LM25005 NSC | Alldatasheet
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Technical content
Features
n Integrated 42V, 160mΩN-channel MOSFET n Ultra-wide input voltage range from 7V to 42V n Internal bias regulator n Adjustable output voltage from 1.225V n 1.5% feedback reference accuracy n Current mode control with emulated inductor current ramp n Single resistor oscillator frequency setting n Oscillator synchronization input n Programmable soft-start n Shutdown / Standby input n Wide bandwidth error amplifier n Thermal Shutdown Package n TSSOP-20EP (Exposed Pad) Simplified Application Schematic 20170001 February 2006 LM25005 42V, 2.5A Step-Down Switching Regulator © 2006 National Semiconductor Corporation DS201700 www.national.com
Ordering Information
73 Units in Rail
2500 Units on Tape and Reel
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Pin(S) Name
Description
Application Information
Output of the bias regulator Vcc tracks Vin up to 9V. Beyond 9V, Vcc is regulated to 7 Volts. A 0.1uF to 1uF ceramic decoupling capacitor is required. An external voltage (7.5V – 14V) can be applied to this pin to reduce internal power dissipation. SD Shutdown or UVLO input If the SD pin voltage is below 0.7V the regulator will be in a low power state. If the SD pin voltage is between 0.7V and 1.225V the regulator will be in standby mode. If the SD pin voltage is above 1.225V the regulator will be operational. An external voltage divider can be used to set a line undervoltage shutdown threshold. If the SD pin is left open circuit, a 5µA pull-up current source configures the regulator fully operational. 3, 4 Vin Input supply voltage Nominal operating range: 7V to 42V SYNC Oscillator synchronization input or output The internal oscillator can be synchronized to an external clock with an external pull-down device. Multiple LM25005 devices can be synchronized together by connection of their SYNC pins. COMP Output of the internal error amplifier The loop compensation network should be connected between this pin and the FB pin. FB Feedback signal from the regulated output This pin is connected to the inverting input of the internal error amplifier. The regulation threshold is 1.225V. RT Internal oscillator frequency set input The internal oscillator is set with a single resistor, connected between this pin and the AGND pin. The recommended frequency range is 50KHz to 500KHz. RAMP Ramp control signal An external capacitor connected between this pin and the AGND pin sets the ramp slope used for current mode control. Recommended capacitor range 50pF to 2000pF. AGND Analog ground Internal reference for the regulator control functions SS Soft-start An external capacitor and an internal 10µA current source set the time constant for the rise of the error amp reference. The SS pin is held low during standby, Vcc UVLO and thermal shutdown. OUT Output voltage connection Connect directly to the regulated output voltage. 13, 14 PGND Power ground Low side reference for the PRE switch and the IS sense resistor. 15, 16 IS Current sense Current measurement connection for the re-circulating diode. An internal sense resistor and a sample/hold circuit sense the diode current near the conclusion of the off-time. This current measurement provides the DC level of the emulated current ramp. 17, 18 SW Switching node The source terminal of the internal buck switch. The SW pin should be connected to the external Schottky diode and to the buck inductor. PRE Pre-charge assist for the bootstrap capacitor This open drain output can be connected to SW pin to aid charging the bootstrap capacitor during very light load conditions or in applications where the output may be pre-charged before the LM25005 is enabled. An internal pre-charge MOSFET is turned on for 250ns each cycle just prior to the on-time interval of the buck switch. LM25005 www.national.com
(Continued) Pin(S) Name Boost input for bootstrap capacitor An external capacitor is required between the BST and the SW pins. A 0.022µF ceramic capacitor is recommended. The capacitor is charged from Vcc via an internal diode during the off-time of the buck switch. NA EP Exposed Pad Exposed metal pad on the underside of the device. It is recommended to connect this pad to the PWB ground plane, in order to aid in heat dissipation. LM25005 www.national.com
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 GND 45V BST to GND 60V PRE to GND 45V SW to GND (Steady State) -1.5V BST to VCC 45V VCC to GND 14V BST to SW 14V OUT to GND Limited to Vin SD, SYNC, SS, FB to GND ESD Rating (Note 2) Human Body Model 2kV Storage Temperature Range -65˚C to +150˚C Operating Ratings (Note 1) VIN 7V to 42V Operation Junction Temperature −40˚C to + 125˚C
