LM2578A/LM3578A Switching Regulator General Description Features The LM2578A is a switching regulator which can easily be set up for such DC-to-DC voltage conversion circuits as the buck, boost, and inverting configurations. The LM2578A features a unique comparator input stage which not only has separate pins for both the inverting and non-inverting inputs, but also provides an internal 1.0V reference to each input, thereby simplifying circuit design and p.c. board layout. The output can switch up to 750 mA and has output pins for its collector and emitter to promote design flexibility. An external current limit terminal may be referenced to either the ground or the Vin terminal, depending upon the application. In addition, the LM2578A has an on board oscillator, which sets the switching frequency with a single external capacitor from < 1 Hz to 100 kHz (typical). The LM2578A is an improved version of the LM2578, offering higher maximum ratings for the total supply voltage and output transistor emitter and collector voltages. n n n n n n Inverting and non-inverting feedback inputs 1.0V reference at inputs Operates from supply voltages of 2V to 40V Output current up to 750 mA, saturation less than 0.9V Current limit and thermal shut down Duty cycle up to 90% Applications n Switching regulators in buck, boost, inverting, and single-ended transformer configurations n Motor speed control n Lamp flasher Connection Diagram and Ordering Information Dual-In-Line Package 00871129 Order Number LM3578AM, LM2578AN or LM3578AN See NS Package Number M08A or N08E © 2005 National Semiconductor Corporation DS008711 www.national.com LM2578A/LM3578A Switching Regulator February 2005 LM2578A/LM3578A Functional Diagram 00871101 www.national.com 2 ESD Tolerance (Note 4) (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Total Supply Voltage Operating Ratings Ambient Temperature Range 50V Collector Output to Ground −0.3V to +50V Emitter Output to Ground (Note 2) LM2578A −40˚C ≤ TA ≤+85˚C LM3578A 0˚C ≤ TA ≤+70˚C −1V to +50V Power Dissipation (Note 3) Internally limited Output Current Junction Temperature Range 750 mA Storage Temperature 2 kV −65˚C to +150˚C Lead Temperature (soldering, 10 seconds) Maximum Junction Temperature 260˚C LM2578A −40˚C ≤ TJ ≤+125˚C LM3578A 0˚C ≤ TJ ≤+125˚C 150˚C Electrical Characteristics These specifications apply for 2V ≤ VIN ≤ 40V (2.2V ≤ VIN ≤ 40V for TJ ≤ −25˚C), timing capacitor CT = 3900 pF, and 25% ≤ duty cycle ≤ 75%, unless otherwise specified. Values in standard typeface are for TJ = 25˚C; values in boldface type apply for operation over the specified operating junction temperature range. LM2578A/ Symbol Parameter Conditions Typical LM3578A (Note 5) Limit (Note 6) Units OSCILLATOR fOSC ∆fOSC/∆T Frequency 20 Frequency Drift with Temperature Amplitude kHz 24 kHz (max) 16 kHz (min) −0.13 %/˚C 550 mVp-p REFERENCE/COMPARATOR (Note 7) VR Input Reference I1 = I2 = 0 mA and Voltage I1 = I2 = 1 mA ± 1% (Note 8) 1.0 V 1.050/1.070 0.950/0.930 V (max) V (min) ∆VR/∆VIN Input Reference Voltage Line Regulation I1 = I2 = 0 mA and IINV Inverting Input Current I1 = I2 = 0 mA, duty cycle = 25% 0.5 µA Level Shift Accuracy Level Shift Current = 1 mA 1.0 % 0.003 I1 = I2 = 1 mA ± 1% (Note 8) %/V 0.01/0.02 10/13 ∆VR/∆t Input Reference Voltage Long Term Stability %/V (max) % (max) 100 ppm/1000h V OUTPUT VC (sat) Collector Saturation Voltage IC = 750 mA pulsed, Emitter grounded 0.7 VE (sat) Emitter Saturation Voltage IO = 80 mA pulsed, 1.4 0.90/1.2 VIN = VC = 40V ICES BVCEO(SUS) Collector Leakage Current VIN = VCE = 40V, Emitter grounded, Output OFF Collector-Emitter Sustaining Voltage ISUST = 0.2A (pulsed), VIN = 0 V (max) V 1.7/2.0 0.1 V (max) µA 200/250 60 µA (max) V 50 V (min) CURRENT LIMIT VCL Sense Voltage Shutdown Level Referred to VIN or Ground (Note 9) 3 110 mV 80 mV (min) 160 mV (max) www.national.com LM2578A/LM3578A Absolute Maximum Ratings LM2578A/LM3578A Electrical Characteristics (Continued) These specifications apply for 2V ≤ VIN ≤ 40V (2.2V ≤ VIN ≤ 40V for TJ ≤ −25˚C), timing