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LM2688M-ADJ データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM2688M-ADJ データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 8 page Application Hints GENERAL This is a cost effective non-synchronous buck solution for powering Pentium and similar microprocessors (AMD’s K5, Motorola’s PowerPC, etc.) that draws large current at a low voltage. Fairly high efficiency of 86% or better at 10A load can be achieved by properly selecting the components. PIN FUNCTIONS +V IN— This is the positive input supply for the controller to work. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the cur- rent needed to drive the external MOSFET. Ground—Circuit ground. Output—Sources pulsed current up to 1.4A maximum to drive external MOSFET. To minimize turn-on delay of the ex- ternal MOSFET, copper trace between this pin and the gate of the MOSFET should be kept as short as possible. Feedback—Senses the regulator output voltage so that a feedback control loop is achieved. A resistor divider can be used to choose the desired output voltage. ON/OFF—Allows the controller to be shut down by a logic level signal. If the shutdown feature is not needed, the pin can be either grounded or left open, both of which enable the controller. COMPONENTS SELECTION Input Filter Depending on the system requirement, an inductor may or may not be necessary. When there is no explicit input di/dt limitation, the filter can be composed of capacitors only. The inductor plays an important role in limiting the input di/dt so that the input power rail appears a quiet supply for other loads. It also limits the inrush current during power on. The best inductance value is too complicated to calculate but can be easily determined by experiment. For a typical Pentium motherboard application, 2µH is recommended. The bulk ca- pacitors in the input filter should be of low ESR type or oth- erwise the input di/dt requirement may not be met. A low cost low ESR electrolytic capacitor manufactured by United Chemicon, LXV16VB102M, is recommended here. Output Filter The output filter plays an extremely important role in meeting the load transient requirement. From the viewpoint of load transient response, it is desirable to have a low inductance and a high output capacitance. However, too low an induc- tance causes huge ripple current through the MOSFETs which translates into lower efficiency. Also too big a bank of output capacitors may not be economical in terms of both cost of the regulator and motherboard real estate. It may also require a larger input capacitor bank to stay within input di/dt specification. The output voltage ripple is determined by the amount of ripple current in the inductor and the ESR of the output capacitor bank. For the typical situation where Pentium processor is the load, a 2.5µH inductor and a bank of three 1500µF electrolytic capacitors are recommended. Sanyo’s ultra low-ESR electrolytic capacitor, the 6MV1500GX, is recommended. MOSFET Selection It is desirable to have the on-resistance of the MOSFET as low as possible so that its conduction loss is minimized and high efficiency can be maintained. A good idea is to use two low-R dson MOSFETs in parallel so that the total conduction loss is halved compared with using one MOSFET. In the typi- cal application circuit, two Fairchild low R dson (10 mΩ) MOS- FETs, the NDS8410A, are used in parallel. Diode Selection Since current flows through the freewheeling diode when the MOSFETs are off, efficiency is affected greatly by the for- ward voltage drop of the diode. Generally a Schottky diode is used here. Motorola MBR2515L Schottky diode has a for- ward voltage drop of less than 0.4V at 20A which fits very well in this application. Output Voltage Adjustment Different output voltages can be obtained by using different combinations of feedback resistors. The formula to calculate output voltage is VCORE=(R2/R3+1)X1.235V. In the “typical application” circuit, two jumpers are used to adjust the value of R2 so that four output voltages can be obtained. When necessary, C9 and C10 can be used to further compensate the converter to result in a more stable circuit, the penalty is a slower transient recovery speed. Bill of Material Label Value Type Part Number Manufacturer C1 1000µ Capacitor LXV16VB102M United Chemi-Con C2 1000µ Capacitor LXV16VB102M United Chemi-Con C3 1000µ Capacitor LXV16VB102M United Chemi-Con C4 10µ Capacitor, Tantalum C5 0.1µ Capacitor, Ceramic C6 1500µ Capacitor 6MV1500GX Sanyo C7 1500µ Capacitor 6MV1500GX Sanyo C8 1500µ Capacitor 6MV1500GX Sanyo C9 1n Capacitor, Ceramic C10 15n Capacitor, Ceramic L1 2.5µ Inductor R1 300, 0.5W Resistor R2 3.65k 1% Resistor R3 2k 1% Resistor 5 www.national.com |
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