Electrical Characteristics
Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 24V, RT = 32.4kΩunless otherwise stated. (Note 3) Symbol Parameter Conditions Min Typ Max Units STARTUP REGULATOR VccReg Vcc Regulator Output 6.85 7.15 7.45 V Vcc LDO Mode turn-off V Vcc Current Limit Vcc = 0V mA VCC SUPPLY Vcc UVLO Threshold (Vcc increasing) 5.95 6.35 6.75 V Vcc Undervoltage Hysteresis V Bias Current (Iin) FB = 1.3V 4.5 mA Shutdown Current (Iin) SD = 0V µA SHUTDOWN THRESHOLDS Shutdown Threshold (SD Increasing) 0.5 0.7 0.9 V Shutdown Hysteresis 0.1 V Standby Threshold (Standby Increasing) 1.18 1.225 1.27 V Standby Hysteresis 0.1 V SD Pull-up Current Source µA SWITCH CHARACTERSICS Buck Switch Rds(on) 160 320 mΩ BOOST UVLO 3.8 V BOOST UVLO Hysteresis 0.56 V Pre-charge Switch Rds(on) Ω Pre-charge Switch on-time 275 ns CURRENT LIMIT Cycle by Cycle Current Limit RAMP = 0V 3.5 4.25 A Cycle by Cycle Current Limit Delay RAMP = 2.5V 100 ns SOFT-START SS Current Source µA OSCILLATOR Frequency1 180 200 220 KHz Frequency2 RT = 11kΩ 425 485 525 KHz SYNC Source Impedance kΩ SYNC Sink Impedance 160 Ω SYNC Threshold (falling) 1.4 V Upper SYNC Frequency 550 KHz SYNC Pulse Width Minimum ns LM25005 www.national.com
Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 24V, RT = 32.4kΩunless otherwise stated. (Note 3) (Continued) Symbol Parameter Conditions Min Typ Max Units RAMP GENERATOR Ramp Current 1 Vin = 36V, Vout=10V 136 160 184 µA Ramp Current 2 Vin = 10V, Vout=10V µA PWM COMPARATOR Forced Off-time 500 ns Min On-time ns COMP to PWM Comparator Offset 0.7 V ERROR AMPLIFIER Feedback Voltage Vfb = COMP 1.207 1.225 1.243 V FB Bias Current nA DC Gain dB COMP Sink / Source Current mA Unity Gain Bandwidth MHz THERMAL SHUTDOWN Tsd Thermal Shutdown Threshold 165 Thermal Shutdown Hysteresis THERMAL RESISTANCE θJC Junction to Case ˚C/W θJA Junction to Ambient ˚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 100pF capacitor discharged through a 1.5kΩresistor into each pin. Note 3: Min and Max limits are 100% production tested at 25˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). LM25005 www.national.com
Typical Performance Characteristics Oscillator Frequency vs RT Oscillator Frequency vs Temperature FOSC = 200kHz 20170020 20170021 Soft Start Current vs Temperature VCC vs ICC VIN = 12V 20170022 20170023 VCC vs VIN RL = 7kΩ Error Amplifier Gain/Phase AVCL = 101 20170024 20170025 LM25005 www.national.com
Typical Performance Characteristics (Continued) Demoboard Efficiency vs IOUT and VIN 20170026 LM25005 www.national.com
Typical Application Circuit and Block Diagram 20170003 FIGURE 1. LM25005 www.national.com
featuring an exposed pad to aid thermal dissipation. through the use of this dual mode regulator. The output of the Vcc regulator is current limited to 20mA. Vin that should not be forward biased in normal operation. Vin and GND pins are essential. FIGURE 2. Vin and Vcc Sequencing
output voltage and an internal precision reference (1.225V). nents, generally a type II network, as illustrated in Figure 1. FIGURE 5. Simplified Oscillator Block Diagram and SYNC I/O Circuit
hold provide the dc level of the reconstructed current signal. creased to increase the ramp slope compensation. FIGURE 6. Composition of Current Sense Signal
The LM25005 contains a unique current monitoring scheme for control and over-current protection. When set correctly, the emulated current sense signal provides a signal which is proportional to the buck switch current with a scale factor of 0.5 V / A. The emulated ramp signal is applied to the current limit comparator. If the emulated ramp signal exceeds 1.75V (3.5A) the present current cycle is terminated (cycle-by-cycle current limiting). In applications with small output inductance and high input voltage the switch current may overshoot due to the propagation delay of the current limit comparator. If an overshoot should occur, the diode current sampling circuit will detect the excess inductor current during the off-time of the buck switch. If the Sample and Hold DC Level exceeds the 1.75V current limit threshold, the buck switch will be disabled and skip pulses until the diode current sampling circuit detects the inductor current has decayed below the current limit threshold. This approach prevents current run- away conditions due to propagation delays or inductor satu- ration since the inductor current is forced to decay following any current overshoot. Soft-Start The soft-start feature allows the regulator to gradually reach the initial steady state operating point, thus reducing start-up stresses and