LM2592HVT-3.3

LM2592HV SIMPLE SWITCHER Power Converter 150 kHz 2A Step-Down Voltage Regulator August 2001 LM2592HV SIMPLE SWITCHER ® Power Converter 150 kHz 2A Step-Down Voltage Regulator General Description The LM2592HV series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 2A 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 LM2593HV, 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 LM2592HV 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 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 2A 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. Typical Application (Fixed Output Voltage Versions) 10129401 SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation. © 2001 National Semiconductor Corporation DS101294 www.national.com LM2592HV Absolute Maximum Ratings (Note 1) Human Body Model (Note 2) Lead Temperature S Package Vapor Phase (60 sec.) Infrared (10 sec.) T Package (Soldering, 10 sec.) Maximum Junction Temperature 2 kV If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Supply Voltage (VIN) ON/OFF Pin Voltage Feedback Pin Voltage Output Voltage to Ground (Steady State) Power Dissipation Storage Temperature Range ESD Susceptibility −1V Internally limited −65˚C to +150˚C 63V −0.3 ≤ V ≤ +25V −0.3 ≤ V ≤ +25V +215˚C +245˚C +260˚C +150˚C Operating Conditions Temperature Range Supply Voltage −40˚C ≤ TJ ≤ +125˚C 4.5V to 60V LM2592HV-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 Typ (Note 3) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VOUT Output Voltage 4.75V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 2A 3.3 3.168/3.135 3.432/3.465 η Efficiency VIN = 12V, ILOAD = 2A 76 V V(min) V(max) LM2592HV-3.3 Limit (Note 4) Units (Limits) LM2592HV-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 Typ (Note 3) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VOUT Output Voltage 7V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 2A 5 4.800/4.750 5.200/5.250 η Efficiency VIN = 12V, ILOAD = 2A 81 V V(min) V(max) % LM2592HV-5.0 Limit (Note 4) Units (Limits) LM2592HV-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 LM2592HV-ADJ Typ (Note 3) SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1 VFB Feedback Voltage 4.5V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 2A VOUT programmed for 3V. Circuit of Figure 1. η Efficiency VIN = 12V, VOUT = 3V, ILOAD = 2A 75 1.230 1.193/1.180 1.267/1.280 V V(min) V(max) % Limit (Note 4) Units (Limits) www.national.com 2 LM2592HV 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 LM2592HV-XX Typ (Note 3) DEVICE PARAMETERS Ib fO Feedback Bias Current Oscillator Frequency Adjustable Version Only, VFB = 1.3V (Note 6) 10 50/100 150 127/110 173/173 VSAT DC ICLIM Saturation Voltage Max Duty Cycle (ON) Min Duty Cycle (OFF) Switch current Limit IOUT = 2A (Note 7) (Note 8) (Note 8) (Note 9) Peak Current, (Note 7) (Note 8) 1.10 1.3/1.4 100 0 3.0 2.4/2.3 3.7/4.0 IL Output Leakage Current (Note 7) (Note 9) (Note 10) Output = 0V Output = −1V IQ ISTBY θJC θJA θJA θJA θJA ON/OFF CONTROL Test Circuit Figure 1 ON /OFF Pin Logic Input VIH VIL IH IL ON /OFF Pin Input Current Threshold Voltage Low (Regulator ON) High (Regulator OFF) VLOGIC = 2.5V (Regulator OFF) VLOGIC = 0.5V (Regulator ON) 5 15 0.02 5 1.3 0.6 2.0 V V(max) V(min) µA µA(max) µA µA(max) Operating Quiescent Current Standby Quiescent Current Thermal Resistance TO220 or TO263 Package, Junction to Case TO220 Package, Juncton to Ambient (Note 11) TO263 Package, Juncton to Ambient (Note 12) TO263 Package, Juncton to Ambient (Note 13) TO263 Package, Juncton to Ambient (Note 14) 2 50 50 30 20 SD /SS pin = 0V (Note 10) 90 200/250 SD /SS Pin Open (Note 9) 5 30 5 10 50 A A(min) A(max) µA(max) mA mA(max) mA mA(max) µA µA(max) ˚C/W ˚C/W ˚C/W ˚C/W ˚C/W nA nA (max) kHz kHz(min) kHz(max) V V(max) % Limit (Note 4) Units (Limits) 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 LM2592HV 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 LM2592HV 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 LM2592HVS 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 LM2592HV Typical Performance Characteristics Normalized Output Voltage (Circuit of Figure 1) Efficiency Line Regulation 10129402 10129403 10129404 Switch Saturation Voltage Switch Current Limit Dropout Voltage 10129405 10129406 10129407 Operating Quiescent Current Shutdown Quiescent Current Minimum Operating Supply Voltage 10129408 10129409 10129410 5 www.national.com LM2592HV Typical Performance Characteristics Feedback Pin Bias Current (Circuit of Figure 1) (Continued) Switching Frequency ON/OFF Threshold Voltage 10129411 10129413 10129479 ON/OFF Pin Current (Sinking) Internal Gain-Phase Characteristics 10129480 10129478 www.national.com 6 LM2592HV Typical Performance Characteristics Continuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 2A L = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ (Circuit of Figure 1) (Continued) Discontinuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 500 mA L = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩ 10129420 10129419 Horizontal Time Base: 2 µs/div. A: Output Pin Voltage, 10V/div. B: Inductor Current 1A/div. C: Output Ripple Voltage, 50 mV/div. Horizontal Time Base: 2 µs/div. A: Output