capacitor CT = 3900 pF, and 25% ≤ duty cycle ≤ 75%, unless otherwise specified. Values in standard typeface are for TJ = 25˚C; values in boldface type apply for operation over the specified operating junction temperature range. LM2578A/ Symbol Parameter Conditions Typical LM3578A (Note 5) Limit (Note 6) Units CURRENT LIMIT ∆VCL/∆T Sense Voltage Temperature Drift ICL Sense Bias Current 0.3 %/˚C Referred to VIN 4.0 µA Referred to ground 0.4 µA Output OFF, VE = 0V 2.0 mA Output ON, IC = 750 mA pulsed, 14 DEVICE POWER CONSUMPTION IS Supply Current 3.5/4.0 mA (max) mA VE = 0V Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: For TJ ≥ 100˚C, the Emitter pin voltage should not be driven more than 0.6V below ground (see Application Information). Note 3: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the 8-pin DIP must be derated at 95˚C/W, junction to ambient. The device in the surface-mount package must be derated at 150˚C/W, junction-to-ambient. Note 4: Human body model, 1.5 kΩ in series with 100 pF. Note 5: Typical values are for TJ = 25˚C and represent the most likely parametric norm. Note 6: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). Room temperature limits are 100% production tested. Limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate AOQL. Note 7: Input terminals are protected from accidental shorts to ground but if external voltages higher than the reference voltage are applied, excessive current will flow and should be limited to less than 5 mA. Note 8: I1 and I2 are the external sink currents at the inputs (refer to Test Circuit). Note 9: Connection of a 10 kΩ resistor from pin 1 to pin 4 will drive the duty cycle to its maximum, typically 90%. Applying the minimum Current Limit Sense Voltage to pin 7 will not reduce the duty cycle to less than 50%. Applying the maximum Current Limit Sense Voltage to pin 7 is certain to reduce the duty cycle below 50%. Increasing this voltage by 15 mV may be required to reduce the duty cycle to 0%, when the Collector output swing is 40V or greater (see Ground-Referred Current Limit Sense Voltage typical curve). Typical Performance Characteristics Oscillator Frequency Change with Temperature Oscillator Voltage Swing 00871132 www.national.com 00871133 4 LM2578A/LM3578A Typical Performance Characteristics (Continued) Collector Saturation Voltage (Sinking Current, Emitter Grounded) Input Reference Voltage Drift with Temperature 00871134 00871135 Emitter Saturation Voltage (Sourcing Current, Collector at Vin) Ground Referred Current Limit Sense Voltage 00871136 00871137 Current Limit Sense Voltage Drift with Temperature Current Limit Response Time for Various Over Drives 00871139 00871138 5 www.national.com LM2578A/LM3578A Typical Performance Characteristics (Continued) Current Limit Sense Voltage vs Supply Voltage Supply Current 00871141 00871140 Collector Current with Emitter Output Below Ground Supply Current 00871142 00871143 The Current Limit Sense Voltage is measured by connecting an adjustable 0-to-1V floating power supply in series with the current limit terminal and referring it to either the ground or the Vin terminal. Set the duty cycle to 90% and monitor test point TP5 while adjusting the floating power supply voltage until the LM2578A’s duty cycle just reaches 0%. This voltage is the Current Limit Sense Voltage. The Supply Current should be measured with the duty cycle at 0% and S1 in the I1 = I2 = 0 mA position. *LM2578A specifications are measured using automated test equipment. This circuit is provided for the customer’s convenience when checking parameters. Due to possible variations in testing conditions, the measured values from these testing procedures may not match those of the factory. Test Circuit* Parameter tests can be made using the test circuit shown. Select the desired Vin, collector voltage and duty cycle with adjustable power supplies. A digital volt meter with an input resistance greater than 100 MΩ should be used to measure the following: Input Reference Voltage to Ground; S1 in either