surges. The internal soft-start current source, set to 10µA, gradually increases the voltage of an external soft-start capacitor connected to the SS pin. The soft-start capacitor voltage is connected to the reference input of the error amplifier. Various sequencing and tracking schemes can be implemented using external circuits that limit or clamp the voltage level of the SS pin. In the event a fault is detected (over-temperature, Vcc UVLO, SD) the soft-start capacitor will be discharged. When the fault condition is no longer present a new soft-start sequence will commence. Boost Pin The LM25005 integrates an N-Channel buck switch and associated floating high voltage level shift / gate driver. This gate driver circuit works in conjunction with an internal diode and an external bootstrap capacitor. A 0.022µF ceramic capacitor, connected with short traces between the BST pin and SW pin, is recommended. During the off-time of the buck switch, the SW pin voltage is approximately - 0.5V and the bootstrap capacitor is charged from Vcc through the internal bootstrap diode. When operating with a high PWM duty cycle, the buck switch will be forced off each cycle for 500ns to ensure that the bootstrap capacitor is recharged. Under very light load conditions or when the output voltage is pre-charged, the SW voltage will not remain low during the off-time of the buck switch. If the inductor current falls to zero and the SW pin rises, the bootstrap capacitor will not receive sufficient voltage to operate the buck switch gate driver. For these applications, the PRE pin can be connected to the SW pin to pre-charge the bootstrap capacitor. The internal pre- charge MOSFET and diode connected between the PRE pin and PGND turns on each cycle for 250ns just prior to the onset of a new switching cycle. If the SW pin is at a normal negative voltage level (continuous conduction mode), then no current will flow through the pre-charge MOSFET/diode. Thermal Protection Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction tem- perature is exceeded. When activated, typically at 165 de- grees Celsius, the controller is forced into a low power reset state, disabling the output driver and the bias regulator. This feature is provided to prevent catastrophic failures from ac- cidental device overheating. LM25005 www.national.com
The procedure for calculating the external components is illustrated with the following design example. The Bill of Materials for this design is listed in Table1. The circuit shown in Figure 1 is configured for the following specifications: VOUT = 5V VIN = 7V to 42V Fs = 300 KHz Minimum load current (for CCM) = 250 mA Maximum load current = 2.5A R3 (RT) RT sets the oscillator switching frequency. Generally, higher frequency applications are smaller but have higher losses. Operation at 300KHz was selected for this example as a reasonable compromise for both small size and high effi- ciency. The value of RT for 300KHz switching frequency can be calculated as follows: The nearest standard value of 21 kΩwas chosen for RT. The inductor value is determined based on the operating frequency, load current, ripple current, and the minimum and maximum input voltage (VIN(min), VIN(max)). To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less than twice the minimum load current, or 0.5 Ap-p. Using this value of ripple current, the value of inductor (L1) is calculated using the following: This procedure provides a guide to select the value of L1. The nearest standard value (33 µH) will be used. L1 must be rated for the peak current (IPK+) to prevent saturation. During normal loading conditions, the peak current occurs at maxi- mum load current plus maximum ripple. During an overload condition the peak current is limited to 3.5A nominal (4.25A maximum). The selected inductor (see Table 1) has a con- servative 6.2 Amp saturation current rating. For this manu- facturer, the saturation rating is defined as the current nec- essary for the inductance to reduce by 30%, at 20˚C. C3 (CRAMP) With the inductor value selected, the value of C3 (CRAMP) necessary for the emulation ramp circuit is: CRAMP = L x 10-5 Where L is in Henrys With L1 selected for 33µH the recommended value for C3 is 330pF. C9, C10 The output capacitors C9, and C10, smooth the inductor ripple current and provide a source of charge for transient loading conditions. For this design a 22µF ceramic capacitor and a 150µF SP organic capacitor were selected. The ce- ramic capacitor provides ultra low ESR to reduce the output ripple voltage and noise spikes, while the SP capacitor pro- vides a large bulk capacitance in a small volume for transient