which time PGD switches high. at the FB pin is above the thresholds mentioned above.

• 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 R FB1: These resistors set the output voltage, and their ratio is calculated from: RFB2/RFB1 = (VOUT/2.51V) - 1 (7) For this example, R FB2/RFB1 = 0.992. R FB1 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 switch- ing frequency. First check that the desired frequency does not require an on-time or off-time shorter than the minimum al- lowed values (90 ns and 150, respectively). The minimum on- time occurs at the maximum input voltage. For this example: The minimum off-time occurs at the minimum input voltage. For this example: 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: A standard value 118 k Ω 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 (I OR), and the ripple voltage ampli- tude 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 (8) 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 I OUT(max) for IOUT (min) in equation 8. The ripple amplitude calculated in Equation 8 is then used in the following equation: 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- RS: The minimum current limit threshold is calculated at max- imum load current, using the minimum ripple current calcu- lated above. The current limit threshold is the lower peak of the inductor current waveform when in current limit (see Fig- ure 2). www.national.com 12 LM25011

Current limit detection occurs when the voltage across the sense resistor (RS) reaches the current limit threshold. To al- low for tolerances, the sense resistor value is calculated using the minimum threshold specification: RS = 115 mV/1.4A = 82 mΩ 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 10mVp-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 If the ripple voltage had calculated to less than 10 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 R S 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 cur- rent. Under normal load conditions, the maximum power dis- sipation in R S occurs at maximum load current, and at maximum input voltage where the on-time duty cycle is min- imum. In this design example, the minimum on-time duty cycle is: At maximum load current, the power dissipation in RS is equal to: When in current limit the maximum power dissipation in R S calculates to P(RS) = (1.83A + 0.472A/4)2 x 0.080Ω = 304 mW 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. C OUT should be a low ESR good quality ceramic capacitor. Experimentation is usually necessary to determine the minimum value for C OUT, 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 non-varying load. CIN and C BYP: The purpose of C IN 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 up- per 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, C IN 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: (9) where tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN. The purpose of CBYP is to minimize tran- sients and ringing due to long lead inductance leading to the VIN pin. A low ESR 0.1 µF ceramic chip capacitor is recom- mended, and C BYP must be located close to the VIN and SGND pins. CBST: The recommended value for C BST 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 1). For a soft-start time of 5 ms, the capacitor value is determined from the following: 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) 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 5, and its performance is shown in Figure 6 and Figure 7. The current limit measured approximately 1.62A at Vin = 8V, and 1.69A at Vin = 36V. 13 www.national.com LM25011

Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead MSSOP-EP Package 15 www.national.com LM25011

LM25011 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy PowerWise® Solutions www.national.com/powerwise Solutions www.national.com/solutions Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless PowerWise® Design University www.national.com/training 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. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. Copyright© 2009 National Semiconductor Corporation For the most current product information visit us at www.national.com National Semiconductor Americas Technical Support Center Email: support@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Technical Support Center Email: europe.support@nsc.com National Semiconductor Asia Pacific Technical Support Center Email: ap.support@nsc.com National Semiconductor Japan Technical Support Center Email: jpn.feedback@nsc.com www.national.com