position. Level Shift Accuracy (%) = (TP3(V)/1V) x 100%; S1 at I1 = I2 = 1 mA Input Current (mA) = (1V − Tp3 (V))/1 MΩ: S1 at I1 = I2 = 0 mA. Oscillator parameters can be measured at Tp4 using a frequency counter or an oscilloscope. www.national.com 6 LM2578A/LM3578A Test Circuit* (Continued) 00871103 Op amp supplies are ± 15V DVM input resistance > 100 MΩ *LM2578 max duty cycle is 90% Current Limit Sense Voltage: The voltage at the Current Limit pin, referred to either the supply or the ground terminal, which (via logic circuitry) will cause the output transistor to turn OFF and resets cycle-by-cycle at the oscillator frequency. Current Limit Sense Current: The bias current for the Current Limit terminal with the applied voltage equal to the Current Limit Sense Voltage. Definition of Terms Input Reference Voltage: The voltage (referred to ground) that must be applied to either the inverting or non-inverting input to cause the regulator switch to change state (ON or OFF). Input Reference Current: The current that must be drawn from either the inverting or non-inverting input to cause the regulator switch to change state (ON or OFF). Supply Current: The IC power supply current, excluding the current drawn through the output transistor, with the oscillator operating. Input Level Shift Accuracy: This specification determines the output voltage tolerance of a regulator whose output control depends on drawing equal currents from the inverting and non-inverting inputs (see the Inverting Regulator of Figure 21, and the RS-232 Line Driver Power Supply of Figure 23). Level Shift Accuracy is tested by using two equal-value resistors to draw current from the inverting and non-inverting input terminals, then measuring the percentage difference in the voltages across the resistors that produces a controlled duty cycle at the switch output. Collector Saturation Voltage: With the inverting input terminal grounded thru a 10 kΩ resistor and the output transistor’s emitter connected to ground, the Collector SaturationVoltage is the collector-to-emitter voltage for a given collector current. Emitter Saturation Voltage: With the inverting input terminal grounded thru a 10 kΩ resistor and the output transistor’s collector connected to Vin, the Emitter Saturation Voltage is the collector-to-emitter voltage for a given emitter current. Collector Emitter Sustaining Voltage: The collectoremitter breakdown voltage of the output transistor, measured at a specified current. Functional Description The LM2578A is a pulse-width modulator designed for use as a switching regulator controller. It may also be used in other applications which require controlled pulse-width voltage drive. A control signal, usually representing output voltage, fed into the LM2578A’s comparator is compared with an internallygenerated reference. The resulting error signal and the oscillator’s output are fed to a logic network which determines when the output transistor will be turned ON or OFF. The following is a brief description of the subsections of the LM2578A. COMPARATOR INPUT STAGE The LM2578A’s comparator input stage is unique in that both the inverting and non-inverting inputs are available to the user, and both contain a 1.0V reference. This is accomplished as follows: A 1.0V reference is fed into a modified voltage follower circuit (see FUNCTIONAL DIAGRAM). When both input pins are open, no current flows through R1 7 www.national.com LM2578A/LM3578A Functional Description 110 mV above ground (see FUNCTIONAL DIAGRAM). The current limit is activated whenever the current limit terminal is pulled 110 mV away from either Vin or ground. (Continued) and R2. Thus, both inputs to the comparator will have the potential of the 1.0V reference, VA. When one input, for example the non-inverting input, is pulled ∆V away from VA, a current of ∆V/R1 will flow through R1. This same current flows through R2, and the comparator sees a total voltage of 2∆V between its inputs. The high gain of the system, through feedback, will correct for this imbalance