loading conditions. An approximation for the output ripple voltage is: A Schottky type re-circulating diode is required for all LM25005 applications. Ultra-fast diodes are not recom- mended and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage and low output voltage applications common to the LM25005. The reverse recovery characteristic determines how long the current surge lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes minimize the peak instantaneous power in the buck switch occurring during turn-on each cycle. The resulting switching losses of the buck switch are significantly reduced when using a Schottky diode. The reverse breakdown rating should be selected for the maximum VIN, plus some safety margin. The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a low output voltage. “Rated” current for diodes vary widely from various manufactures. The worst case is to assume a short circuit load condition. In this case the diode will carry the output current almost continuously. For the LM25005 this current can be as high as 3.5A. Assuming a worst case 1V drop across the diode, the maximum diode power dissipation can be as high as 3.5W. For the reference design a 60V Schottky in a DPAK package was selected. C1, C2 The regulator supply voltage has a large source impedance at the switching frequency. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. 20170010 FIGURE 7. Inductor Current Waveform
(Continued) When the buck switch turns on, the current into the VIN pin steps to the lower peak of the inductor current waveform, ramps up to the peak value, then drops to zero at turn-off. The average current into VIN during the on-time is the load current. The input capacitance should be selected for RMS current rating and minimum ripple voltage. A good approxi- mation for the required ripple current rating necessary is IRMS > IOUT / 2. Quality ceramic capacitors with a low ESR should be se- lected for the input filter. To allow for capacitor tolerances and voltage effects, two 2.2 µF, 100V ceramic capacitors will be used. If step input voltage transients are expected near the maximum rating of the LM25005, a careful evaluation of ringing and possible spikes at the device VIN pin should be completed. An additional damping network or input voltage clamp may be required in these cases. The capacitor at the VCC pin provides noise filtering and stability for the VCC regulator. The recommended value of C8 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. A value of 0.47 µF was selected for this design. The bootstrap capacitor between the BST and the SW pins supplies the gate current to charge the buck switch gate at turn-on. The recommended value of C7 is 0.022 µF, and should be a good quality, low ESR, ceramic capacitor. The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage and the output voltage, to reach the final regulated value. The time is determined from: For this application, a C4 value of 0.01 µF was chosen which corresponds to a soft-start time of 1 ms. R5, R6 R5 and R6 set the output voltage level, the ratio of these resistors is calculated from: R5/R6 = (VOUT / 1.225V) - 1 For a 5V output, the R5/R6 ratio calculates to 3.082. The resistors should be chosen from standard value resistors, a good starting point is selection in the range of 1.0 kΩ- 10 kΩ. Values of 5.11 kΩfor R5, and 1.65 kΩfor R6 were selected. R1, R2, C12 A voltage divider can be connected to the SD pin to set a minimum operating voltage Vin(min) for the regulator. If this feature is required, the easiest approach to select the divider resistor values is to select a value for R1 (between 10 kΩ and 100 kΩrecommended) then calculate R2 from: Capacitor C12 provides filtering for the divider. The voltage at the SD pin should never exceed 8V, when using an external set-point divider it may be necessary to clamp the SD pin at high input voltage conditions. The reference de- sign utilizes the full range of the LM25005 (7V to 42V); therefore these components can be omitted. With the SD pin open circuit the LM25005 responds once the Vcc UVLO threshold is satisfied. R7, C11 A snubber network across the power diode reduces ringing and spikes at the switching node. Excessive ringing and spikes can cause erratic operation and couple spikes and noise to the output. In the limit, spikes beyond the rating of the LM25005 or the re-circulating diode can damage these devices. Selecting the values for the snubber is best accom- plished through empirical methods. First, make sure the lead lengths for the snubber connections are very short. For the current levels typical for the LM25005 a resistor value be- tween 