AN1781 Datasheet, PDF (1/4 Pages) Maxim Integrated Products – Graphics Chip Supply Delivers 1.65V at 26A

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