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MAX861MJA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX861MJA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 11 page MAX860/MAX861 50mA, Frequency-Selectable, Switched-Capacitor Voltage Converters 8 Maxim Integrated MAX860 MAX861 “n” MAX860 MAX861 “1” 8 7 5 VOUT C2 8 7 +VIN C1 C1 22 33 44 5 VOUT = -VIN ROUT = ROUT OF SINGLE DEVICE NUMBER OF DEVICES … … Figure 2. Paralleling MAX860s or MAX861s to Reduce Output Resistance MAX860 MAX861 “n” MAX860 MAX861 “1” 8 7 5 VOUT C2 8 7 +VIN C1 C2 C1 22 33 44 5 VOUT = -nVIN … … Figure 1. Cascading MAX860s or MAX861s to Increase Output Voltage Flying Capacitor, C1 Increasing the size of the flying capacitor reduces the output resistance. Output Capacitor, C2 Increasing the size of the output capacitor reduces the output ripple voltage. Decreasing its ESR reduces both output resistance and ripple. Smaller capacitance val- ues can be used if one of the higher switching frequen- cies is selected, if less than the maximum rated output current (50mA) is required, or if higher ripple can be tolerated. The following equation for peak-to-peak rip- ple applies to both the inverter and doubler circuits. IOUT VRIPPLE = ———————— + 2 x IOUT x ESRC2 2 x fS x C2 Bypass Capacitor Bypass the incoming supply to reduce its AC impedance and the impact of the MAX860/MAX861’s switching noise. The recommended bypassing depends on the cir- cuit configuration and where the load is connected. When the inverter is loaded from OUT to GND or the doubler is loaded from VDD to GND, current from the supply switches between 2 x IOUT and zero. Therefore, use a large bypass capacitor (e.g., equal to the value of C1) if the supply has a high AC impedance. When the inverter and doubler are loaded from VDD to OUT, the circuit draws 2 x IOUT constantly, except for short switching spikes. A 0.1µF bypass capacitor is sufficient. Cascading Devices Two devices can be cascaded to produce an even larger negative voltage, as shown in Figure 1. The unloaded output voltage is nominally -2 x VIN, but this is reduced slightly by the output resistance of the first device multiplied by the quiescent current of the sec- ond. The output resistance of the complete circuit is approximately five times the output resistance of a sin- gle MAX860/MAX861. Three or more devices can be cascaded in this way, but output resistance rises dramatically, and a better solution is offered by inductive switching regulators (such as the MAX755, MAX759, MAX764, or MAX774). Connect LV as with a standard inverter circuit (see Pin Description). The maximum load current and startup current of nth cascaded circuit must not exceed the maximum output current capability of (n-1)th circuit to ensure proper startup. Paralleling Devices Paralleling multiple MAX860s or MAX861s reduces the output resistance. As illustrated in Figure 2, each device requires its own pump capacitor (C1), but the reservoir capacitor (C2) serves all devices. C2’s value should be increased by a factor of n, where n is the number of devices. Figure 2 shows the equation for cal- culating output resistance. An alternative solution is to use the MAX660 or MAX665, which are capable of sup- plying up to 100mA of load current. Connect LV as with a standard inverter circuit (see Pin Description). Combined Doubler/Inverter In the circuit of Figure 3, capacitors C1 and C2 form the inverter, while C3 and C4 form the doubler. C1 and C3 are the pump capacitors; C2 and C4 are the reservoir capacitors. Because both the inverter and doubler use part of the charge-pump circuit, loading either output causes both outputs to decline towards GND. Make |
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