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MAX726ECK Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX726ECK Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page 5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators 10 ______________________________________________________________________________________ 0.2 µs would be needed to provide a narrow enough duty cycle that could control current when the output is shorted. Since 0.6 µs is too long (at 100kHz), the f OSC is lowered to 20kHz once FB (and hence the output) drops below about 1.3V (see Frequency vs. VFB Voltage graph in the Typical Operating Characteristics). This way, the MAX724/MAX726's 0.6 µs minimum t ON allows a sufficiently small duty cycle (at the reduced fOSC) so that current can still be limited. Compensation Network A series RC network connected from VC to ground compensates the MAX724/MAX726. Compensation RC values are shown in the applications circuits. RC and CC shape error-amplifier gain as follows: At DC, RC and CC have no effect, so the error-amplifier's gain is the product of its transconductance (approxi- mately 5000 µmhos) and an internal 400kΩ load impedance (rINT) at VC. So at DC, AV(DC) = gM(rINT) = approximately 2000 µmhos. R C and CC then add a low-frequency pole and a high-frequency zero, as shown in Figure 5. Output Overshoot The MAX724/MAX726 error-amplifier design minimizes overshoot, but precautions against overshoot should still be exercised in sensitive applications. Worst-case overshoot typically occurs when recovering from an output short because VC slews down from its highest voltage. This can be checked by simply shorting and releasing the output. Reduce objectional overshoot by increasing the com- pensation resistor (to 3k Ω or 4kΩ) at V C. This allows the error-amplifier output, VC, to move more rapidly in the negative direction. In some cases, loop stability may suffer with a high-value compensation resistor. An option, then, is to add output filter capacitance, which reduces short-circuit recovery overshoot by limiting out- put rise time. Lowering the compensation capacitor to below 0.05 µF may also help by allowing V C to slew fur- ther before the output rises too far. Optional Output Filters Though not shown in the application circuits in Figures 2, 7, and 8, additional filtering can easily be added to reduce output ripple to levels below 2%. It is more effective to add an LC type filter rather than additional output capacitance alone. A small-value inductor (2 µH to 10 µH) and between 47µF and 220µF of filter capaci- tance should suffice (Figure 6). Although the inductor does not need to be of high quality (it is not switching), it must still be rated for the full load current. When an LC filter is added, do not move the connection of the feedback resistor to the LC output. It should be left connected to the main output filter capacitor (C1 in Figure 2). If the feedback connection is moved to the LC filter point, the added phase shift may impact stability. LF CF TO LOAD FEEDBACK RESISTOR MAIN FILTER CAP Figure 6. Optional LC Output Filter FREQUENCY 90° PHASE SHIFT fPOLE = 1/[2π(400kΩ)]CC -AV(MID) = gM / (2π f CC) fZERO = 1 / (2π RC CC) AV(HI) = gMRC AV(DC) = gM(400kΩ) ≈ 2000 Figure 5. Error-Amplifier Gain as Set by RC and CC at VC Pin |
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