and return both inputs to the 1.0V level. Applications Information CURRENT LIMIT As mentioned in the functional description, the current limit terminal may be referenced to either the Vin or the ground terminal. Resistor R3 converts the current to be sensed into a voltage for current limit detection. This unusual comparator input stage increases circuit flexibility, while minimizing the total number of external components required for a voltage regulator system. The inverting switching regulator configuration, for example, can be set up without having to use an external op amp for feedback polarity reversal (see TYPICAL APPLICATIONS). OSCILLATOR The LM2578A provides an on-board oscillator which can be adjusted up to 100 kHz. Its frequency is set by a single external capacitor, C1, as shown in Figure 1, and follows the equation fOSC = 8x10−5/C1 The oscillator provides a blanking pulse to limit maximum duty cycle to 90%, and a reset pulse to the internal circuitry. 00871115 FIGURE 2. Current Limit, Ground Referred 00871104 FIGURE 1. Value of Timing Capacitor vs Oscillator Frequency 00871116 FIGURE 3. Current Limit, Vin Referred OUTPUT TRANSISTOR The output transistor is capable of delivering up to 750 mA with a saturation voltage of less than 0.9V. (see Collector Saturation Voltage and Emitter Saturation Voltage curves). The emitter must not be pulled more than 1V below ground (this limit is 0.6V for TJ ≥ 100˚C). Because of this limit, an external transistor must be used to develop negative output voltages (see the Inverting Regulator Typical Application). Other configurations may need protection against violation of this limit (see the Emitter Output section of the Applications Information). CURRENT LIMIT TRANSIENT SUPPRESSION When noise spikes and switching transients interfere with proper current limit operation, R1 and C1 act together as a low pass filter to control the current limit circuitry’s response time. Because the sense current of the current limit terminal varies according to where it is referenced, R1 should be less than 2 kΩ when referenced to ground, and less than 100Ω when referenced to Vin. CURRENT LIMIT The LM2578A’s current limit may be referenced to either the ground or the Vin pins, and operates on a cycle-by-cycle basis. The current limit section consists of two comparators: one with its non-inverting input referenced to a voltage 110 mV below Vin, the other with its inverting input referenced www.national.com 8 LM2578A/LM3578A Applications Information (Continued) 00871120 FIGURE 7. Current Limit Sense Voltage Multiplication, Vin Referred 00871117 FIGURE 4. Current Limit Transient Suppressor, Ground Referred UNDER-VOLTAGE LOCKOUT Under-voltage lockout is accomplished with few external components. When Vin becomes lower than the zener breakdown voltage, the output transistor is turned off. This occurs because diode D1 will then become forward biased, allowing resistor R3 to sink a greater current from the noninverting input than is sunk by the parallel combination of R1 and R2 at the inverting terminal. R3 should be one-fifth of the value of R1 and R2 in parallel. 00871118 FIGURE 5. Current Limit Transient Suppressor, Vin Referred C.L. SENSE VOLTAGE MULTIPLICATION When a larger sense resistor value is desired, the voltage divider network, consisting of R1 and R2, may be used. This effectively multiplies the sense voltage by (1 + R1/R2). Also, R1 can be replaced by a diode to increase current limit sense voltage to about 800 mV (diode Vf + 110 mV). 00871122 FIGURE 8. Under-Voltage Lockout MAXIMUM DUTY CYCLE LIMITING The maximum duty cycle can be externally limited by adjusting the charge to discharge ratio of the oscillator capacitor with a single external resistor. Typical values are 50 µA for the charge current, 450 µA for the discharge current, and a voltage swing from 200 mV to 750 mV. Therefore, R1 is selected for the desired charging and discharging slopes and C1 is readjusted to set the oscillator frequency. 