LM2591HV SIMPLE SWITCHER ® Power Converter 150 kHz 1A Step-Down Voltage Regulator General Description The LM2591HV series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 1A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, and an adjustable output version. This series of switching regulators is similar to the LM2590HV, but without some of the supervisory and performance features of the latter. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation†, improved line and load specifications and a fixed-frequency oscillator. The LM2591HV operates at a switching frequency of 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 5-lead TO-220 package with several different lead bend options, and a 5-lead TO-263 Surface mount package. Other features include a guaranteed ± 4% tolerance on output voltage under all conditions of input voltage and output load conditions, and ± 15% on the oscillator frequency. External shutdown is included, featuring typically 90 µA standby current. Self protection features include a two stage Typical Application current limit for the output switch and an over temperature shutdown for complete protection under fault conditions. Features n 3.3V, 5V, and adjustable output versions n Adjustable version output voltage range, 1.2V to 57V ± 4% max over line and load conditions n Guaranteed 1A output load current n Available in 5-pin TO-220 and TO-263 (surface mount) Package n Input voltage range up to 60V n 150 kHz fixed frequency internal oscillator n On/Off control n Low power standby mode, IQ typically 90 µA n High Efficiency n Thermal shutdown and current limit protection Applications n n n n Simple high-efficiency step-down (buck) regulator Efficient pre-regulator for linear regulators On-card switching regulators Positive to Negative converter Note: † Patent Number 5,382,918. (Fixed Output Voltage Versions) 10129301 SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation. © 2003 National Semiconductor Corporation DS101293 www.national.com LM2591HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator July 2003 LM2591HV Absolute Maximum Ratings Human Body Model (Note 2) (Note 1) Lead Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Supply Voltage (VIN) S Package 63V ON/OFF Pin Voltage −0.3 ≤ V ≤ +25V Feedback Pin Voltage −0.3 ≤ V ≤ +25V 2 kV Vapor Phase (60 sec.) +215˚C Infrared (10 sec.) +245˚C T Package (Soldering, 10 sec.) +260˚C Maximum Junction Temperature +150˚C Output Voltage to Ground (Steady State) −1V Power Dissipation Internally limited Storage Temperature Range −65˚C to +150˚C Operating Conditions −40˚C ≤ TJ ≤ +125˚C Temperature Range Supply Voltage ESD Susceptibility 4.5V to 60V LM2591HV-3.3 Electrical Characteristics Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2591HV-3.3 Typ Limit (Note 3) (Note 4) Units (Limits) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VOUT η Output Voltage Efficiency 4.75V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A VIN = 12V, ILOAD = 1A 3.3 V 3.168/3.135 V(min) 3.432/3.465 V(max) 77 LM2591HV-5.0 Electrical Characteristics Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2591HV-5.0 Typ Limit (Note 3) (Note 4) Units (Limits) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VOUT η Output Voltage Efficiency 7V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A VIN = 12V, ILOAD = 1A 5 V 4.800/4.750 V(min) 5.200/5.250 V(max) 82 % LM2591HV-ADJ Electrical Characteristics Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2591HV-ADJ Typ Limit (Note 3) (Note 4) Units (Limits) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VFB Feedback Voltage 4.5V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A 1.230 VOUT programmed for 3V. Circuit of Figure 1. η Efficiency www.national.com VIN = 12V, VOUT = 3V, ILOAD = 1A 2 76 V 1.193/1.180 V(min) 1.267/1.280 V(max) % Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version. ILOAD = 500 mA Symbol Parameter Conditions LM2591HV-XX Typ Limit (Note 3) (Note 4) Units (Limits) DEVICE PARAMETERS Feedback Bias Current Ib fO VSAT DC ICLIM IL Oscillator Frequency Saturation Voltage Adjustable Version Only, VFB = 1.3V (Note 6) IOUT = 1A (Note 7) (Note 8) (Note 8) 100 (Note 9) 0 Switch current Limit Peak Current, (Note 7) (Note 8) (Note 7) (Note 9) (Note 10) Output = 0V SD /SS Pin Open (Note 9) 5 SD /SS pin = 0V 90 (Note 10) Current θJC Thermal Resistance kHz(min) 173/173 kHz(max) 1.2/1.3 V(max) kHz V % A 1.3/1.2 A(min) 2.8/3.0 A(max) 50 µA(max) 30 mA(max) 5 Current Standby Quiescent 127/110 1.9 Output = −1V ISTBY nA (max) 0.95 Max Duty Cycle (ON) Operating Quiescent nA 50/100 150 Min Duty Cycle (OFF) Output Leakage Current IQ 10 mA mA 10 mA(max) 200/250 µA(max) µA TO220 or TO263 Package, Junction to Case 2 ˚C/W θJA TO220 Package, Juncton to Ambient (Note 11) 50 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 