5 and 20 Ohms is adequate. Increasing the value of the snubber capacitor results in more damping but higher losses. Select a minimum value of C11 that provides ad- equate damping of the SW pin waveform at high load. R4, C5, C6 These components configure the error amplifier gain char- acteristics to accomplish a stable overall loop gain. One advantage of current mode control is the ability to close the loop with only two feedback components, R4 and C5. The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC modulator gain of the LM25005 is as follows: DC Gain(MOD) = Gm(MOD) x RLOAD = 2 x RLOAD The dominant low frequency pole of the modulator is deter- mined by the load resistance (RLOAD,) and output capaci- tance (COUT). The corner frequency of this pole is: fp(MOD) = 1 / (2π RLOAD COUT) For RLOAD = 5 Ωand COUT = 177 µF then fp(MOD) = 180Hz DC Gain(MOD) = 2 x 5 = 10 = 20 dB For the design example of Figure 1 the following modulator gain vs. frequency characteristic was measured as shown in Figure 8. LM25005 www.national.com
(Continued) PCB LAYOUT AND THERMAL CONSIDERATIONS The circuit in Figure 1 serves as both a block diagram of the LM25005 and a typical application board schematic for the LM25005. In a buck regulator there are two loops where currents are switched very fast. The first loop starts from the input capacitors, to the regulator VIN pin, to the regulator SW pin, to the inductor then out to the load. The second loop starts from the output capacitor ground, to the regulator PGND pins, to the regulator IS pins, to the diode anode, to the inductor and then out to the load. Minimizing the loop area of these two loops reduces the stray inductance and minimizes noise and possible erratic operation. A ground plane in the PC board is recommended as a means to connect the input filter capacitors to the output filter capaci- tors and the PGND pins of the regulator. Connect all of the low power ground connections (CSS, RT, CRAMP) directly to the regulator AGND pin. Connect the AGND and PGND pins together through the topside copper area covering the entire underside of the device. Place several vias in this underside copper area to the ground plane. The two highest power dissipating components are the re- circulating diode and the LM25005 regulator IC. The easiest method to determine the power dissipated within the LM25005 is to measure the total conversion losses (Pin – Pout) then subtract the power losses in the Schottky diode, output inductor and snubber resistor. An approximation for the Schottky diode loss is P = (1-D) x Iout x Vfwd. An approximation for the output inductor power is P = IOUT 2 x R x 1.1, where R is the DC resistance of the inductor and the 1.1 factor is an approximation for the ac losses. If a snubber is used, the power loss can be estimated with an oscillo- scope by observation of the resistor voltage drop at both turn-on and turn-off transitions. The regulator has an ex- posed thermal pad to aid power dissipation. Adding several vias under the device to the ground plane will greatly reduce the regulator junction temperature. Selecting a diode with an exposed pad will aid the power dissipation of the diode. TABLE 1. 5V, 2.5A Demo Board Bill of Materials C C4532X7R2A225M CAPACITOR, CER, TDK 2.2µ, 100V C C4532X7R2A225M CAPACITOR, CER, TDK 2.2µ, 100V C C0805C331G1GAC CAPACITOR, CER, KEMET 330p, 100V C C2012X7R2A103K CAPACITOR, CER, TDK 0.01µ, 100V C C2012X7R2A103K CAPACITOR, CER, TDK 0.01µ, 100V C OPEN NOT USED C C2012X7R2A223K CAPACITOR, CER, TDK 0.022µ, 100V C C2012X7R1C474M CAPACITOR, CER, TDK 0.47µ, 16V C C3225X7R1C226M CAPACITOR, CER, TDK 22µ, 16V C EEFHE0J151R CAPACITOR, SP, PANASONIC 150µ, 6.3V C C0805C331G1GAC CAPACITOR, CER, KEMET 330p, 100V C OPEN NOT USED D CSHD6-60C DIODE, 60V, CENTRAL 6CWQ10FN DIODE, 100V, IR (D1-ALT) L DR127-330 INDUCTOR, COOPER 33µH R OPEN NOT USED R OPEN NOT USED R CRCW08052102F RESISTOR 21K R CRCW08054992F RESISTOR 49.9K R CRCW08055111F RESISTOR 5.11K R CRCW08051651F RESISTOR 1.65K R CRCW2512100J RESISTOR 10, 1W U LM25005 REGULATOR, NATIONAL SEMICONDUCTOR LM25005 www.national.com
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inches (millimeters) unless otherwise noted 20-Lead TSSOP Package National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems 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. 2. A critical component is any component of 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. BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. Leadfree products are RoHS compliant. National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 www.national.com LM25005 42V, 2.5A Step-Down Switching Regulator