00871119 FIGURE 6. Current Limit Sense Voltage Multiplication, Ground Referred 9 www.national.com LM2578A/LM3578A Applications Information (Continued) 00871121 00871124 FIGURE 9. Maximum Duty Cycle Limiting FIGURE 11. Shutdown Occurs when VL is High DUTY CYCLE ADJUSTMENT EMITTER OUTPUT When the LM2578A output transistor is in the OFF state, if the Emitter output swings below the ground pin voltage, the output transistor will turn ON because its base is clamped near ground. The Collector Current with Emitter Output Below Ground curve shows the amount of Collector current drawn in this mode, vs temperature and Emitter voltage. When the Collector-Emitter voltage is high, this current will cause high power dissipation in the output transistor and should be avoided. This situation can occur in the high-current high-voltage buck application if the Emitter output is used and the catch diode’s forward voltage drop is greater than 0.6V. A fastrecovery diode can be added in series with the Emitter output to counter the forward voltage drop of the catch diode (see Figure 2). For better efficiency of a high output current buck regulator, an external PNP transistor should be used as shown in Figure 16. When manual or mechanical selection of the output transistor’s duty cycle is needed, the cirucit shown below may be used. The output will turn on with the beginning of each oscillator cycle and turn off when the current sunk by R2 and R3 from the non-inverting terminal becomes greater than the current sunk from the inverting terminal. With the resistor values as shown, R3 can be used to adjust the duty cycle from 0% to 90%. When the sum of R2 and R3 is twice the value of R1, the duty cycle will be about 50%. C1 may be a large electrolytic capacitor to lower the oscillator frequency below 1 Hz. 00871123 FIGURE 10. Duty Cycle Adjustment REMOTE SHUTDOWN The LM2578A may be remotely shutdown by sinking a greater current from the non-inverting input than from the inverting input. This may be accomplished by selecting resistor R3 to be approximately one-half the value of R1 and R2 in parallel. 00871130 FIGURE 12. D1 Prevents Output Transistor from Improperly Turning ON due to D2’s Forward Voltage SYNCHRONIZING DEVICES When several devices are to be operated at once, their oscillators may be synchronized by the application of an external signal. This drive signal should be a pulse waveform with a minimum pulse width of 2 µs. and an amplitude from www.national.com 10 LM2578A/LM3578A Applications Information Vout = D x Vin = Vin x (ton)/(ton + toff). (Continued) 1.5V to 2.0V. The signal source must be capable of 1.) driving capacitive loads and 2.) delivering up to 500 µA for each LM2578A. Capacitors C1 thru CN are to be selected for a 20% slower frequency than the synchronization frequency. 00871105 FIGURE 14. Basic Buck Regulator Figure 15 is a 15V to 5V buck regulator with an output current, Io, of 350 mA. The circuit becomes discontinuous at 20% of Io(max), has 10 mV of output voltage ripple, an efficiency of 75%, a load regulation of 30 mV (70 mA to 350 mA) and a line regulation of 10 mV (12 ≤ Vin ≤ 18V). Component values are selected as follows: R1 = (Vo − 1) x R2 where R2 = 10 kΩ R3 = V/Isw(max) R3 = 0.15Ω where: V is the current limit sense voltage, 0.11V Isw(max) is the maximum allowable current thru the output transistor. 00871125 FIGURE 13. Synchronizing Devices Typical Applications The LM2578A may be operated in either the continuous or the discontinuous conduction mode. The following applications (except for the Buck-Boost Regulator) are designed for continuous conduction operation. That is, the inductor current is not allowed to fall to zero. This mode of operation has higher efficiency and lower EMI characteristics than the discontinuous mode. L1 is the inductor and may be found from the inductance calculation chart (Figure 16) as follows: Given Vin = 15V Vo = 5V Io(max) = 350 mA fOSC = 50 kHz Discontinuous at 20% of Io(max). BUCK REGULATOR The buck configuration is used to step an input voltage down to a lower level. Transistor Q1 