LM26484 Power Management Unit
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The document introduces the LM26484 Power Management Unit which integrates 2 step-down DC/DC converters, 1 LDO regulator, and other features. It can supply digital cores and I/Os from a 3.0-5.5V input with externally adjustable outputs. The buck regulators use PWM control with phase shift between clocks and can operate from light to full loads with high efficiency. The LDO regulator has a low dropout voltage and can operate from 3.0-5.5V inputs with an externally adjustable output.
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LM26484 Power Management Unit
- 5. Application Circuit Comm DC Input Voltage Comm DC Input Voltage VOUT1 VOUT2 Comm DC Input Voltage PVIN LDO_OUT
- 7. Configuration of the LDO Regulator V OUT = V FB * (R 5 +R 6 ) / R 6 NFET
- 11. Configuration of the Bucks
Editor's Notes
- Welcome to the training module on National Semiconductor LM26484 Power Management Unit. This training module introduces LM26484 Power Management Unit and it key features.
- The LM26484 is a multi-function, configurable Power Management Unit. This device integrates two highly efficient 2.0A Step-Down DC/DC converters and one LDO Controller. All regulator output voltages are externally adjustable. The LDO controller is a low-voltage n-channel Metal-Oxide-Semiconductor NMOS voltage regulator. Because the LM26484 is externally configurable and has many voltage options, it is possible to change the feedback network to get a different regulator output voltage.
- The highly integrated and complete power management solution is specially designed for core and I/O supply, such as FPGAs, ASICs, and DSPs. The other applications include automotive infotainment, set-top-box, cordless phone base station, networking routers, and printers.
- Here shows the typical application circuit of the LM26484. The detailed explanation is introduced in the next few pages.
- The LDO is a linear regulator which targets analog loads characterized by low noise requirements. The LDO is the best regulator when the difference between input and output load (Iout) is low or the efficiency is a secondary concern compared to the output noise, ripple, cost and solution size. Drop out voltage is the minimum difference between unregulated input voltage and the regulated output voltage for which the regulator will operate within specifications. The lower dropout voltage is the reason LDO regulators dominate battery-powered applications, since they maximize the utilization of the available input voltage and can operate with higher efficiency.
- The output voltage of the LDO on the LM26484 is established by the feed back resistor divider R5 and R6 as shown. The LDO is enabled through the ENLDO pin, which enables LDO when it is a logic high. The LDO control loop will force the voltage on V FB to be 0.50V. Therefore, based on the equation given in the page it is able to calculate the resistor values. The table shows ideal resistor values to establish LDO voltages from 0.8V to 1.5V. A Feedback capacitor (C11) is required for stability; recommended values can be seen in the table. The output capacitor (C10) must meet the requirement for the minimum value of capacitance and also have an ESR value that is within the range 50 mΩ to 500 mΩ for stability. A 10.0μF ceramic capacitor is recommended for proper operation. when selecting an N –Field Effect Transistor for the LM26484 controller, it is important for the N –Field Effect Transistor to have a maximum power rating larger than the application will need.
- The Buck regulator is used to down-convert a DC voltage to a lower DC voltage. The LM26484 incorporates two high efficiency synchronous switching buck regulators which are 180 º out of phase, SW1 and SW2 that deliver voltages from a single DC input voltage. Using a voltage mode architecture with synchronous rectification, both bucks have the ability to deliver up to 2A depending on the input voltage and output voltage, and the inductor chosen. Both SW1 and SW2 can operate up to a 100% duty cycle for low drop out control of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.
- There are three modes of operation depending on the current required - PWM, PFM, and shutdown. PWM mode handles current loads of approximately 70mA or higher, delivering voltage precision with high efficiency. Lighter output current loads cause the device to automatically switch into PFM for reduced current consumption and a longer battery life. The Standby operating mode turns off the device, offering the lowest current consumption.
- When the converter is enabled, soft start is activated. The soft-start feature allows the power converter to gradually reach the initial steady state operating point, thus reducing start-up stresses and surges. The two LM26484 buck converters have a soft-start circuit that limits in-rush current during start-up or ramps up output voltage over 500μs. The two buck regulators can operate at 100% duty cycle for low dropout support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. When the device operates near 100% duty cycle, output voltage ripple is approximately 25 mV.
- The Buck control loop will force the voltage on VFB to be 0.50V. The table shows ideal resistor values to establish buck voltages from 0.8V to 3.5V. In addition to the resistor feedback, feedback capacitors (C3/C4/C6/C8) are also required. There are several design considerations related to the selection of output inductors and capacitors, including load transient response, stability, efficiency, output ripple voltage, and over-current ruggedness. The LM26484 has been optimized for use with nominal values 0.5 μH and 22 μF.
- Thank you for taking the time to view this presentation on “ LM26484 Power Management Unit” . 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.