LM3578AMX/NOPB

LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 LM2578A/LM3578A Switching Regulator Check for Samples: LM2578A, LM3578A FEATURES DESCRIPTION • • • • 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 100 MΩ *LM2578 max duty cycle is 90% Definition of Terms Input Reference Voltage: The voltage (referred to ground) that must be applied to either the inverting or noninverting 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). Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 7 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com 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 34, and the RS-232 Line Driver Power Supply of Figure 36). Level Shift Accuracy is tested by using two equal-value resistors to draw current from the inverting and noninverting 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 collector-emitter breakdown voltage of the output transistor, measured at a specified current. 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. Supply Current: The IC power supply current, excluding the current drawn through the output transistor, with the oscillator operating. 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 internally-generated 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 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 ΔV away from VA, a current of ΔV/R1 will flow through R1. This same current flows through R2, and the comparator sees a total voltage of 2ΔV 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. 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 14, and follows the equation fOSC = 8×10−5/C1 The oscillator provides a blanking pulse to limit maximum duty cycle to 90%, and a reset pulse to the internal circuitry. 8 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 Figure 14. Value of Timing Capacitor vs Oscillator Frequency 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 The LM2578A's current limit may be referenced to either the ground or the Vin pins, and operates on a cycle-bycycle 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 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. 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. Figure 15. Current Limit, Ground Referred Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 9 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com Figure 16. Current Limit, Vin Referred 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. Figure 17. Current Limit Transient Suppressor, Ground Referred Figure 18. 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). 10 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 Figure 19. Current Limit Sense Voltage Multiplication, Ground Referred Figure 20. Current Limit Sense Voltage Multiplication, Vin 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 non-inverting 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. Figure 21. 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. Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 11 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com Figure 22. Maximum Duty Cycle Limiting DUTY CYCLE ADJUSTMENT 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. Figure 23. 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. Figure 24. Shutdown Occurs when VL is High 12 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 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 fast-recovery diode can be added in series with the Emitter output to counter the forward voltage drop of the catch diode (see Figure 15). For better efficiency of a high output current buck regulator, an external PNP transistor should be used as shown in Figure 29. Figure 25. 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 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. Figure 26. 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. Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 13 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com BUCK REGULATOR The buck configuration is used to step an input voltage down to a lower level. Transistor Q1 in Figure 27 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: Vout = D × Vin = Vin × (ton)/(ton + toff). Figure 27. Basic Buck Regulator Figure 28 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) × 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. L1 is the inductor and may be found from the inductance calculation chart (Figure 29) as follows: Given Vin = 15V Vo = 5V Io(max) = 350 mA fOSC = 50 kHz Discontinuous at 20% of Io(max). 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. 14 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 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 28. Buck or Step-Down Regulator Figure 29. DC/DC Inductance Calculator Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 15 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com 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) =(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. 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)/(ΔIL Vin fosc) BOOST L = Vin (Vo − Vin)/(ΔIL fosc Vo) INVERT L = Vin |Vo|/[ΔIL(Vin + |Vo|)fosc] where ΔIL is the current ripple through the inductor. ΔIL 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, ΔIL = 2 • IO(min) ΔIL = 140 mA for this circuit. ΔIL can also be interpreted as ΔIL = 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. The remainder of the components of Figure 28 are chosen as follows: C1 is the timing capacitor found in Figure 14. C2 ≥ Vo (Vin − Vo)/(8fosc2VinVrippleL1) 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. BUCK WITH BOOSTED OUTPUT CURRENT For applications requiring a large output current, an external transistor may be used as shown in Figure 30. 