LM Price breaks and distributor sample stock. The input voltage from 3. The IC has a wide datasheey voltage range 3. The UC — a pin to pin compatible replacement available from Texas Instruments can also be used.

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The device is available in three different output voltage versions: 12V, 15V, and adjustable. 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. Included on the chip is a 3. 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. Features Requires few external components NPN output switches 3.

Ordering Information Temperature Range? Maximum Junction Temperature 45V 65V 6. C ? C, and those in bold type face apply over full Operating Temperature Range.

A min? A max? 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. To prevent damage to the switch, its current must be externally limited to 6. Note 3: All limits guaranteed at room temperature standard type face and at temperature extremes boldface type.

Note 5: All limits guaranteed at room temperature standard type face and at temperature extremes boldface type. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control SQC methods.

Note 6: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. Note 7: A 1. Note 8: Junction to ambient thermal resistance with approximately 1 square inch of pc board copper surrounding the leads.

Additional copper area will lower thermal resistance further. Note 9: If the TO package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using 0. Reference Voltage vs Supply Voltage? This is a switching regulator used for producing an output voltage greater than the input supply voltage. A basic explanation of how it works is as follows. 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.

The error amp output voltage is compared to a voltage proportional to the switch current i. 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.

Inductor Selection L A. Voltage Options: 1. For 12V or 15V output 13 www. From here, proceed 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. Find where E? T intersects this inductor value to determine if it has an L or H prefix. T intersects both the L and H regions, select the inductor with an H prefix. Identify Inductor Value: 1.

From Figure 9, identify the inductor code for the region indicated by the intersection of E? This code gives the inductor value in microhenries. The L or H prefix signifies whether the inductor is rated for a maximum E? LM Inductor Selection Guide www.

Greater ripple current causes higher peak switch currents and greater output ripple voltage; lower ripple current is achieved with larger-value inductors.

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. 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.

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.

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 Schott Corp. 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 First, calculate the maximum value for RC.

Select a capacitor with ESR, at 52 kHz, that is less than or equal to the lower value calculated. Also, be aware that ESR increases by a factor of 2 when operating at? F , and capacitors with high WVDC, or by paralleling smaller-value capacitors.

Select a resistor less than or equal to this value, and it should also be no greater than 3 k?. Calculate the minimum value for COUT using the following two equations. The larger of these two values is the minimum value that ensures stability. Calculate the minimum value of CC. Typical performance of this regulator is shown in Figure 14 and Figure The switching waveforms observed during the operation of this circuit are shown in Figure Input Capacitor Selection CIN The switching action in the step-up regulator causes a triangular ripple current to be drawn from the supply source.

This in turn causes noise to appear on the supply voltage. For proper operation of the LM, the input voltage should be decoupled. F capacitor leads as short as possible is normally sufficient. Aluminum Electrolytic Capacitors Recommended for Switching Regulators If the LM is located far from the supply source filter capacitors, an additional large electrolytic capacitor e.

F is often required. 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 LM 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 12 for recommended part numbers and voltage ratings of 1A and 3A diodes. AC-coupled B: Load current, 0. A Flyback regulator can produce single or multiple output voltages that are lower or greater than the input supply voltage.

Its operation is similar to a step-up regulator, except the output switch contols the primary current of a flyback transformer. Note that the primary and secondary windings are out of phase, so no current flows through secondary when current flows through the primary. This allows the primary to charge up the transformer core when the switch is on. When the switch turns off, the core discharges by sending current through the secondary, and this produces voltage at the outputs. The output voltages are controlled by adjusting the peak primary current, as described in the step-up regulator section.

Voltage and current waveforms for this circuit are shown in Figure 17, and formulas for calculating them are given in Figure Figure 20lists these transformers with the input voltage, output voltages and maximum load current they are designed for. The following procedure is for a dual output flyback regulator with equal turns ratios for each secondary i. Select a resistor less than or equal to this value, and no greater than 3 k?.

The larger of these two values must be used to ensure regulator stability. Flyback Regulator Formulas C. VOUT output capacitors in parallel. Diode Selection The switching diode in a flyback converter must withstand the reverse voltage specified by the following equation. This formula can also be used to calculate the maximum ESR of a single output regulator.





LM2577T-ADJ National Semiconductor, LM2577T-ADJ Datasheet


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