12) 50 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 13) 30 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 14) 20 ˚C/W 1.3 V ON/OFF CONTROL Test Circuit Figure 1 ON /OFF Pin Logic Input VIH Threshold Voltage VIL IH ON /OFF Pin Input Current IL Low (Regulator ON) 0.6 V(max) High (Regulator OFF) 2.0 V(min) VLOGIC = 2.5V (Regulator OFF) 5 VLOGIC = 0.5V (Regulator ON) 0.02 µA 15 µA(max) 5 µA(max) µA Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin. Note 3: Typical numbers are at 25˚C and represent the most likely norm. Note 4: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2591HV is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics. Note 6: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current overload. Note 7: No diode, inductor or capacitor connected to output pin. Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON. Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version to force the output transistor switch OFF. Note 10: VIN = 60V. 3 www.national.com LM2591HV All Output Voltage Versions Electrical Characteristics LM2591HV All Output Voltage Versions Electrical Characteristics (Continued) Note 11: Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the leads soldered to a printed circuit board with (1 oz.) copper area of approximately 1 in2. Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2 of (1 oz.) copper area. Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2 of (1 oz.) copper area. Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2 of (1 oz.) copper area on the LM2591HVS side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model in Switchers Made Simple available at http://power.national.com. Typical Performance Characteristics (Circuit of Figure 1) Normalized Output Voltage Line Regulation 10129302 10129303 Switch Saturation Voltage Efficiency 10129305 10129304 www.national.com 4 Switch Current Limit LM2591HV Typical Performance Characteristics (Circuit of Figure 1) (Continued) Dropout Voltage 10129306 10129307 Operating Quiescent Current Shutdown Quiescent Current 10129308 10129309 Minimum Operating Supply Voltage Feedback Pin Bias Current 10129311 10129310 5 www.national.com LM2591HV Typical Performance Characteristics (Circuit of Figure 1) Switching Frequency (Continued) ON/OFF Threshold Voltage 10129313 10129379 ON/OFF Pin Current (Sinking) Internal Gain-Phase Characteristics 10129380 10129378 Discontinuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 250 mA L = 15 µH, COUT = 150 µF, COUT ESR = 90 mΩ Continuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 1A L = 52 µH, COUT = 100 µF, COUT ESR = 100 mΩ 10129320 10129319 A: Output Pin Voltage, 10V/div. A: Output Pin Voltage, 10V/div. B: Inductor Current 0.5A/div. B: Inductor Current 0.25A/div. C: Output Ripple Voltage, 50 mV/div. C: Output Ripple Voltage, 100 mV/div. Horizontal Time Base: 2 µs/div. www.national.com Horizontal Time Base: 2 µs/div. 6 (Continued) Load Transient Response for Discontinuous Mode VIN = 20V, VOUT = 5V, ILOAD = 250 mA to 1A L = 15 µH, COUT = 150 µF, COUT ESR = 90 mΩ Load Transient Response for Continuous Mode VIN = 20V, VOUT = 5V, ILOAD = 250 mA to 1A L = 52 µH, COUT = 100 µF, COUT ESR = 100 mΩ 10129322 A: Output Voltage, 100 mV/div. (AC) 10129321 B: 250 mA to 1A Load Pulse A: Output Voltage, 100 mV/div. (AC) Horizontal Time Base: 200 µs/div. B: 250 mA to 1A Load Pulse Horizontal Time Base: 50 µs/div. Connection Diagrams and Order Information Bent and Staggered Leads, Through Hole Package 5-Lead TO-220 (T) Surface Mount Package 5-Lead TO-263 (S) 10129382 10129381 Order Number LM2591HVS-3.3, LM2591HVS-5.0, or LM2591HVS-ADJ See NS Package Number TS5B Order Number LM2591HVT-3.3, LM2591HVT-5.0, or LM2591HVT-ADJ See NS Package Number T05D 7 www.national.com LM2591HV Typical Performance Characteristics (Circuit of Figure 1) LM2591HV Test Circuit and Layout Guidelines Fixed Output Voltage Versions 10129324 Component Values shown are for VIN = 15V, VOUT = 5V, ILOAD = 1A. — 470 µF, 50V, Aluminum Electrolytic Nichicon “PM Series” CIN — COUT 220 µF, 25V Aluminum Electrolytic, Nichicon “PM Series” D1 — 2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier) L1 — 68 µH, See Inductor Selection Procedure Adjustable Output Voltage Versions 10129325 Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability. Component Values shown are for VIN = 20V, VOUT = 10V, ILOAD = 1A. CIN: — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series” COUT: — 220 µF, 35V Aluminum Electrolytic, Nichicon “PM Series” D1 — 2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier) L1 — 100 µH, See Inductor Selection Procedure R1 — 1 kΩ, 1% R2 — 7.15k, 1% CFF — 3.3 nF Typical Values CSS — 0.1 µF CDELAY — 0.1 µF RPULL UP — 4.7k (use 22k if VOUT is ≥ 45V) † Small signal Schottky diode to prevent damage to feedback