Pin Voltage, 10V/div. B: Inductor Current 0.5A/div. C: Output Ripple Voltage, 100 mV/div. Load Transient Response for Continuous Mode VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A L = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ Load Transient Response for Discontinuous Mode VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A L = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩ 10129422 Horizontal Time Base: 200 µs/div. 10129421 Horizontal Time Base: 50 µs/div. A: Output Voltage, 100 mV/div. (AC) B: 500 mA to 2A Load Pulse A: Output Voltage, 100 mV/div. (AC) B: 500 mA to 2A Load Pulse 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) 10129481 10129482 Order Number LM2592HVT-3.3, LM2592HVT-5.0, or LM2592HVT-ADJ See NS Package Number T05D Order Number LM2592HVS-3.3, LM2592HVS-5.0, or LM2592HVS-ADJ See NS Package Number TS5B 7 www.national.com LM2592HV Test Circuit and Layout Guidelines Fixed Output Voltage Versions 10129424 Component Values shown are for VIN = 15V, CIN D1 L1 VOUT = 5V, ILOAD = 2A. — 470 µF, 50V, Aluminum Electrolytic Nichicon “PM Series” — — — 220 µF, 25V Aluminum Electrolytic, Nichicon “PM Series” 3.3A, 60V Schottky Rectifier, 31DQ06 (International Rectifier) 33 µH, See Inductor Selection Procedure COUT Adjustable Output Voltage Versions 10129425 Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability. Component Values shown are for VIN = 20V, CIN: VOUT = 10V, ILOAD = 2A. — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series” — 220 µF, 35V Aluminum Electrolytic, Nichicon “PM Series” COUT: D1 — 3.3A, 60V Schottky Rectifier, 31DQ06 (International Rectifier) L1 — 47 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 LM2592HV Block Diagram 10129483 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 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. 9 www.national.com LM2592HV INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) 10129465 FIGURE 2. LM2592HV-3.3 10129466 FIGURE 3. LM2592HV-5.0 www.national.com 10 LM2592HV INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued) 10129467 FIGURE 4. LM2592HV-ADJ 10129468 FIGURE 5. Current Ripple Ratio 11 www.national.com LM2592HV INDUCTOR VALUE SELECTION GUIDES Coilcraft Inc. Coilcraft Inc., Europe Pulse Engineering Inc. Pulse Engineering Inc., Europe Renco Electronics Inc. Schott Corp. Cooper Electronic Tech. (Coiltronics) Phone Web Address Phone Web Address Phone Web Address Phone Web Address Phone Web Address Phone Web Address Phone Web Address (For Continuous Mode Operation) (Continued) (USA): 1-800-322-2645 http://www.coilcraft.com (UK): 1-236-730595 http://www.coilcraft-europe.com (USA): 1-858-674-8100 http://www.pulseeng.com (UK): 1-483-401700 http://www.pulseeng.com (USA): 1-321-637-1000 http://www.rencousa.com (USA): 1-952-475-1173 http://www.shottcorp.com (USA): 1-888-414-2645 http://www.cooperet.com FIGURE 6. Contact Information for Suggested Inductor Manufacturers www.national.com 12 LM2592HV 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 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. The Energy under steady operation is 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) LM2592HV-5.0, VIN = 24V, Output 5V @ 1A 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 68µH/1A 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) LM2592HV-5.0, VIN = 48V, Output 5V @ 1.5A 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 68µH/4A 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: 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 Example 3: (VIN ≤ 40V) LM2592HV-ADJ, VIN = 20V, Output 10V @ 2A 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 If VIN > 40V, the inductor should be sized to handle eCLIM instead of the steady energy values. The worst case ICLIM for the LM2592HV is 4A. 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 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 13 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 www.national.com LM2592HV Application Information (Continued) 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 LM2592HV 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). 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. Therefore, looking at Figure 2 we quick-select a 47µH/2A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle 200 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). 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. 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. 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 LM2592HV Application Information (Continued) 10129436 FIGURE 7. Delayed Startup UNDERVOLTAGE LOCKOUT 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 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. 10129437 FIGURE 8. Undervoltage Lockout for Buck Regulator 10129484 This circuit has an ON/OFF threshold of approximately 13V. FIGURE 9. Undervoltage Lockout for Inverting Regulator 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 LM2592HV-5.0 to generate a −5V output, but other output voltages are possible by selecting other output voltage versions, including the adjustable ver15 www.national.com LM2592HV Application Information (Continued) sion. 