LM2577S-ADJ

LM2577S/T-ADJ General Description The LM2577S/T-ADJ is a monolithic integrated circuit that provide all of the power and control functions for step-up (boost), flyback, and forward converter switching regulators. TO-220 Requiring a minimum number of external components, these regulators are cost effective, and simple to use. Listed in this data sheet are a family of standard inductors and flyback transformers designed to work with these switching regulators. TO-263 Included on the chip is a 3.0A NPN switch and its associated protection circuitry, consisting of current and thermal limiting, and undervoltage lockout. Other features include a 52 kHz fixed-frequency oscillator that requires no external components, a soft start mode to reduce in-rush current during start-up, and current mode control for improved rejection of input voltage and output load transients. The LM2577S is available in TO263-5L package. The LM2577T is available in TO220-5L package. Features
Requires few external components
NPN output switches 3.0A, can stand off 65V
Wide input voltage range: 3.5V to 40V
Current-mode operation for improved transient response, line regulation, and current limit
52 kHz internal oscillator
Soft-start function reduces in-rush current during start-up
Output switch protected by current limit, under-voltage lockout, and thermal shutdown Applications
Simple boost regulator
Flyback and forward regulators
Multiple-output regulator www.slkormicro.com 1 LM2577S/T-ADJ Pin Connection Pin Description LM2577S Absolute Maximum Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Characteristic Limit Unit Supply voltage 45 V Output switch voltage 65 V 6.0 A Output switch current *2 Power dissipation Internally limited -65～+150 ℃ Lead temperature (soldering, 10 sec.) 260 ℃ Maximum junction temperature 150 ℃ 2 kV Storage temperature range Minimum ESD rating (C=100pF, R=1.5kΩ) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions the device is intended to be functional, but device parameter specifications may not be guaranteed under these conditions. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: Due to timing considerations of the LM2577S/T_ADJ current limit circuit, output current cannot be internally limited when the LM2577S/T_ADJ is used as a step-up regulator. To prevent damage to the switch, its current must be externally limited to 6.0A. However, output current is internally limited when the LM2577S/T_ADJ is used as a flyback or forward converter regulator in accordance to the Application Hints. Recommended Operating Rating Characteristic Limit Unit 3.5～40 V Output switch voltage 0～60 V Output switch current ≤3.0 A -40～+125 ℃ Supply voltage Junction temperature range www.slkormicro.com 2 LM2577S/T-ADJ Electrical Characteristics: (Specifications with standard type face are for TJ=25°C, and those in bold type face apply over full Operating Temperature Range. Unless otherwise specified, VIN = 5V, and ISWITCH = 0.) LM2577S/T_ADJ VFEEDBACK=VREF Characteristic Symbol System Parameters circuit of figure 3 Output voltage VOUT Line regulation Load regulation Efficiency Device Parameters Input supply current Input supply undervoltage lockout Oscillator frequency Output reference voltage Output reference voltage line regulator Error amp input bias current Error amp transconductance η IS VUV fO VREF Test conditions *4 VIN=5V to 10V ILOAD=100mA to 800mA *5 VIN=3.5V to 10V ILOAD=300mA VIN=5V, ILOAD=100mA to 800mA VIN=5V,ILOAD=800mA Min. *3 Typ. Max *3 Unit 11.60/ 11.40 12 12.40/ 12.60 V 20 50/ 100 mV 20 50/ 100 mV 80 VFEEDBACK=1.5V(switch off) 7.5 ISWITCH=2.0A VCOMP=2.0V(max dutycycle) 25 ISWITCH=100mA Measured at switch pin ISWITCH=100mA Measured at feedback pin VIN=3.5V to 40V VCOMP=1.0V 2.70/ 2.65 48/ 42 1.214/ 1.206 2.90 52 1.230 % 10.0/ 14.0 50/ 85 3.10/ 3.15 56/ 62 1.246/ 1.254 mA mA V kHz V VIN=3.5V to 40V 0.5 mV IB VCOMP=1.0V 100 300/8 00 nA GM ICOMP=-30µ A to +30µ A VCOMP=1.0V 2400/1 3700 5800 µ mho 500/ 800 V/V 2.2/ 2.0 2.4 V Device Parameters Error amp voltage gain AVOL Error amplifier output swing Error amplifier output current Soft start current ISS Maximum duty cycle D VCOMP=1.1V to 1.9V RCOMP=1.0MΩ *6 Upper limit VFEEDBACK=1.0V Lower limit VFEEDBACK=1.5V VFEEDBACK=12.0V to 18.0V VCOMP=1.0V VFEEDBACK=1.0V VCOMP=0V VCOMP=1.5V ISWITCH=100mA Switch transconductance www.slkormicro.com 3 0.3 0.40/ V ±130/ ±200 ±300/ µA 2.5/ 1.5 5.0 7.5/ 9.5 µA 93/ 90 95 % 12.5 A/V LM2577S/T-ADJ Switch leakage current IL Switch saturation current VSAT NPN switch current limit VSWITCH=65V VFEEDBACK=1.5V(switch off) ISWITCH=2.0A VCOMP=2.0V(max duty cycle) VCOMP=2.0V 10 0.5 3.7/ 3.0 4.3 Min. Typ 300/ 600 µA 0.7/ 0.9 5.3/ 6.0 V A Thermal Paramters (All Versions) Characteristic Thermal resistance Symbol θJA θJC θJA Test conditions TO-220, Junction to ambient TO-220, Junction to case TO-263, Junction to