LM2590HV SIMPLE SWITCHER ® Power Converter 150 kHz 1A Step-Down Voltage Regulator, with Features General Description Features The LM2590HV 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 LM2591HV with additional supervisory and performance features. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation†, improved line and load specifications, fixed-frequency oscillator, Shutdown/Soft-start, output error flag and flag delay. The LM2590HV 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 7-lead TO-220 package with several different lead bend options, and a 7-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 current limit for the output switch and an over temperature shutdown for complete protection under fault conditions. 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 7-pin TO-220 and TO-263 (surface mount) Package n Input voltage range up to 60V n 150 kHz fixed frequency internal oscillator n Shutdown/Soft-start n Out of regulation error flag n Error flag delay n Low power standby mode, IQ typically 90 µA n High Efficiency n Thermal shutdown and current limit protection Typical Application 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) 10134701 SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation. © 2001 National Semiconductor Corporation DS101347 www.national.com LM2590HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with Features December 2001 LM2590HV Absolute Maximum Ratings ESD Susceptibility (Note 1) Human Body Model (Note 3) 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 SD /SS Pin Input Voltage (Note 2) 6V Delay Pin Voltage (Note 2) 1.5V +215˚C Infrared (10 sec.) +245˚C +260˚C Maximum Junction Temperature +150˚C −0.3 ≤ V ≤+25V Feedback Pin Voltage Output Voltage to Ground (Steady State) Operating Conditions −1V Internally limited Temperature Range −65˚C to +150˚C Supply Voltage Power Dissipation Storage Temperature Range Vapor Phase (60 sec.) T Package (Soldering, 10 sec.) −0.3 ≤ V ≤45V Flag Pin Voltage 2 kV Lead Temperature −40˚C ≤ TJ ≤ +125˚C 4.5V to 60V LM2590HV-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 LM2590HV-3.3 Typ Limit (Note 4) (Note 5) Units (Limits) SYSTEM PARAMETERS (Note 6) 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 LM2590HV-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 LM2590HV-5.0 Typ Limit (Note 4) (Note 5) Units (Limits) SYSTEM PARAMETERS (Note 6) 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 % LM2590HV-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 LM2590HV-ADJ Typ Limit (Note 4) (Note 5) Units (Limits) SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1 VFB Feedback Voltage 4.5V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A VOUT programmed for 3V. Circuit of Figure 1. www.national.com 2 1.230 V 1.193/1.180 V(min) 1.267/1.280 V(max) (Continued) Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol η Parameter Efficiency Conditions LM2590HV-ADJ Units (Limits) Typ Limit (Note 4) (Note 5) VIN = 12V, VOUT = 3V, ILOAD = 1A 76 % All Output Voltage Versions Electrical Characteristics 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 LM2590HV-XX Typ Limit (Note 4) (Note 5) Units (Limits) DEVICE PARAMETERS Ib Feedback Bias Current Adjustable Version Only, VFB = 1.3V 10 nA 50/100 fO Oscillator Frequency (Note 7) 150 VSAT Saturation Voltage IOUT = 1A (Note 8) (Note 9) 0.95 DC Max Duty Cycle (ON) (Note 9) 100 Min Duty Cycle (OFF) (Note 10) 0 Switch current Limit Peak Current, (Note 8) (Note 9) kHz 127/110 kHz(min) 173/173 kHz(max) V 1.2/1.3 ICLIM IL Output Leakage Current (Note 8) (Note 10) (Note 11) Output = −1V IQ Operating Quiescent ISTBY Standby Quiescent 5 SD /SS pin = 0V 90 Current (Note 11) Current θJC Thermal Resistance A 1.3/1.2 A(min) 2.8/3.0 A(max) 50 µA(max) 30 mA(max) 5 SD /SS Pin Open (Note 10) V(max) % 1.9 Output = 0V nA (max) 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 12) 50 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 13) 50 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 14) 30 ˚C/W θJA TO263 Package, Juncton to Ambient (Note 15) 20 ˚C/W SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1 VSD Shutdown Threshold Voltage 1.3 Low, (Shutdown Mode) High, (Soft-start Mode) VSS ISD Soft-start Voltage Shutdown Current VOUT = 20% of Nominal Output Voltage 2 VOUT = 100% of Nominal Output Voltage 3 VSHUTDOWN = 0.5V 5 V 0.6 V(max) 2 V(min) V µA 10 ISS Soft-start Current VSoft-start = 2.5V 1.5 µA 5 3 µA(max) µA(max) www.national.com LM2590HV LM2590HV-ADJ Electrical Characteristics LM2590HV All Output Voltage Versions Electrical Characteristics (Continued) 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 LM2590HV-XX Typ Limit (Note 4) (Note 5) Units (Limits) FLAG/DELAY CONTROL Test Circuit of Figure 1 Regulator Dropout Detector Low (Flag ON) 96 Threshold Voltage VFSAT IFL Flag Output Saturation ISINK = 3 mA Voltage VDELAY = 0.5V Flag Output Leakage Current VFLAG = 60V Delay Pin Source Current Delay Pin Saturation %(min) 98 %(max) 0.7/1.0 V(max) 0.3 V 0.3 Delay Pin Threshold Voltage % 92 µA 1.25 V Low (Flag ON) 1.21 V(min) High (Flag OFF) and VOUT Regulated 1.29 V(max) 6 µA(max) VDELAY = 0.5V 3 Low (Flag ON) µA 70 mV 350/400 mV(max) 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: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin. Note 4: Typical numbers are at 25˚C and represent the most likely norm. Note 5: 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 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2590HV is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics. Note 7: 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 8: No diode, inductor or capacitor connected to output pin. Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON. Note 10: 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 11: VIN = 60V. Note 12: 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 13: 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 14: 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 15: 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 LM2590HVS 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. www.national.com 4 NormalizedOutput Voltage (Circuit of Figure 1) Line Regulation Efficiency 10134702 Switch SaturationVoltage 10134703 Switch Current Limit 10134704 Dropout Voltage 10134706 10134705 Operating Quiescent Current LM2590HV Typical Performance Characteristics Shutdown Quiescent Current 10134708 10134709 5 10134707 Minimum Operating Supply Voltage 10134710 www.national.com LM2590HV Typical Performance Characteristics Feedback Pin Bias Current (Circuit of Figure 1) (Continued) Flag Saturation Voltage 10134711 10134714 Soft-start Response 10134713 10134712 Shutdown /Soft-start Current Soft-start Switching Frequency 10134715 Shutdown/Soft-start Threshold Voltage 10134718 Delay Pin Current 10134716 Internal Gain-Phase Characteristics 10134753 10134778 www.national.com 6 (Circuit of Figure 1) (Continued) 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 10134720 10134719 Horizontal Time Base: 2 µs/div. Horizontal Time Base: 2 µs/div. 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. 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 10134722 Horizontal Time Base: 200 µs/div. 10134721 A: Output Voltage, 100 mV/div. (AC) Horizontal Time Base: 50 µs/div. B: 250 mA to 1A Load Pulse A: Output Voltage, 100 mV/div. (AC) B: 250 mA to 1A Load Pulse Connection Diagrams and Order Information Bent and Staggered Leads, Through Hole Package 7-Lead TO-220 (T) Surface Mount Package 7-Lead TO-263 (S) 10134750 10134723 Order Number LM2590HVT-3.3, LM2590HVT-5.0, or LM2590HVT-ADJ See NS Package Number TA07B Order Number LM2590HVS-3.3, LM2590HVS-5.0, or LM2590HVS-ADJ See NS Package Number TS7B 7 www.national.com LM2590HV Typical Performance Characteristics LM2590HV Test Circuit and Layout Guidelines Fixed Output Voltage Versions 10134724 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 10134725 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. — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series” CIN: 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) † Resistive divider is required to aviod exceeding maximum rating of 45V/3mA on/into flag pin. †† 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 LM2590HV Block Diagram 10134730 Feedback (Pin 6) — 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 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. Shutdown /Soft-start (Pin 7) — The regulator is in shutdown mode, drawing about 90 µA, when this pin is driven to a low level (≤ 0.6V), and is in normal operation when this Pin is left floating (internal-pullup) or driven to a high level (≥ 2.0V). The typical value of the threshold is 1.3V and the pin is internally clamped to a maximum of about 7V. If it is driven higher than the clamp voltage, it must be ensured by means of an external resistor that the current into the pin does not exceed 1mA. The duty cycle is minimum (0%) if this Pin is below 1.8V, and increases as the voltage on the pin is increased. The maximum duty cycle (100%) occurs when this pin is at 2.8V or higher. So adding a capacitor to this pin produces a softstart feature. An internal current source will charge the capacitor from zero to its internally clamped value. The charging current is about 5 µA when the pin is below 1.3V but is reduced to only 1.6 µA above 1.3V, so as to allow the use of smaller softstart capacitors. 