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: 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. For example, when converting +20V to −12V, the regulator would see 32V between the input pin and ground pin. The LM2592HV 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 2.3A for the LM2592HV). 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 as- 10129439 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 10129440 CIN — 68 µF/25V Tant. Sprague 595D 470 µF/50V Elec. Panasonic HFQ COUT — 47 µF/20V Tant. Sprague 595D 220 µF/25V Elec. Panasonic HFQ FIGURE 11. Inverting −5V Regulator with Delayed Startup www.national.com 16 LM2592HV Application Information (Continued) 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, 4A 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 LM2592HV 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 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. 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 10129442 FIGURE 12. Inverting Regulator Ground Referenced Shutdown 10129486 FIGURE 13. Inverting Regulator Ground Referenced Shutdown using Opto Device THERMAL CONSIDERATIONS The LM2592HV is available in two packages, a 5-pin TO-220 (T) and a 5-pin surface mount TO-263 (S). The TO-220 package needs a heat sink under most conditions. The size of the heatsink depends on the input voltage, the output voltage, the load current and the ambient temperature. Higher ambient temperatures require more heat sinking. The TO-263 surface mount package tab is designed to be soldered to the copper on a printed circuit board. The copper and the board are the heat sink for this package and the other heat producing components, such as the catch diode and inductor. The PC board copper area that the package is 17 soldered to should be at least 0.4 in2, and ideally should have 2 or more square inches of 2 oz. (0.0028) in) copper. Additional copper area improves the thermal characteristics, but with copper areas greater than approximately 6 in2, only small improvements in heat dissipation are realized. If further thermal improvements are needed, double sided, multilayer PC board with large copper areas and/or airflow are recommended. The curves shown in Figure 14 show the LM2592HVS (TO-263 package) junction temperature rise above ambient temperature with a 2A load for various input and output voltages. This data was taken with the circuit operating as a buck switching regulator with all components mounted on a www.national.com LM2592HV Application Information (Continued) PC board to simulate the junction temperature under actual operating conditions. This curve can be used for a quick check for the approximate junction temperature for various conditions, but be aware that there are many factors that can affect the junction temperature. When load currents higher than 2A are used, double sided or multilayer PC boards with large copper areas and/or airflow might be needed, especially for high ambient temperatures and high output voltages. For the best thermal performance, wide copper traces and generous amounts of printed circuit board copper should be used in the board layout. (One exception to this is the output (switch) pin, which should not have large areas of copper.) Large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and moving air lowers the thermal resistance even further. Package thermal resistance and junction temperature rise numbers are all approximate, and there are many factors that will affect these numbers. Some of these factors include board size, shape, thickness, position, location, and even board temperature. Other factors are, trace width, total printed circuit copper area, copper thickness, single- or double-sided, multilayer board and the amount of solder on the board. The effectiveness of the PC board to dissipate heat also depends on the size, quantity and spacing of other components on the board, as well as whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode will add heat to the PC board and the heat can vary as the input voltage changes. For the inductor, depending on the physical size, type of core material and the DC resistance, it could either act as a heat sink taking heat away from the board, or it could add heat to the board. 10129438 FIGURE 14. Junction Temperature Rise, TO-263 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. 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. www.national.com 18 LM2592HV Physical Dimensions unless otherwise noted inches (millimeters) 5-Lead TO-220 Bent and Staggered Package Order Number LM2592HVT-3.3, LM2592HVT-5.0 or LM2592HVT-ADJ NS Package Number T05D 19 www.national.com LM2592HV SIMPLE SWITCHER Power Converter 150 kHz 2A Step-Down Voltage Regulator Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 5-Lead TO-263 Bent and Formed Package Order Number LM2592HVS-3.3, LM2592HVS-5.0 or LM2592HVS-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 Corporation Americas Email: support@nsc.com National Semiconductor Europe 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 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 Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 www.national.com 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.