ambient *7 65 2 Max Unit °C/W 37 *1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions the device is intended to be functional, but device parameter specifications may not be guaranteed under these conditions. For guaranteed specifications and test conditions, see the Electrical Characteristics. *2: Due to timing considerations of the LM2577S/T_ADJ current limit circuit, output current cannot be internally limited when the LM2577S/T_ADJ is used as a step-up regulator. To prevent damage to the switch, its current must be externally limited to 6.0A. However, output current is internally limited when the LM2577S/T _ADJ is used as a flyback or forward converter regulator in accordance to the Application Hints. *3: All limits guaranteed at room temperature (standard type face) and at temperature extremes (boldface type). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. *4: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. When the LM2577S/T_ADJ is used as shown in the Test Circuit, system performance will be as specified by the system parameters. *5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (boldface type). All limits are used to calculate Outgoing Quality Level, and are 100% production tested. *6: A 1.0 MΩ resistor is connected to the compensation pin (which is the error amplifier’s output) to ensure accuracy in measuring AVOL. In actual applications, this pin’s load resistance should be ≥10 MΩ, resulting in AVOL that is typically twice the guaranteed minimum limit. *7: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using 0.5 square inches of copper area, θJA is 50°C/W; with 1 square inch of copper area, θJA is 37°C/W; and with 1.6 or more square inches of copper area, θJA is 32°C/W. www.slkormicro.com 4 LM2577S/T-ADJ Test Circuit FIGURE 3. Circuit Used to Specify System Parameters for ADJ Versions L = 415-0930 (AIE) D = any manufacturer COUT = Sprague Type 673D Electrolytic 680 µF, 20V R1 = 48.7k in series with 511Ω (1%) www.slkormicro.com 5 R2 = 5.62k (1%) LM2577S/T-ADJ D2577-ADJ FIGURE 4. LM2577S/T_ADJ Block Diagram and Boost Regulator Application Step-Up (boost) Regulator Figure 4 shows the LM2577S/T_ADJ-ADJ used as a Step-Up Regulator. This is a switching regulator used for producing an output voltage greater than the input supply voltage. The LM2577S/T_ADJ-12 and LM2577S/T_ADJ-15 can also be used for step-up regulators with 12V or 15V outputs (respectively), by tying the feedback pin directly to the regulator output. A basic explanation of how it works is as follows. The LM2577S/T_ADJ turns its output switch on and off at a frequency of 52 kHz, and this creates energy in the inductor (L). When the NPN switch turns on, the inductor current charges up at a rate of VIN/L, storing current in the inductor.When the switch turns off, the lower end of www.slkormicro.com 6 LM2577S/T-ADJ the inductor flies above VIN, discharging its current through diode (D) into the output capacitor (COUT) at a rate of (VOUT − VIN)/L. Thus, energy stored in the inductor during the switch on time is transferred to the output during the switch off time. The output voltage is controlled by the amount of energy transferred which, in turn, is controlled by modulating the peak inductor current. This is done by feeding back a portion of the output voltage to the error amp, which amplifies the difference between the feedback voltage and a 1.230V reference.The error amp output voltage is compared to a voltage proportional to the switch current (i.e., inductor current during the switch on time). The comparator terminates the switch on time when the two voltages are equal, thereby controlling the peak switch current to maintain a constant output voltage. Voltage and current waveforms for this circuit are shown in Figure 5, and formulas for calculating them are given in Figure 6. FIGURE 5. Step-Up Regulator Waveforms FIGURE 6. Step-Up Regulator Formulas Step-Up Regulator Design Procedure The following design procedure can be used to select the appropriate external components for the circuit in www.slkormicro.com 7 LM2577S/T-ADJ Figure 4, based on these system requirements. Given: VIN (min) = Minimum input supply voltage VOUT = Regulated output voltage ILOAD(max) = Maximum output load current Before proceeding any further, determine if the LM2577S/T_ADJ can provide these values of VOUT and ILOAD(max) when operating with the minimum value of VIN. The upper limits for VOUT and ILOAD(max) are given by the following equations. VOUT ≤ 60V and VOUT ≤ 10 x VIN(min) These limits must be greater than or equal to the values specified in this application. 