pin by negative spike when output is shorted (CFF not being able to discharge immediately will drag feedback pin below ground). Required if VIN > 40V FIGURE 1. Standard Test Circuits and Layout Guides www.national.com 8 LM2591HV Block Diagram 10129383 on this pin whenever the output is shorted. This happens because the feedforward capacitor cannot discharge fast enough, and since one end of it is dragged to Ground, the other end goes momentarily negative. To prevent the energy rating of this pin from being exceeded, a small-signal Schottky diode to Ground is recommended for DC input voltages above 40V whenever a feedforward capacitor is present (See Figure 1). Feedforward capacitor values larger than 0.1 µF are not recommended for the same reason, whatever be the DC input voltage. ON /OFF (Pin 5) — The regulator is in shutdown mode, drawing about 90 µA, when this pin is driven to a high level (≥ 2.0V), and is in normal operation when this Pin is left floating or driven to a low level (≤ 0.6V). The typical value of the threshold is 1.3V and the voltage on this pin must not exceed 25V. Pin Functions +VIN (Pin 1) — This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator. Output (Pin 2) — Internal switch. The voltage at this pin switches between approximately (+VIN − VSAT) and approximately −0.5V, with a duty cycle of VOUT/VIN. Ground (Pin 3) — Circuit ground. Feedback (Pin 4) — Senses the regulated output voltage to complete the feedback loop. This pin is directly connected to the Output for the fixed voltage versions, but is set to 1.23V by means of a resistive divider from the output for the Adjustable version. If a feedforward capacitor is used (Adjustable version), then a negative voltage spike is generated 9 www.national.com LM2591HV Inductor Value Selection Guides (For Continuous Mode Operation) 10129365 FIGURE 2. LM2591HV-3.3 10129366 FIGURE 3. LM2591HV-5.0 www.national.com 10 LM2591HV Inductor Value Selection Guides (For Continuous Mode Operation) (Continued) 10129367 FIGURE 4. LM2591HV-ADJ 10129368 FIGURE 5. Current Ripple Ratio 11 www.national.com LM2591HV Inductor Value Selection Guides (For Continuous Mode Operation) Coilcraft Inc. (Continued) Phone (USA): 1-800-322-2645 Web Address http://www.coilcraft.com Phone (UK): 1-236-730595 Web Address http://www.coilcraft-europe.com Phone (USA): 1-858-674-8100 Web Address http://www.pulseeng.com Pulse Engineering Inc., Phone (UK): 1-483-401700 Europe Web Address http://www.pulseeng.com Renco Electronics Inc. Phone (USA): 1-321-637-1000 Web Address http://www.rencousa.com Phone (USA): 1-952-475-1173 Web Address http://www.shottcorp.com Cooper Electronic Tech. (Coiltronics) Phone (USA): 1-888-414-2645 Web Address http://www.cooperet.com TDK Phone (USA): 1-847-803-6100 Web Address http://www.componet.tdk.com Coilcraft Inc., Europe Pulse Engineering Inc. Schott Corp. FIGURE 6. Contact Information for Suggested Inductor Manufacturers www.national.com 12 INDUCTOR SELECTION PROCEDURE Application Note AN-1197 titled "Selecting Inductors for Buck Converters" provides detailed information on this topic. For a quick-start the designer may refer to the nomographs provided in Figure 2 to Figure 4. To widen the choice of the Designer to a more general selection of available inductors, the nomographs provide the required inductance and also the energy in the core expressed in microjoules (µJ), as an alternative to just prescribing custom parts. The following points need to be highlighted: 1. The Energy values shown on the nomographs apply to steady operation at the corresponding x-coordinate (rated maximum load current). However under start-up, without soft-start, or a short-circuit on the output, the current in the inductor will momentarily/repetitively hit the current limit ICLIM of the device, and this current could be much higher than the rated load, ILOAD. This represents an overload situation, and can cause the Inductor to saturate (if it has been designed only to handle the energy of steady operation). However most types of core structures used for such applications have a large inherent air gap (for example powdered iron types or ferrite rod inductors), and so the inductance does not fall off too sharply under an overload. The device is usually able to protect itself by not allowing the current to ever exceed ICLIM. But if the DC input voltage to the regulator is over 40V, the current can slew up so fast under core saturation, that the device may not be able to act fast enough to restrict the current. The current can then rise without limit till destruction of the device takes place. Therefore to ensure reliability, it is recommended, that if the DC Input Voltage exceeds 40V, the inductor must ALWAYS be sized to handle an instantaneous current equal to ICLIM without saturating, irrespective of the type of core structure/material. 