in Figure 14 chops the input DC voltage into a squarewave. This squarewave is then converted back into a DC voltage of lower magnitude by the low pass filter consisting of L1 and C1. The duty cycle, D, of the squarewave relates the output voltage to the input voltage by the following equation: Note that since the circuit will become discontinuous at 20% of Io(max), the load current must not be allowed to fall below 70 mA. 11 www.national.com LM2578A/LM3578A Typical Applications (Continued) 00871106 Vin = 15V R3 = 0.15Ω Vo = 5V C1 = 1820 pF Vripple = 10 mV C2 = 220 µF Io = 350 mA C3 = 20 pF fosc = 50 kHz L1 = 470 µH R1 = 40 kΩ D1 = 1N5818 R2 = 10 kΩ FIGURE 15. Buck or Step-Down Regulator www.national.com 12 LM2578A/LM3578A Typical Applications (Continued) 00871131 FIGURE 16. DC/DC Inductance Calculator 13 www.national.com LM2578A/LM3578A Typical Applications ∆IL = 2 • IO(min) ∆IL = 140 mA for this circuit. ∆IL can also be interpreted as ∆IL = 2 • (Discontinuity Factor) • IL where the Discontinuity Factor is the ratio of the minimum load current to the maximum load current. For this example, the Discontinuity Factor is 0.2. (Continued) Step 1: Calculate the maximum DC current through the inductor, IL(max). The necessary equations are indicated at the top of the chart and show that IL(max) = Io(max) for the buck configuration. Thus, IL(max) = 350 mA. Step 2: Calculate the inductor Volts-sec product, E-Top, according to the equations given from the chart. For the Buck: E-Top = (Vin − Vo) (Vo/Vin) (1000/fosc) The remainder of the components of Figure 15 are chosen as follows: C1 is the timing capacitor found in Figure 1. C2 ≥ Vo (Vin − Vo)/(8fosc 2VinVrippleL1) where Vripple is the peak-to-peak output voltage ripple. C3 is necessary for continuous operation and is generally in the 10 pF to 30 pF range. D1 should be a Schottky type diode, such as the 1N5818 or 1N5819. =(15 − 5) (5/15) (1000/50) = 66V-µs. with the oscillator frequency, fosc, expressed in kHz. Step 3: Using the graph with axis labeled “Discontinuous At % IOUT” and “IL(max, DC)” find the point where the desired maximum inductor current, IL(max, DC) intercepts the desired discontinuity percentage. BUCK WITH BOOSTED OUTPUT CURRENT For applications requiring a large output current, an external transistor may be used as shown in Figure 17. This circuit steps a 15V supply down to 5V with 1.5A of output current. The output ripple is 50 mV, with an efficiency of 80%, a load regulation of 40 mV (150 mA to 1.5A), and a line regulation of 20 mV (12V ≤ Vin ≤ 18V). Component values are selected as outlined for the buck regulator with a discontinuity factor of 10%, with the addition of R4 and R5: R4 = 10VBE1Bf/Ip R5 = (Vin − V − VBE1 − Vsat) Bf/(IL(max, DC) + IR4) where: VBE1 is the VBE of transistor Q1. Vsat is the saturation voltage of the LM2578A output transistor. In this example, the point of interest is where the 0.35A line intersects with the 20% line. This is nearly the midpoint of the horizontal axis. Step 4: This last step is merely the translation of the point found in Step 3 to the graph directly below it. This is accomplished by moving straight down the page to the point which intercepts the desired E-Top. For this example, E-Top is 66V-µs and the desired inductor value is 470 µH. Since this example was for 20% discontinuity, the bottom chart could have been used directly, as noted in step 3 of the chart instructions. For a full line of standard inductor values, contact Pulse Engineering (San Diego, Calif.) regarding their PE526XX series, or A. I. E. Magnetics (Nashville, Tenn.). A more precise inductance value may be calculated for the Buck, Boost and Inverting Regulators as follows: BUCK L = Vo (Vin − Vo)/(∆IL Vin fosc) BOOST L = Vin (Vo − Vin)/(∆IL fosc Vo) INVERT L = Vin |Vo|/[∆IL(Vin + |Vo|)fosc] where ∆IL is the current ripple through the inductor. ∆IL is usually chosen based on the minimum load current expected of the circuit. For the buck regulator, since the inductor current IL equals the load current IO, www.national.com