May 2005
Beginner to intermediate
398 pages
12h 12m
English
Content preview from Designing Embedded Hardware, 2nd Edition
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MAX724 Regulator
For higher-current situations, the MAX724 can supply up to 5 A from an input supply voltage between 8 V and 40 V. Its output is adjustable from 2.5 V to 35 V, and its quiescent current is 8.5 mA. It comes in a 5-pin TO-220 package, with an attachment point for an external heatsink. The basic circuit for a MAX724 is shown in Figure 5-8. Note that it is a step-down regulator only.
Figure 5-8. MAX724 circuit
The inductor is nominally 50 uH, but can be any value in the range of 5 uH to 200 uH. When the output pin VSW turns off, the diode provides a path to ground for the inductor current. The inductor chosen should have a high saturation current. If it doesn't, the effects will be disastrous. The Maxim datasheet provides detailed information on selecting an appropriate inductor.
It is recommended by Maxim that a Schottky diode, such as an MBR745, be used due to the fast switching times required. Both the regulator's input and output require large decoupling capacitors to filter out ripple. The capacitors must have low ESR (Equivalent Series Resistance) over the expected temperature range and operating lifetime.
The output voltage is set by resistors R1 and R2. The equation for calculating the output voltage is:
VOUT = 2.21 * (R1 + R2) / R2
R2 should be less than 4 kΩ. Maxim recommends choosing 2.21 kΩ for R2, as it simplifies the above equation:
VOUT = (R1 + 2.21k) / 1000 ...
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