2. consider the rather wide tolerance on the nominal inductance of commercial inductors. 5. Figure 4 shows the inductor selection curves for the Adjustable version. The y-axis is ’Et’, in Vµsecs. It is the applied volts across the inductor during the ON time of the switch (VIN-VSAT-VOUT) multiplied by the time for which the switch is on in µsecs. See Example 3 below. Example 1: (VIN ≤ 40V) LM2591HV-5.0, VIN = 24V, Output 5V @ 0.8A 1. A first pass inductor selection is based upon Inductance and rated max load current. We choose an inductor with the Inductance value indicated by the nomograph (Figure 3) and a current rating equal to the maximum load current. We therefore quick-select a 100µH/0.8A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle 50 µJ (see Figure 3) by either estimating the peak current or by a detailed calculation as shown in AN-1197, and also that the losses are acceptable. Example 2: (VIN > 40V) LM2591HV-5.0, VIN = 48V, Output 5V @ 1A 1. A first pass inductor selection is based upon Inductance and the switch currrent limit. We choose an inductor with the Inductance value indicated by the nomograph (Figure 3) and a current rating equal to ICLIM. We therefore quick-select a 100µH/3A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle eCLIM by the procedure shown in AN-1197 and that the losses are acceptable. Here eCLIM is: The Energy under steady operation is Example 3: (VIN ≤ 40V) LM2591HV-ADJ, VIN = 20V, Output 10V @ 1A 1. Since input voltage is less than 40V, a first pass inductor selection is based upon Inductance and rated max load current. We choose an inductor with the Inductance value indicated by the nomograph Figure 4 and a current rating equal to the maximum load. But we first need to calculate Et for the given application. The Duty cycle is where L is in µH and IPEAK is the peak of the inductor current waveform with the regulator delivering ILOAD. These are the energy values shown in the nomographs. See Example 1 below. 3. The Energy under overload is If VIN > 40V, the inductor should be sized to handle eCLIM instead of the steady energy values. The worst case ICLIM for the LM2591HV is 3A. The Energy rating depends on the Inductance. See Example 2 below. 4. where VD is the drop across the Catch Diode () 0.5V for a Schottky) and VSAT the drop across the switch ()1.5V). So The nomographs were generated by allowing a greater amount of percentage current ripple in the Inductor as the maximum rated load decreases (see Figure 5). This was done to permit the use of smaller inductors at light loads. Figure 5 however shows only the ’median’ value of the current ripple. In reality there may be a great spread around this because the nomographs approximate the exact calculated inductance to standard available values. It is a good idea to refer to AN-1197 for detailed calculations if a certain maximum inductor current ripple is required for various possible reasons. Also And the switch ON time is where f is the switching frequency in Hz. So 13 www.national.com LM2591HV Application Information LM2591HV Application Information OUTPUT CAPACITOR COUT — An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter. The ESR should generally not be less than 100 mΩ or there will be loop instability. If the ESR is too large, efficiency and output voltage ripple are effected. So ESR must be chosen carefully. (Continued) Therefore, looking at Figure 2 we quick-select a 100µH/1A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle 100 µJ (see Figure 4) by the procedure shown in AN-1197 and that the losses are acceptable. (If the DC Input voltage had been greater than 40V we would need to consider eCLIM as in Example 2 above). Note that we have taken VSAT as 1.5V which includes an estimated resistive drop across the inductor. CATCH DIODE Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This must be a fast diode and must be located close to the LM2591HV using short leads and short printed circuit traces. This completes the simplified inductor selection procedure. For more general applications and better optimization, the designer should refer to AN-1197. Figure 6 provides helpful contact information on suggested Inductor manufacturers who may be able to recommend suitable parts, if the requirements are known. Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best performance, especially in low output voltage applications (5V and lower). Ultra-fast recovery, or High-Efficiency rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. The diode must be chosen for its average/RMS current rating and maximum voltage rating. The voltage rating of the diode must be greater than the DC input voltage (not the output voltage). FEEDFORWARD CAPACITOR (Adjustable Output Voltage Version) CFF - A Feedforward Capacitor CFF, shown across R2 in Figure 1 is used when the output voltage is greater than 10V or when COUT has a very low ESR. This capacitor adds lead compensation to the feedback loop and increases the phase margin for better loop stability. If the output voltage ripple is large ( > 5% of the nominal output voltage), this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation, adding a resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at the feedback pin. DELAYED STARTUP The circuit in Figure 7 uses the the ON /OFF pin to provide a time delay between the time the input voltage is applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start up is shown). As the input voltage rises, the charging of capacitor C1 pulls the ON /OFF pin high, keeping the regulator off. Once the input voltage reaches its final value and the capacitor stops charging, and resistor R2 pulls the ON /OFF pin low, thus allowing the circuit to start switching. Resistor R1 is included to limit the maximum voltage applied to the ON /OFF pin (maximum of 25V), reduces power supply noise sensitivity, and also limits the capacitor, C1, discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple can be coupled into the ON /OFF pin and cause problems. This delayed startup feature is useful in situations where the input power source is limited in the amount of current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating. Buck regulators require less input current at higher input voltages. INPUT CAPACITOR CIN — A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It must be located near the regulator using short leads. This capacitor prevents large voltage transients from appearing at the input, and provides the instantaneous current needed each time the switch turns on. The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of the relatively high RMS currents flowing in a buck regulator’s input capacitor, this capacitor should be chosen for its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage rating are directly related to the RMS current rating. The voltage rating of the capacitor and its RMS ripple current capability must never be exceeded. www.national.com 14 LM2591HV Application Information (Continued) 10129336 FIGURE 7. Delayed Startup UNDERVOLTAGE LOCKOUT needed, the circuit in Figure 10 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is approximately equal to the value of the output voltage. If zener voltages greater than 25V are used, an additional 47 kΩ resistor is needed from the ON /OFF pin to the ground pin to stay within the 25V maximum limit of the ON /OFF pin. Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. An undervoltage lockout feature applied to a buck regulator is shown in Figure 8, while Figure 9 and Figure 10 applies the same feature to an inverting circuit. The circuit in Figure 9 features a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is 10129337 FIGURE 8. Undervoltage Lockout for Buck Regulator 15 www.national.com LM2591HV Application Information (Continued) 10129384 This circuit has an ON/OFF threshold of approximately 13V. FIGURE 9. Undervoltage Lockout for Inverting Regulator ances, we should take the min value of Inductance for L in the equation above (typically 20% less than the nominal value). Further, the above equation disregards the drop across the Switch and the diode. This is equivalent to assuming 100% efficiency, which is never so. Therefore expect IPEAK to be an additional 10-20% higher than calculated from the above equation. lNVERTING REGULATOR The circuit in Figure 11 converts a positive input voltage to a negative output voltage with a common ground. The circuit operates by bootstrapping the regulator’s ground pin to the negative output voltage, then grounding the feedback pin, the regulator senses the inverted output voltage and regulates it. This example uses the LM2591HV-5.0 to generate a −5V output, but other output voltages are possible by selecting other output voltage versions, including the adjustable version. Since this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maximum output current greatly depends on both the input and output voltage. To determine how much load current is possible before the internal device current limit is reached (and power limiting occurs), the system must be evaluated as a buck-boost configuration rather than as a buck. The peak switch current in Amperes, for such a configuration is given as: The reader is also referred to Application Note AN-1157 for examples based on positive to negative configuration. The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 60V. For example, when converting +20V to −12V, the regulator would see 32V between the input pin and ground pin. The LM2591HV has a maximum input voltage spec of 60V. Additional diodes are