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. Error Flag (Pin 3) — Open collector output that goes active low (≤ 1.0V) when the output of the switching regulator is out of regulation (less than 95% of its nominal value). In this state it can sink maximum 3mA. When not low, it can be pulled high to signal that the output of the regulator is in regulation (power good). During power-up, it can be programmed to go high after a certain delay as set by the Delay pin (Pin 5). The maximum rating of this pin should not be exceeded, so if the rail to which it will be pulled-up to is higher than 45V, a resistive divider must be used instead of a single pull-up resistor, as indicated in Figure 1. Ground (Pin 4) — Circuit ground. Delay (Pin 5) — This sets a programmable power-up delay from the moment that the output reaches regulation, to the high signal output (power good) on Pin 3. A capacitor on this pin starts charging up by means on an internal () 3 µA) current source when the regulated output rises to within 5% of its nominal value. Pin 3 goes high (with an external pull-up) when the voltage on the capacitor on Pin 5 exceeds 1.3V. The voltage on this pin is clamped internally to about 1.7V. If the regulated output drops out of regulation (less than 95% of its nominal value), the capacitor on Pin 5 is rapidly discharged internally and Pin 3 will be forced low in about 1/1000th of the set power-up delay time. 9 www.national.com LM2590HV PIN FUNCTIONS (Continued) Note If any of the above three features (Shutdown /Soft-start, Error Flag, or Delay) are not used, the respective pins can be left open. 10134731 FIGURE 2. Soft-Start, Delay, Error Output www.national.com 10 LM2590HV 10134732 FIGURE 3. Timing Diagram for 5V Output INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) 10134726 FIGURE 4. LM2590HV-3.3 11 www.national.com LM2590HV INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued) 10134727 FIGURE 5. LM2590HV-5.0 10134729 FIGURE 6. LM2590HV-ADJ www.national.com 12 (For Continuous Mode Operation) (Continued) 10134765 FIGURE 7. Current Ripple Ratio Coilcraft Inc. Coilcraft Inc., Europe Pulse Engineering Inc. 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 Schott Corp. Cooper Electronic Tech. (Coiltronics) Phone (USA): 1-888-414-2645 Web Address http://www.cooperet.com FIGURE 8. Contact Information for Suggested Inductor Manufacturers 13 www.national.com LM2590HV INDUCTOR VALUE SELECTION GUIDES LM2590HV Application Information 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 4 to Figure 6. 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. The Energy under steady operation is consider the rather wide tolerance on the nominal inductance of commercial inductors. 5. Figure 6 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) LM2590HV-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 5) and a current rating equal to the maximum load current. We therefore quick-select a 100µH/0.8 A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle 50 µJ (see Figure 5) 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) LM2590HV-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 5) 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: Example 3: (VIN ≤ 40V) LM2590HV-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 6 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 LM2590HV is 3A. The Energy rating depends on the Inductance. See Example 2 below. 4. The nomographs were generated by allowing a greater amount of percentage current ripple in the Inductor as the maximum rated load decreases (see Figure 7). This was done to permit the use of smaller inductors at light loads. Figure 7 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 www.national.com where VD is the drop across the Catch Diode () 0.5V for a Schottky) and VSAT the drop across the switch ()1.5V). So And the switch ON time is where f is the switching frequency in Hz. So 14 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. (Continued) Therefore, looking at Figure 4 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 6) 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. This completes the simplified inductor selection procedure. For more general applications and better optimization, the designer should refer to AN-1197. Figure 8 provides helpful contact information on suggested Inductor manufacturers who may be able to recommend suitable parts, if the requirements are known. 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. 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 LM2590HV using short leads and short printed circuit traces. 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. SHUTDOWN /SOFT-START This reduction in start up current is useful in situations where the input power source is limited in the amount of current it can deliver. In some applications Soft-start can be used to replace undervoltage lockout or delayed startup functions. If a very slow output voltage ramp is desired, the Soft-start capacitor can be made much larger. Many seconds or even minutes are possible. If only the shutdown feature is needed, the Soft-start capacitor can be eliminated. 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 15 www.national.com LM2590HV Application Information LM2590HV Application Information (Continued) 10134742 FIGURE 9. Typical Circuit Using Shutdown /Soft-start and Error Flag Features 10134743 FIGURE 10. Inverting −5V Regulator With Shutdown and Soft-start lNVERTING REGULATOR The circuit in Figure 10 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 LM2590HV-5 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 www.national.com 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: 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 LM2590HV). Also to account for inductor tolerances, 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 as16 LM2590HV Application Information (Continued) suming 100% efficiency, which is never so. Therefore expect IPEAK to be an additional 10-20% higher than calculated from the above equation. 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. In this example, when converting +20V to −5V, the regulator would see 25V between the input pin and ground pin. The LM2590HV has a maximum input voltage rating of 60V. An additional diode is 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 closely 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 IN5400 diode could be used. 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 LM2590HV current limit (approximately 3.0A) are needed for 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 Soft-Start feature shown in Figure 10 is recommended. Also shown in Figure 10 are several shutdown methods for the inverting configuration. With the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown signals. 10134745 FIGURE 11. Undervoltage Lockout for a Buck Regulator Figure 12 and Figure 13 apply the same feature to an inverting circuit. Figure 12 features a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in Figure 13 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. Since the SD /SS pin has an internal 7V zener clamp, R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on. 10134747 FIGURE 12. Undervoltage Lockout Without Hysteresis for an Inverting Regulator UNDERVOLTAGE LOCKOUT Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. Figure 11 contains a undervoltage lockout circuit for a buck configuration, while Figure 12 and Figure 13 are for the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 11 uses a zener diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than the zener voltage, resistors R1 and R2 hold the Shutdown /Soft-start pin low, keeping the regulator in the shutdown mode. As the input voltage exceeds the zener voltage, the zener conducts, pulling the Shutdown /Soft-start pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout feature is approximately 1.5V greater than the zener voltage. 10134746 FIGURE 13. Undervoltage Lockout With Hysteresis for an Inverting Regulator 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 17 www.national.com LM2590HV Application Information 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. (Continued) 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. 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. www.national.com 18 LM2590HV Physical Dimensions inches (millimeters) unless otherwise noted 7-Lead TO-220 Bent and Staggered Package Order Number LM2590HVT-3.3, LM2590HVT-5.0 or LM2590HVT-ADJ NS Package Number TA07B 19 www.national.com LM2590HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with Features Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 7-Lead TO-263 Bent and Formed Package Order Number LM2590HVS-3.3, LM2590HVS-5.0 or LM2590HVS-ADJ NS Package Number TS7B 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. 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