V is the current limit sense voltage. Bf is the forced current gain of transistor Q1 (Bf = 30 for Figure 17 ). IR4 = VBE1/R4 Ip = IL(max, DC) + 0.5∆IL 14 LM2578A/LM3578A Typical Applications (Continued) 00871108 Vin = 15V R4 = 200Ω fosc = 50 kHz C3 = 20 pF Vo = 5V R5 = 330Ω R1 = 40 kΩ L1 = 220 µH Vripple = 50 mV C1 = 1820 pF R2 = 10 kΩ D1 = 1N5819 Io = 1.5A C2 = 330 µF R3 = 0.05Ω Q1 = D45 FIGURE 17. Buck Converter with Boosted Output Current BOOST REGULATOR The boost regulator converts a low input voltage into a higher output voltage. The basic configuration is shown in Figure 18. Energy is stored in the inductor while the transistor is on and then transferred with the input voltage to the output capacitor for filtering when the transistor is off. Thus, Vo = Vin + Vin(ton/toff). 00871111 00871109 FIGURE 18. Basic Boost Regulator The circuit of Figure 19 converts a 5V supply into a 15V supply with 150 mA of output current, a load regulation of 14 mV (30 mA to 140 mA), and a line regulation of 35 mV (4.5V ≤ Vin ≤ 8.5V). Vin = 5V R4 = 200 kΩ Vo = 15V C1 = 1820 pF Vripple = 10 mV C2 = 470 µF Io = 140 mA C3 = 20 pF fosc = 50 kHz C4 = 0.0022 µF R1 = 140 kΩ L1 = 330 µH R2 = 10 kΩ D1 = 1N5818 R3 = 0.15Ω FIGURE 19. Boost or Step-Up Regulator R1 = (Vo − 1) R2 where R2 = 10 kΩ. R3 = V/(IL(max, DC) + 0.5 ∆IL) where: ∆IL = 2(ILOAD(min))(Vo/Vin) ∆IL is 200 mA in this example. R4, C3 and C4 are necessary for continuous operation and are typically 220 kΩ, 20 pF, and 0.0022 µF respectively. C1 is the timing capacitor found in Figure 1. C2 ≥ Io (Vo − Vin)/(fosc Vo Vripple). 15 www.national.com LM2578A/LM3578A Typical Applications (Continued) Figure 21 shows an LM2578A configured as a 5V to −15V polarity inverter with an output current of 300 mA, a load regulation of 44 mV (60 mA to 300 mA) and a line regulation of 50 mV (4.5V ≤ Vin ≤ 8.5V). R1 = (|Vo| +1) R2 where R2 = 10 kΩ. R3 = V/(IL(max, DC) + 0.5 ∆IL). R4 = 10VBE1Bf/(IL (max, DC) + 0.5 ∆IL) D1 is a Schottky type diode such as a 1N5818 or 1N5819. L1 is found as described in the buck converter section, using the inductance chart for Figure 16 for the boost configuration and 20% discontinuity. INVERTING REGULATOR Figure 20 shows the basic configuration for an inverting regulator. The input voltage is of a positive polarity, but the output is negative. The output may be less than, equal to, or greater in magnitude than the input. The relationship between the magnitude of the input voltage and the output voltage is Vo = Vin x (ton/toff). where: V, VBE1, Vsat, and Bf are defined in the “Buck Converter with Boosted Output Current” section. ∆IL = 2(ILOAD(min))(Vin +|Vo|)/VIN R5 is defined in the “Buck with Boosted Output Current” section. R6 serves the same purpose as R4 in the Boost Regulator circuit and is typically 220 kΩ. C1, C3 and C4 are defined in the “Boost Regulator” section. C2 ≥ Io |Vo|/[fosc(|Vo| + Vin) Vripple] L1 is found as outlined in the section on buck converters, using the inductance chart of Figure 16 for the invert configuration and 20% discontinuity. 00871110 FIGURE 20. Basic Inverting Regulator 00871112 Vin = 5V R4 = 190Ω fosc = 50 kHz C3 = 20 pF Vo = −15V R5 = 82Ω R1 = 160 kΩ C4 = 0.0022 µF Vripple = 5 mV R6 = 220 kΩ R2 = 10 kΩ L1 = 150 µH Io = 300 mA C1 = 1820 pF R3 = 0.01Ω D1 = 1N5818 Imin = 60 mA C2 = 1000 µF FIGURE 21. Inverting Regulator D1 and D2 are Schottky type diodes such as the 1N5818 or 1N5819. BUCK-BOOST REGULATOR The Buck-Boost Regulator, shown in Figure 22, may step a voltage up or down, depending upon whether or not the desired output voltage is greater or less than the input voltage. In this case, the output voltage is 12V with an input voltage from 9V to 15V. The circuit exhibits an efficiency of 75%, with a load regulation of 60 mV (10 mA to 100 mA) and a line regulation of 52 mV. R1 = (Vo − 1) R2 where R2 = 10 kΩ R3 = V/0. 