1. Inductor Selection (L) A. Voltage Options: 1. For 12V or 15V output From Figure 7 (for 12V output) or Figure 8 (for 15V output), identify inductor code for region indicated by VIN (min) and ILOAD (max). The shaded region indicates conditions for which the LM2577S/T_ADJ output switch would be operating beyond its switch current rating. The minimum operating voltage for the LM2577S/T_ADJ is 3.5V. From here, proceed to step C. 2. For Adjustable version Preliminary calculations: The inductor selection is based on the calculation of the following three parameters: D(max), the maximum switch duty cycle (0 ≤ D ≤ 0.9): where VF = 0.5V for Schottky diodes and 0.8V for fast recovery diodes (typically); E •T, the product of volts x time that charges the inductor: IIND,DC, the average inductor current under full load; B. Identify Inductor Value: 1. From Figure 9, identify the inductor code for the region indicated by the intersection of E•T and IIND,DC.This www.slkormicro.com 8 LM2577S/T-ADJ code gives the inductor value in microhenries. The L or H prefix signifies whether the inductor is rated for a maximum E•T of 90 V•µs (L) or 250 V•µs (H). 2. If D < 0.85, go on to step C. If D ≥ 0.85, then calculate the minimum inductance needed to ensure the switching regulator’s stability: If LMIN is smaller than the inductor value found in step B1, go on to step C. Otherwise, the inductor value found in step B1 is too low; an appropriate inductor code should be obtained from the graph as follows: 1. Find the lowest value inductor that is greater than LMIN. 2. Find where E•T intersects this inductor value to determine if it has an L or H prefix. If E•T intersects both the L and H regions, select the inductor with an H prefix. FIGURE 7. LM2577S/T_ADJ-12 Inductor Selection Guide Inductor Selection Guide www.slkormicro.com 9 FIGURE 8.LM2577S/T_ADJ-15 LM2577S/T-ADJ Note: These charts assume that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current (when the regulator is under full load). Greater ripple current causes higher peak switch currents and greater output ripple voltage; lower ripple current is achieved with larger-value inductors. The factor of 20 to 30% is chosen as a convenient balance between the two FIGURE 9. LM2577S/T_ADJ-ADJ Inductor Selection Graph C. Select an inductor from the table of Figure 10 which cross-references the inductor codes to the part numbers of three different manufacturers. Complete specifications for these inductors are available from the respective manufacturers. The inductors listed in this table have the following characteristics: AIE: ferrite, pot-core inductors; Benefits of this type are low electro-magnetic interference (EMI), small physical size, and very low power dissipation (core loss). Be careful not to operate these inductors too far beyond their maximum ratings for E•T and peak current, as this will saturate the core. Pulse: powdered iron, toroid core inductors; Benefits are low EMI and ability to withstand E•T and peak current above rated value better than ferrite cores. Renco: ferrite, bobbin-core inductors; Benefits are low cost and best ability to withstand E•T and peak current above rated value. Be aware that these inductors generate more EMI than the other types, and this may interfere with signals sensitive to noise. www.slkormicro.com 10 LM2577S/T-ADJ FIGURE 10. Table of Standardized Inductors and Manufacturer’s Part Numbers 2. Compensation Network (RC, CC) and Output Capacitor(COUT) Selection RC and CC form a pole-zero compensation network that stabilizes the regulator. The values of RC and CC are mainly dependant on the regulator voltage gain, ILOAD(max), L and COUT. The following procedure calculates values for RC, CC, and COUT that ensure regulator stability. Be aware that this procedure doesn’t necessarily result in RC and CC that provide optimum compensation. In order to guarantee optimum compensation, one of the standard procedures for testing loop stability must be used, such as measuring VOUT transient response when pulsing ILOAD (see Figure 15). A. First, calculate the maximum value for RC. Select a resistor less than or equal to this value, and it should also be no greater than 3 kΩ. B. Calculate the minimum value for COUT using the following two equations. www.slkormicro.com 11 LM2577S/T-ADJ The larger of these two values is the minimum value that ensures stability. C. Calculate the minimum value of CC . The compensation capacitor is also part of the soft start circuitry. When power to the regulator is turned on, the switch duty cycle is allowed to rise at a rate controlled by this capacitor (with no control on the duty cycle, it would immediately rise to 90%, drawing huge currents from the input power supply). In order to operate properly, the soft start circuit requires CC ≥ 0.22 µF. The value of the output filter capacitor is normally large enough to require the use of aluminum electrolytic capacitors. Figure 11 lists several different types that are recommended for switching regulators, and the following parameters are used to select the proper capacitor. Working Voltage (WVDC): Choose a capacitor with a working voltage at least 20% higher than the regulator output voltage. Ripple Current: This is the maximum RMS value of current that charges the capacitor during each switching cycle. For step-up and flyback regulators, the formula for ripple current is Choose a capacitor that is rated at least 50% higher than this value at 52 kHz. Equivalent Series Resistance (ESR) : This is the primary cause of output ripple voltage, and it also affects the values of RC and CC needed to stabilize the regulator. As a result, the preceding calculations for CC and RC are only valid if ESR doesn’t exceed the maximum value specified by the following equations. Select a capacitor with ESR, at 52 kHz, that is less than or equal to the lower value calculated. Most electrolytic capacitors specify ESR at 120 Hz which is 15% to 30% higher than at 52 kHz. Also, be aware that ESR increases by a factor of 2 when operating at −20°C. In general, low values of ESR are achieved by using large value capacitors (C ≥ 470 µF), and capacitors with high WVDC, or by paralleling smaller-value capacitors. 3. Output Voltage Selection (R1 and R2) This section is for applications using the LM2577S/T_ADJ-ADJ. Skip this section if the LM2577S/T_ADJ12 or LM2577S/T_ADJ-15 is being www.slkormicro.com 12 LM2577S/T-ADJ used. With the LM2577S/T_ADJ-ADJ, the output voltage is given by VOUT = 1.23V (1 + R1/R2) Resistors R1 and R2 divide the output down so it can be compared with the LM2577S/T_ADJ-ADJ internal 1.23V reference. For a given desired output voltage VOUT, select R1 and R2 so that 4. Diode Selection (D) The switching diode used in the boost regulator must withstand a reverse voltage equal to the circuit output voltage,and must conduct the peak output current of the LM2577S/T_ADJ. A suitable diode must have a minimum reverse breakdown voltage greater than the circuit output voltage, and should be rated for average and peak current greater than ILOAD(max) and ID(PK). Schottky barrier diodes are often favored for use in switching regulators. Their low forward voltage drop allows higher regulator efficiency than if a (less expensive) fast recovery diode was used. See Figure 11 for recommended part numbers and voltage ratings of 1A and 3A diodes. FIGURE 11. Diode Selection Chart www.slkormicro.com 13 LM2577S/T-ADJ Flyback Regulator Design Procedure 1. Transformer Selection A family of standardized flyback transformers is available for creating flyback regulators that produce dual output voltages, from ±10V to ±15V, as shown in Figure 17. Figure 19 lists these transformers with the input voltage, output voltages and maximum load current they are designed for. T1 = Pulse Engineering, PE-65300 D1, D2 = 1N5821 FIGURE 17. LM2577S/T_ADJ-ADJ Flyback Regulator with ± Outputs www.slkormicro.com 14 LM2577S/T-ADJ FIGURE 18. Flyback Regulator Formulas C. Calculate the minimum value of CC D. Calculate the maximum ESR of the +VOUT and −VOUT output capacitors in parallel. This formula can also be used to calculate the maximum ESR of a single output regulator. At this point, refer to this same section in the Step-Up Regulator Design Procedure for more information regarding the selection of COUT. www.slkormicro.com 15 LM2577S/T-ADJ 3. Output Voltage Selection This section is for applications using the LM2577S/T_ADJ-ADJ. Skip this section if the LM2577S/T_ADJ12 or LM2577S/T_ADJ-15 is being used.With the LM2577S/T_ADJ-ADJ, the output voltage isgiven by VOUT = 1.23V (1 + R1/R2) Resistors R1 and R2 divide the output voltage down so it can be compared with the LM2577S/T_ADJ-ADJ internal 1.23V reference. For a desired output voltage VOUT, select R1 and R2 so that 4. Diode Selection The switching diode in a flyback converter must withstand the reverse voltage specified by the following equation. A suitable diode must have a reverse voltage rating greater than this. In addition it must be rated for more than the average and peak diode currents listed in Figure 18. 5. Input Capacitor Selection The primary of a flyback transformer draws discontinuous pulses of current from the input supply. As a result, a flyback regulator generates more noise at the input supply than a step-up regulator, and this requires a larger bypass capacitor to decouple the LM2577S/T_ADJ VIN pin from this noise. For most applications, a low ESR, 1.0 µF cap will be sufficient, if it is connected very close to the VIN and Ground pins. www.slkormicro.com 16 LM2577S/T-ADJ FIGURE 19. Flyback Transformer Selection Guide In addition to this bypass cap, a larger capacitor (≥ 47 µF) should be used where the flyback transformer connects to the input supply. This will attenuate noise which may interfere with other circuits connected to the same input supply voltage. www.slkormicro.com 17 LM2577S/T-ADJ Outline Drawing TO-220 Unit:mm TO-263 Unit: mm www.slkormicro.com 18