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MAX653MJA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX653MJA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 12 page capacitor, even when the switch is off. This will usually not be a problem for a 5V output at room temperature, since the diode’s reverse leakage current and the feedback resistors’ current typically drain the excess. However, if the diode leakage is very low (which can occur at low temperatures and/or small output voltages), charge may build up on the output capacitor, making VOUT rise above its set point. If this happens, add a small load resistor (typically 1M Ω) to the output to pull a few extra microamps of current from the output capacitor. Layout Several of the external components in a MAX639/ MAX640/MAX653 circuit experience peak currents up to 600mA. Wherever one of these components con- nects to ground, there is a potential for ground bounce. Ground bounce occurs when high currents flow through the parasitic resistances of PC board traces. What one component interprets as ground can differ from the IC’s ground by several millivolts. This may increase the MAX639/MAX640/MAX653’s output ripple, since the error comparator (which is referenced to ground) will generate extra switching pulses when they are not needed. It is essential that the input filter capac- itor’s ground lead, the MAX639/MAX640/MAX653’s GND pin, the diode’s anode, and the output filter capacitor’s ground lead are as close together as possi- ble, preferably at the same point. Figure 5 shows a suggested through-hole printed circuit layout that mini- mizes ground bounce. Inverter Configuration Figure 6 shows the MAX639/MAX640/MAX653 in a floating ground configuration. By tying what would nor- mally be the output to the supply-voltage ground, the IC’s GND pin is forced to a regulated -5V (MAX639), -3.3V (MAX640), or -3V (MAX653). Avoid exceeding the maximum differential voltage of 11.5V from V+ to VOUT. Other negative voltages can be generated by placing a voltage divider across COUT and connecting the tap point to VFB in the same manner as the normal step- down configuration. Two AA Batteries to 5V, 3.3V, or 3V For battery-powered applications, where the signal ground does not have to correspond to the power-supply ground, the circuit in Figure 6 generates 5V (MAX639), 3.3V (MAX640), or 3V (MAX653) from a pair of AA batter- ies. Connect the VIN ground point to your system’s input, and connect the output to your system’s ground input. This configuration has the added advantage of reduced on resistance, since the IC’s internal power FET has VIN + VOUT of gate drive (Figures 7 and 8). 5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters ______________________________________________________________________________________ 11 GND MAX639 MAX640 MAX653 SHDN LX VOUT -5V -3.3V OR -3V 4 V+ VFB L = 100 µH 7 1 5 COUT 100 µF VIN CIN 100 µF 8 6 1N5817 Figure 6. Inverting Configuration 0 V+ (V) TA = +25°C L = 100 µH MAX639 0 20 40 60 80 100 160 1 2 3 4 5 120 140 Figure 7. Maximum Current Capability of Figure 6 Circuit 84.0 V+ (V) TA = +25°C VOUT = -5V L = 470 µH IOUT = 10mA 1.5 84.5 85.0 85.5 86.0 86.5 87.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Figure 8. Efficiency of Figure 6 Circuit |
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