Power Converter kHz 1A Step-Down Voltage Regulator General Description The LM series of regulators are monolithic integrated circuits that provide all the active functions for a step-down buck switching regulator, capable of driving a 1A load with excellent line and load regulation. These devices are available in fixed output voltages of 3. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation? The LM series operates at a switching frequency of kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 5-lead TO package with several different lead bend options, and a 5-lead TO surface mount package.
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Power Converter kHz 1A Step-Down Voltage Regulator General Description The LM series of regulators are monolithic integrated circuits that provide all the active functions for a step-down buck switching regulator, capable of driving a 1A load with excellent line and load regulation. These devices are available in fixed output voltages of 3. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation?
The LM series operates at a switching frequency of kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 5-lead TO package with several different lead bend options, and a 5-lead TO surface mount package.
Typically, for output voltages less than 12V, and ambient temperatures less than 50? C, no heat sink is required. A standard series of inductors are available from several different manufacturers optimized for use with the LM series. This feature greatly simplifies the design of switch-mode power supplies. External shutdown is included, featuring typically 85? A stand-by current. Self protection features include a two stage frequency reducing current limit for the output switch and an over temperature shutdown for complete protection under fault conditions.
Features n 3. A n High efficiency n Uses readily available standard inductors 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,, Infrared 10 sec. T Package Soldering, 10 sec. C, and those with boldface type apply over full Operating Temperature Range. A max mA mA max mA mA max? A max? 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 pF capacitor discharged through a 1. 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 limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control SQC methods. Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator system performance.
When the LM 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. Note Junction to ambient thermal resistance no external heat sink for the TO package mounted vertically, with the leads soldered to a printed circuit board with 1 oz.
Note Junction to ambient thermal resistance with the TO package tab soldered to a single printed circuit board with 0. Note Junction to ambient thermal resistance with the TO package tab soldered to a single sided printed circuit board with 2.
Note Junction to ambient thermal resistance with the TO package tab soldered to a double sided printed circuit board with 3 in2 of 1 oz. B: Inductor Current 0. Horizontal Time Base: 2? Horizontal Time Base: ? Rapidly switching currents associated with wiring inductance can generate voltage transients which can cause problems. For minimal inductance and ground loops, 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. See application section for more information. Inductor Selection L1 A. Select the correct inductor value selection guide from Figure 4 , Figure 5, or Figure 6.
Output voltages of 3. For all other voltages, see the design procedure for the adjustable version. From the inductor value selection guide, identify the inductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line.
Each region is identified by an inductance value and an inductor code LXX. F and ? F and low ESR solid tantalum capacitors between 56? F provide the best results. This capacitor should be located close to the IC using short capacitor leads and short copper traces. Do not use capacitors larger than ? For additional information, see section on output capacitors in application information section.
To simplify the capacitor selection procedure, refer to the quick design component selection table shown in Figure 2. This table contains different input voltages, output voltages, and load currents, and lists various inductors and output capacitors that will provide the best design solutions. The capacitor voltage rating for electrolytic capacitors should be at least 1. For computer aided design software, see Switchers Made Simple? Use the inductor selection guide for the 5V version shown in Figure 5.
From the inductor value selection guide shown in Figure 5, the inductance region intersected by the 12V horizontal line and the 1A vertical line is 68? H, and the inductor code is L The inductance value required is 68? From the table in Figure 8, go to the L30 line and choose an inductor part number from any of the four manufacturers shown.
In most instance, both through hole and surface mount inductors are available. See section on output capacitors in application information section. From the quick design component selection table shown in Figure 2, locate the 5V output voltage section.
In the load current column, choose the load current line that is closest to the current needed in your application, for this example, use the 1A line. In the maximum input voltage column, select the line that covers the input voltage needed in your application, in this example, use the 15V line. Continuing on this line are recommended inductors and capacitors that will provide the best overall performance.
The capacitor list contains both through hole electrolytic and surface mount tantalum capacitors from four different capacitor manufacturers. In this example aluminum electrolytic capacitors from several different manufacturers are available with the range of ESR numbers needed. For a 5V output, a capacitor voltage rating at least 7.
But, in this example, even a low ESR, switching grade, ? F 10V aluminum electrolytic capacitor would exhibit approximately m? This amount of ESR would result in relatively high output ripple voltage. A 16V or 25V capacitor will reduce the ripple voltage by approximately half. Procedure continued on next page. Example continued on next page.
Catch Diode Selection D1 A. The catch diode current rating must be at least 1. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM The most stressful condition for this diode is an overload or shorted output condition.
The reverse voltage rating of the diode should be at least 1. This diode must be fast short reverse recovery time and must be located close to the LM using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications.
Ultra-fast recovery, or High-Efficiency rectifiers also provide good results. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less.
Rectifiers such as the 1N series are much too slow and should not be used. Input Capacitor CIN A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin to prevent large voltage transients from appearing at the input.
This capacitor should be located close to the IC using short leads. In addition, the RMS current rating of the input capacitor should be selected to be at least 1? The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded.