Datasheet MAX1852, MAX1853 (Maxim) - 6

SC70 Inverting Charge Pumps with Shutdown
S1 S2 REQUIV IN V+ VOUT 1 C1 REQUIV = fOSC ✕ C1 C2 RL C2 S3 S4 VOUT = -(VIN) Figure 3b. Equivalent Circuit
Paralleling Devices
Paralleling multiple MAX1852/MAX1853s reduces the Figure 2. Ideal Voltage Inverter output resistance. Each device requires its own pump capacitor (C1), but the reservoir capacitor (C2) serves fOSC all devices (Figure 5). Increase C2’s value by a factor of n, where n is the number of parallel devices. Figure 5 V+ VOUT shows the equation for calculating output resistance.
MAX1852/MAX1853 Combined Doubler/Inverter
C2 R C1 L In the circuit of Figure 6, 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 Figure 3a. Switched-Capacitor Model part of the charge-pump circuit, loading either output causes both outputs to decline toward GND. Make sure the sum of the currents drawn from the two outputs IOUT does not exceed 30mA. V = + 2 ×I × ESR RIPPLE OUT C2 2(f )C2 OSC
Heavy Load Connected to a Positive Supply Input Bypass Capacitor (C3)
If necessary, bypass the incoming supply to reduce its Under heavy loads, where a higher supply is sourcing AC impedance and the impact of the MAX1852/ current into OUT, the OUT supply must not be pulled MAX1853s’ switching noise. A bypass capacitor with a above ground. Applications that sink heavy current into value equal to that of C1 is recommended. OUT require a Schottky diode (1N5817) between GND and OUT, with the anode connected to OUT (Figure 7).
Voltage Inverter Layout and Grounding
The most common application for these devices is a charge-pump voltage inverter (Figure 1). This applica- Good layout is important, primarily for good noise per- tion requires only two external components—capacitors formance. To ensure good layout, mount all compo- C1 and C2—plus a bypass capacitor, if necessary. nents as close together as possible, keep traces short Refer to the Capacitor Selection section for suggested to minimize parasitic inductance and capacitance, and capacitor types. use a ground plane.
Cascading Devices
Two devices can be cascaded to produce an even larger negative voltage (Figure 4). The unloaded output voltage is normally -2 ✕ VIN, but this is reduced slightly by the output resistance of the first device multiplied by the quiescent current of the second. When cascading more than two devices, the output resistance rises sig- nificantly. For applications requiring larger negative voltages, see the MAX865 and MAX868 data sheets.
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