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The series consists of fixed output voltages of 3. A high fixed frequency oscillator KHz allows the use of physically smaller sized components. A family of standard inductors for use with the LM are available from several manufacturers to greatly simplify the design process.
Operating Ratings indicate conditions under which of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test condition, see the electrical Characteristics tables.
Note 2: ESD was applied using the human-body model, a pF capacitor discharged through a 1. Note 4: All limits are 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 standard Quality Control SQC methods.
Note 5: Junction to ambient thermal resistance no external heat sink for the 7 lead TO package mounted vertically, with 12 inch leads in a socket, or on a PC board with minimum copper area. Note 6: Junction to ambient thermal resistance no external heat sink for the 7 lead TO package mounted vertically, with 12 inch leads soldered to a PC board containing approximately 4 square inches of 1 oz.
Note 7: Junction to ambient thermal resistance for the 7 lead TO mounted horizontally against a PC board area of 0. Note 8: Junction to ambient thermal resistance for the 7 lead TO mounted horizontally against a PC board area of 0.
Note 9: Junction to ambient thermal resistance for the 7 lead TO mounted horizontally against a PC board copper area of 1. Additional copper area will reduce thermal resistance further.
See the thermal model in Switchers Made Simple software. Note Junction to ambient thermal resistance for the lead LLP mounted on a PC board copper area equal to the die attach paddle. Note Junction to ambient thermal resistance for the lead LLP mounted on a PC board copper area using 12 vias to a second layer of copper equal to die attach paddle. For layout recommendations, refer to Application Note AN A complete design uses a minimum number of external components, which have been predetermined from a variety of manufacturers.
The switch provides energy to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator PWM.
In addition to providing energy to the load the input voltage also provides bias for the internal circuitry of the LM For guaranteed performance the input voltage must be in the range of 8V to 40V.
For best performance of the power supply the input pin should always be bypassed with an input capacitor located close to pin 2. This minimizes conduction losses in the power switch to maintain high efficiency.
The recommended value for C Boost is 0. In fast-switching, high-current applications such as those implemented with the LM, it is recommended that a broad ground plane be used to minimize signal coupling throughout the circuit FEEDBACK This is the input to a two-stage high gain amplifier, which drives the PWM controller.
It is necessary to connect pin 6 to the actual output of the power supply to set the dc output voltage. For the fixed output devices 3. For the adjustable output version two external resistors are required to set the dc output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor flux to the feedback input. Connecting this pin to ground or to any voltage less than 0. The current drain from the input supply when OFF is only 50A.
Pin 7 has an internal pull-up current source of approximately 20A and a protection clamp zener diode of 7V to ground.
Basic circuit for fixed output voltage applications. Basic circuit for adjustable output voltage applications Power supply design using the LM is greatly simplified by using recommended external components. A wide range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in designs that cover the full range of capabilities input voltage, output voltage and load current of the LM A simple design procedure using nomographs and component tables provided in this data sheet leads to a working design with very little effort.
The individual components from the various manufacturers called out for use are still just a small sample of the vast array of components available in the industry. While these components are recommended, they are not exclusively the only components for use in a design. After a close comparison of component specifications, equivalent devices from other manufacturers could be substituted for use in an application.
Important considerations for each external component and an explanation of how the nomographs and selection tables were developed follows. For efficiency the inductor stores energy during the switch ON time and then transfers energy to the load while the switch is OFF. Nomographs are used to select the inductance value required for a given set of operating conditions.
The nomographs assume that the circuit is operating in continuous mode the current flowing through the inductor never falls to zero. The magnitude of inductance is selected to maintain a 11 www. The inductors offered have been specifically manufactured to provide proper operation under all operating conditions of input and output voltage and load current.
Several part types are offered for a given amount of inductance. Both surface mount and through-hole devices are available. The inductors from each of the three manufacturers have unique characteristics. Renco: ferrite stick core inductors; benefits are typically lowest cost and can withstand ripple and transient peak currents above the rated value. These inductors have an external magnetic field, which may generate EMI. Pulse Engineering: powdered iron toroid core inductors; these also can withstand higher than rated currents and, being toroid inductors, will have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest physical size inductors and are available only as surface mount components. These inductors also generate EMI but less than stick inductors. Selection of an output capacitor, with an associated equivalent series resistance ESR , impacts both the amount of output ripple voltage and stability of the control loop. The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current.
The capacitor types recommended in the tables were selected for having low ESR ratings. In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions. Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop.
In addition the capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency compensation circuitry of the LM modify the gain and phase shift of the closed loop system.
As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching frequency. With the fixed KHz switching frequency of the LM, the output capacitor is selected to provide a unity gain bandwidth of 40KHz maximum.
Each recommended capacitor value has been chosen to achieve this result. When parallel combinations of capacitors are required it has been assumed that each capacitor is the exact same part type.
The RMS current and working voltage WV ratings of the output capacitor are also important considerations. The capacitor RMS current rating must be www. The voltage rating of the output capacitor should be greater than 1. If operation of the system at elevated temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature rating.
An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations.
Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum dc load current so the capacitor should be rated to handle this.
Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating should also be selected to be 1. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation and should be considered when selecting an input capacitor. The input capacitor should be placed very close to the input pin of the LM Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry.
It may be necessary in some designs to add a small valued 0. The path for this current is through the diode connected between the switch output and ground. This forward biased diode clamps the switch output to a voltage less than ground.
This negative voltage must be greater than 1V so a low voltage drop particularly at high current levels Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode. The average current through the catch diode is dependent on the switch duty cycle D and is equal to the load current times 1-D.
Use of a diode rated for much higher current than is required by the actual application helps to minimize the voltage drop and power loss in the diode. During the switch ON time the diode will be reversed biased by the input voltage. The reverse voltage rating of the diode should be at least 1. This improves efficiency by minimizing the on resistance of the switch and associated power loss. For all applications it is recommended to use a 0. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to observe a large hysteresis in the current limit.
This can affect the output voltage of the device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself. Under current limiting conditions, the LMx is designed to respond in the following manner: 1.
At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately terminated. This happens for any application condition. Step 3: Determine the inductor required by using one of the four nomographs, Figure 3 through Figure 6. Table 1 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 3 fixed output voltage or Table 6 adjustable output voltage , determine the output capacitance required for stable operation. Table 2 provides the specific capacitor type from the manufacturer of choice. Step 5: Determine an input capacitor from Table 4 for fixed output voltage applications.
LM2676S-ADJ/NOPB National Semiconductor, LM2676S-ADJ/NOPB Datasheet
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