required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode isolation changes the topology to closley resemble a buck configuration thus providing good closed loop stability. A Schottky diode is recommended for low input voltages, (because of its lower voltage drop) but for higher input voltages, a fast recovery diode could be used. Without diode D3, when the input voltage is first applied, the charging current of CIN can pull the output positive by several volts for a short period of time. Adding D3 prevents the output from going positive by more than a diode voltage. where L is in µH and f is in Hz. The maximum possible load current ILOAD is limited by the requirement that IPEAK ≤ ICLIM. While checking for this, take ICLIM to be the lowest possible current limit value (min across tolerance and temperature is 1.2A for the LM2591HV). Also to account for inductor toler- www.national.com 16 LM2591HV Application Information (Continued) 10129339 This circuit has hysteresis Regulator starts switching at VIN = 13V Regulator stops switching at VIN = 8V FIGURE 10. Undervoltage Lockout with Hysteresis for Inverting Regulator 10129340 CIN 470 COUT 220 — 68 µF/25V Tant. Sprague 595D µF/50V Elec. Panasonic HFQ — 47 µF/20V Tant. Sprague 595D µF/25V Elec. Panasonic HFQ FIGURE 11. Inverting −5V Regulator with Delayed Startup 11 is recommended. By delaying the regulator startup, the input capacitor is allowed to charge up to a higher voltage before the switcher begins operating. A portion of the high input current needed for startup is now supplied by the input capacitor (CIN). For severe start up conditions, the input capacitor can be made much larger than normal. Because of differences in the operation of the inverting regulator, the standard design procedure is not used to select the inductor value. In the majority of designs, a 33 µH, 3A inductor is the best choice. Capacitor selection can also be narrowed down to just a few values. This type of inverting regulator can require relatively large amounts of input current when starting up, even with light loads. Input currents as high as the LM2591HV current limit (approx 4A) are needed for at least 2 ms or more, until the output reaches its nominal output voltage. The actual time depends on the output voltage and the size of the output capacitor. Input power sources that are current limited or sources that can not deliver these currents without getting loaded down, may not work correctly. Because of the relatively high startup currents required by the inverting topology, the delayed startup feature (C1, R1 and R2) shown in Figure lNVERTING REGULATOR SHUTDOWN METHODS To use the ON /OFF pin in a standard buck configuration is simple, pull it below 1.3V (@25˚C, referenced to ground) to turn regulator ON, pull it above 1.3V to shut the regulator OFF. With the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now setting at the negative output voltage level. Two different shutdown methods for inverting regulators are shown in Figure 12 and Figure 13 17 www.national.com LM2591HV Application Information (Continued) 10129342 FIGURE 12. Inverting Regulator Ground Referenced Shutdown 10129386 FIGURE 13. Inverting Regulator Ground Referenced Shutdown using Opto Device LAYOUT SUGGESTIONS As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance can generate voltage transients which can cause problems. For minimal inductance and ground loops, with reference to Figure 1, the wires indicated by heavy lines should be wide printed circuit traces and should be kept as short as possible. For best results, external components should be located as close to the switcher lC as possible using ground plane construction or single point grounding. www.national.com If open core inductors are used, special care must be taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback, lC groundpath and COUT wiring can cause problems. When using the adjustable version, special care must be taken as to the location of the feedback resistors and the associated wiring. Physically locate both resistors near the IC, and route the wiring away from the inductor, especially an open core type of inductor. 18 LM2591HV Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead TO-220 Bent and Staggered Package Order Number LM2591HVT-3.3, LM2591HVT-5.0 or LM2591HVT-ADJ NS Package Number T05D 19 www.national.com LM2591HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 5-Lead TO-263 Bent and Formed Package Order Number LM2591HVS-3.3, LM2591HVS-5.0 or LM2591HVS-ADJ NS Package Number TS5B 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. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 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. National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 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.