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). 16 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 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. V is the current limit sense voltage. Bf is the forced current gain of transistor Q1 (Bf = 30 for Figure 30). IR4 = VBE1/R4 Ip = IL(max, DC) + 0.5ΔIL Vin = 15V R4 = 200Ω Vo = 5V R5 = 330Ω Vripple = 50 mV C1 = 1820 pF Io = 1.5A C2 = 330 μF fosc = 50 kHz C3 = 20 pF R1 = 40 kΩ L1 = 220 μH R2 = 10 kΩ D1 = 1N5819 R3 = 0.05Ω Q1 = D45 Figure 30. 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 31. 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). Figure 31. Basic Boost Regulator The circuit of Figure 32 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). Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 17 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com 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 32. Boost or Step-Up Regulator R1 = (Vo − 1) R2 where R2 = 10 kΩ. R3 = V/(IL(max, DC) + 0.5 ΔIL) where: ΔIL = 2(ILOAD(min))(Vo/Vin) ΔIL 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 14. C2 ≥ Io (Vo − Vin)/(fosc Vo Vripple). 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 29 for the boost configuration and 20% discontinuity. INVERTING REGULATOR Figure 33 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 × (ton/toff). Figure 33. Basic Inverting Regulator Figure 34 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). 18 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 R1 = (|Vo| +1) R2 where R2 = 10 kΩ. R3 = V/(IL(max, DC) + 0.5 ΔIL). R4 = 10VBE1Bf/(IL (max, DC) + 0.5 ΔIL) where: V, VBE1, Vsat, and Bf are defined in the Buck Converter with Boosted Output Current section. ΔIL = 2(ILOAD(min))(Vin +|Vo|)/VIN R5 is defined in the Buck Converter 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 29 for the invert configuration and 20% discontinuity. Vin = 5V R4 = 190Ω Vo = −15V R5 = 82Ω Vripple = 5 mV R6 = 220 kΩ Io = 300 mA C1 = 1820 pF Imin = 60 mA C2 = 1000 μF fosc = 50 kHz C3 = 20 pF R1 = 160 kΩ C4 = 0.0022 μF R2 = 10 kΩ L1 = 150 μH R3 = 0.01Ω D1 = 1N5818 Figure 34. Inverting Regulator BUCK-BOOST REGULATOR The Buck-Boost Regulator, shown in Figure 35, 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. D1 and D2 are Schottky type diodes such as the 1N5818 or 1N5819. Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 19 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.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) (1) where: RS-232 LINE DRIVER POWER SUPPLY The power supply, shown in Figure 36, 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. 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. 20 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 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 35. Buck-Boost Regulator 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 36. RS-232 Line Driver Power Supply Capacitor C1 sets the oscillator frequency and is selected from Figure 14. Capacitor C2 serves as a compensation capacitor for synchronous operation and a value of 10 to 50 pF should be sufficient for most applications. Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 21 LM2578A, LM3578A SNVS767E – AUGUST 2000 – REVISED APRIL 2013 www.ti.com A minimum value for an ideal output capacitor C3, could be calculated as C = Io × t/ΔV where Io is the load current, t is the transistor on time (typically 0.4/fosc), and ΔV is the peak-to-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. 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 36 was a Pulse Engineering PE-64287. 22 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A LM2578A, LM3578A www.ti.com SNVS767E – AUGUST 2000 – REVISED APRIL 2013 REVISION HISTORY Changes from Revision D (April 2013) to Revision E • Page Changed layout of National Data Sheet to TI format .......................................................................................................... 22 Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM2578A LM3578A Submit Documentation Feedback 23 PACKAGE OPTION ADDENDUM www.ti.com 26-Aug-2022 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan (2) Lead finish/ Ball material MSL Peak Temp Op Temp (°C) Device Marking (3) Samples (4/5) (6) LM2578AM NRND SOIC D 8 95 Non-RoHS & Green Call TI Level-1-235C-UNLIM -40 to 125 2578 AM LM2578AM/NOPB ACTIVE SOIC D 8 95 RoHS & Green SN Level-1-260C-UNLIM -40 to 125 2578 AM Samples LM2578AMX/NOPB ACTIVE SOIC D 8 2500 RoHS & Green SN Level-1-260C-UNLIM -40 to 125 2578 AM Samples LM2578AN/NOPB ACTIVE PDIP P 8 40 RoHS & Green NIPDAU Level-1-NA-UNLIM -40 to 125 LM2578AN Samples LM3578AM NRND SOIC D 8 95 Non-RoHS & Green Call TI Level-1-235C-UNLIM 0 to 125 3578 AM LM3578AM/NOPB ACTIVE SOIC D 8 95 RoHS & Green SN Level-1-260C-UNLIM 0 to 125 3578 AM LM3578AMX NRND SOIC D 8 2500 Non-RoHS & Green Call TI Level-1-235C-UNLIM 0 to 125 3578 AM LM3578AMX/NOPB ACTIVE SOIC D 8 2500 RoHS & Green SN Level-1-260C-UNLIM 0 to 125 3578 AM Samples LM3578AN/NOPB ACTIVE PDIP P 8 40 RoHS & Green NIPDAU Level-1-NA-UNLIM 0 to 125 LM3578AN Samples (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of