LM2267X: Step-Down Voltage Regulator
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To study the functionalities and application of LM2267X buck regulators
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LM2267X: Step-Down Voltage Regulator
- 6. Internal Block Diagram LM22670 Block Diagram
- 14. Current Flow in Buck Application
Hinweis der Redaktion
- Welcome to the training module on National Semiconductor LM2267X: Step-Down Voltage Regulator. The intent of this training module is to study the functionalities and application of LM2267X buck regulators.
- From the figure, we can see that the output voltage is stepped down by modulating the duty cycle of the MOSFET, Q1. When the MOSFET turns on Vin is applied to the LC output filter. The transfer function of this regulator is Vout = D x Vin, where D is the duty cycle of the modulating switch, Q1. The diode D1 provides a path for the inductor current to flow when Q1 is turned off. Each cycle the inductor current ramps up while Q1 is on and then ramps down while the diode is conducting. The MOSFET and diode currents sum to form the inductor current.
- The SIMPLE SWITCHER ® concept provides for an easy to use complete design using a minimum number of external components and National’s WEBENCH ® design tool. National’s WEBENCH ® tool includes features such as external component calculation, electrical simulation, thermal simulation, and Build-It boards for easy design-in. The switching clock frequency is provided by an internal fixed frequency oscillator which operates at 500 kHz. The switching frequency can also be adjusted with an external resistor or synchronized to an external clock up to 1MHz. It is also possible to self-synchronize multiple regulators to share the same switching frequency.
- The LM2267x Simple Switcher Step-Down Regulator Series is the next generation of highly popular LM267x Simple Switcher Regulators. Numerous enhancements and improvements have been added such as an input range down to 4.5V, tighter output accuracy, adjustable switching frequency, a precision enable pin, and thermally enhanced green packages. These new Simple Switcher Regulators together with WEBENCH Power Designer provide fast and easy design-in while delivering performance and flexibility to meet your design and application requirements.
- The LM2267x switching regulators feature all of the functions necessary to implement an efficient high voltage buck regulator using a minimum of external components. This integrates a 42V N-channel switch with an output current of up to 5A. The regulator control method is based on voltage mode control with input voltage feed forward. The loop compensation is integrated into the LM2267x so that no external compensation components need to be selected or utilized. The operating frequency is fixed at 500Khz to allow for small external components while avoiding excessive switching losses. The switching frequency can be adjusted with an external resistor from 200Khz to 1Mhz or it can be synchronized to an external clock up to 1Mhz.
- The table provides a quick selection guide for the LM2267X family of SIMPLE SWITCHER regulators.
- The precision Enable pin can be used to shut down the power supply. Connecting this pin to GND or to a voltage less than typical 1.6V will completely turn-off the regulator. The enable pin has an internal pull-up current source of approximately 6 µ A. When driving the enable pin, the high voltage level for the on condition should not exceed the 6V limit. When enable control is not required, the EN pin should be left floating. The precision feature enables simple sequencing of multiple power suppliers with a resistor divider from another power supply.
- When the power switch turns on, the slight capacitance loading of Schottky diode (D1) causes a leading-edge current spike with an extended ringing period. This spike can cause the current limit comparator to trip prematurely. When the switch current reaches the current limit threshold, the switch is immediately turned off and the internal switching frequency is reduced. This extends the off time of the switch to prevent a steady state high current condition. As the switch current falls below the current limit threshold, the switch current will attempt to turn on.
- The soft Start feature allows the regulator to gradually reach the initial steady state operating point and reduces start-up stresses and surges. The soft-start can be adjust by an external soft-start capacitor. If no external soft-start capacitor is used, there is an internal soft-start feature with 500 µ s typically. There are 3 different modes for the RT/SYNC pin. It can be left floating for a 500 KHz switching frequency. A resistor from the RT/SYNC pin to GND can be used to adjust the switching frequency between 200Khz and 1Mhz. An external sync pulse can be applied to RT/SYNC pin for switching frequencies up to 1Mhz. The LM2267x integrates an N-Channel FET switch and associated floating high voltage level shift/gate driver. This gate works along with internal Diode and external bootstrap capicator (0.01uF ceramic cap).
- Internal thermal shutdown circuit protects the LM2267x in the event the maximum junction temperature is exceeded. When activated, typically at 150 ° C, the regulator is forced into a low power reset state.
- The typical max duty-cycle is 85% at 500Khz switching frequency. This corresponds to a typical minimum off-time of 300ns. When operating at switching frequencies higher than 500Khz, the 300ns minimum off-time results in a lower maximum duty-cycle limit than 85%. Besides a minimum off-time, there is also Minimum on-time which will effect when the output voltage is adjusted very low and the input voltage is very high.
- Inductor(L1): its value is determined based on the load current, ripple current and min/max input voltage; Input capacitor(c1): this is used to Limit the ripple voltage at the Vin pin while supplying most of the switch current during on-time; Output Capacitor(c4):this limits the output ripple voltage and provide a source of charge for transient loading condition; boot Capacitor(C3): the bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the N-channel MOSFET; Resistor Divider: for –ADJ option, no resistor divider is required for 1.285V output. For other output voltages –ADJ option can be used with resistor divider; Catch Diode(D1): a Schottky diode which has ideal reverse recovery character and low forward voltage drop is required.
- The most important PCB layout rule is to keep the AC current loops as small as possible. The schematic shows the current flow of a buck converter. The top schematic shows a dotted line which represents the current flow during the FET switch on-state. The middle one shows the current flow during the FET switch off-state. The bottom one shows the currents referred to as AC current. These currents are the most critical since it is changing in very short time periods. The dotted line in the schematic are the traces to keep as short as possible this will yield a small loop area reducing the loop inductance.
- Thank you for taking the time to view this presentation on LM2267 Buck Regulators. If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the National Semiconductor site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.