75A R4, C1, C3 and C4 are defined in the “Boost Regulator” section. www.national.com where: Vd is the forward voltage drop of the diodes. Vsat is the saturation voltage of the LM2578A output transistor. Vsat1 is the saturation voltage of transistor Q1. L1 ≥ (Vin − Vsat − Vsat1) (ton/Ip) 16 LM2578A/LM3578A Typical Applications (Continued) where: RS-232 LINE DRIVER POWER SUPPLY The power supply, shown in Figure 23, operates from an input voltage as low as 4.2V (5V nominal), and delivers an output of ± 12V at ± 40 mA with better than 70% efficiency. The circuit provides a load regulation of ± 150 mV (from 10% to 100% of full load) and a line regulation of ± 10 mV. Other notable features include a cycle-by-cycle current limit and an output voltage ripple of less than 40 mVp-p. 00871113 A unique feature of this circuit is its use of feedback from both outputs. This dual feedback configuration results in a sharing of the output voltage regulation by each output so that neither side becomes unbalanced as in single feedback systems. In addition, since both sides are regulated, it is not necessary to use a linear regulator for output regulation. The feedback resistors, R2 and R3, may be selected as follows by assuming a value of 10 kΩ for R1; R2 = (Vo − 1V)/45.8 µA = 240 kΩ R3 = (|Vo| +1V)/54.2 µA = 240 kΩ Actually, the currents used to program the values for the feedback resistors may vary from 40 µA to 60 µA, as long as their sum is equal to the 100 µA necessary to establish the 1V threshold across R1. Ideally, these currents should be equal (50 µA each) for optimal control. However, as was done here, they may be mismatched in order to use standard resistor values. This results in a slight mismatch of regulation between the two outputs. The current limit resistor, R4, is selected by dividing the current limit threshold voltage by the maximum peak current level in the output switch. For our purposes R4 = 110 mV/ 750 mA = 0.15Ω. A value of 0.1Ω was used. 9V ≤ Vin ≤ 15V R5 = 270 Vo = 12V C1 = 1820 pF Io = 100 mA C2 = 220 µF Vripple = 50 mV C3 = 20 pF fosc = 50 kHz C4 = 0.0022 µF R1 = 110k L1 = 220 µH R2 = 10k D1, D2 = 1N5819 R3 = 0.15 Q1 = D44 R4 = 220k FIGURE 22. Buck-Boost Regulator 00871114 Vin = 5V R4 = 0.15Ω Vo ± 12V C1 = 820 pF Io = ± 40 mA C2 = 10 pF fosc = 80 kHz C3 = 220 µF R1 = 10 kΩ D1, D2, D3 = 1N5819 R2 = 240 kΩ T1 = PE-64287 R3 = 240 kΩ FIGURE 23. RS-232 Line Driver Power Supply Capacitor C1 sets the oscillator frequency and is selected from Figure 1. Capacitor C2 serves as a compensation capacitor for synchronous operation and a value of 10 to 50 pF should be sufficient for most applications. 17 www.national.com LM2578A/LM3578A Typical Applications Transformer selection should be picked for an output transistor “on” time of 0.4/fosc, and a primary inductance high enough to prevent the output transistor switch from ramping higher than the transistor’s rating of 750 mA. Pulse Engineering (San Diego, Calif.) and Renco Electronics, Inc. (Deer Park, N.Y.) can provide further assistance in selecting the proper transformer for a specific application need. The transformer used in Figure 23 was a Pulse Engineering PE-64287. (Continued) A minimum value for an ideal output capacitor C3, could be calculated as C = Io x t/∆V where Io is the load current, t is the transistor on time (typically 0.4/fosc), and ∆V is the peakto-peak output voltage ripple. A larger output capacitor than this theoretical value should be used since electrolytics have poor high frequency performance. Experience has shown that a value from 5 to 10 times the calculated value should be used. For good efficiency, the diodes must have a low forward voltage drop and be fast switching. 1N5819 Schottky diodes work well. www.national.com 18 LM2578A/LM3578A Physical Dimensions inches (millimeters) unless otherwise noted Plastic Surface-Mount Package (M) Order Number LM3578AM NS Package Number M08A 19 www.national.com LM2578A/LM3578A Switching Regulator Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Molded Dual-In-Line Package (N) Order Number LM2578AN or LM3578AN NS Package Number N08E 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. 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