Datasheet > AN1781

AN1781

更新时间： 2022-11-29 07:20:45

品牌	Logo	应用领域
美信 - MAXIM		/
页数	文件大小	规格书
4页	32K
描述
Graphics Chip Supply Delivers 1.65V at 26A

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AN1781 数据手册

POWER-SUPPLY CIRCUITS

Nov 11, 2002

Graphics Chip Supply Delivers 1.65V at 26A

Game boxes are using more powerful processors to provide more realistic

and complex graphics and games. As the processor power increases, so

does the current required to power the parts. The MAX1954 in the

application pictured below delivers 1.65V @ 26A from a 5V input.

Game boxes are using more powerful processors to provide more realistic and complex

graphics and games. As the processor power increases, so does the current required to power

the parts. The MAX1954 in the application pictured below delivers 1.65V @ 26A from a 5V input.

The MAX1954 is a 300kHz current-mode step-down controller. The MOSFET drivers are strong

enough to handle 100nC gate charge. Given the nominal input and output voltages, the duty

cycle is 33% and the on-resistance of the high- and low-side FETs are skewed accordingly. Two

IRF7811W with 12m on-resistance each are used for the high-side and two IRF7822 with

6.5m on-resistance each are used for the low-side. Two IRF7811W in parallel have better

switching characteristics than one IRF7822.

Seven 22uF/6V X5R ceramic capacitors are used for input bypassing to handle the 12.5Arms

input ripple current. At 5V input, ceramic capacitors are more cost effective and smaller than

electrolytics. In addition, the MAX1954 senses current by looking at the voltage across the high-

side FET on-resistance and ceramic bypass capacitors provide a better reference for the current-

sense circuitry. A 2.2nF ceramic capacitor is included drain-to-source across each low-side FET

to damp the parasitic ringing of the bond-wire inductance with the drain capacitance. This

ringing can affect the current-sense circuitry.

The pulse engineering inductor is a custom part designed to handle 70A peak current without

saturating. The MAX1954 uses a "valley" current limit based on the on-resistance of the low-side

FET. The current limit is 200mV divided by the low-side FET on-resistance (3.3m ) plus the

inductor ramp current (10A) which equals 70A. A 0.5uH/70A inductor is not readily available and

a custom design is required to provide short-circuit protection.

The Sanyo output capacitors provide a good balance among ESR, ripple current rating, and

cost. The capacitance is large enough for the output ripple to be dominated by the ESR. The

application does not require low-profile.

The catch-diode, D2, limits the negative voltage of the LX node when the high-side FET turns

off. In conjunction with R4, this controls the voltage on the boost capacitor, C8, at high-line and

12 3 4

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