SC70 Inverting Charge Pumps

with Shutdown

6

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Input Bypass Capacitor (C3)

If necessary, bypass the incoming supply to reduce its

AC impedance and the impact of the MAX1852/

MAX1853s’ switching noise. A bypass capacitor with a

value equal to that of C1 is recommended.

Voltage Inverter

The most common application for these devices is a

charge-pump voltage inverter (Figure 1). This applica-

tion requires only two external components—capacitors

C1 and C2—plus a bypass capacitor, if necessary.

Refer to the Capacitor Selection section for suggested

capacitor types.

Cascading Devices

Two devices can be cascaded to produce an even

larger negative voltage (Figure 4). The unloaded output

voltage is normally -2

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.

Paralleling Devices

Paralleling multiple MAX1852/MAX1853s reduces the

output resistance. Each device requires its own pump

capacitor (C1), but the reservoir capacitor (C2) serves

all devices (Figure 5). Increase C2’s value by a factor of

n, where n is the number of parallel devices. Figure 5

shows the equation for calculating output resistance.

Combined Doubler/Inverter

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

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

does not exceed 30mA.

Heavy Load Connected to a

Positive Supply

Under heavy loads, where a higher supply is sourcing

current into OUT, the OUT supply must not be pulled

above ground. Applications that sink heavy current into

OUT require a Schottky diode (1N5817) between GND

and OUT, with the anode connected to OUT (Figure 7).

Layout and Grounding

Good layout is important, primarily for good noise per-

formance. To ensure good layout, mount all compo-

nents as close together as possible, keep traces short

to minimize parasitic inductance and capacitance, and

use a ground plane.

V=

I

2(f

)C2

2 I

ESR

RIPPLE

OUT

OSC

OUT

C2

+×

×

S1

IN

S2

S3

S4

C1

C2

Figure 2. Ideal Voltage Inverter

V+

C1

C2

Figure 3a. Switched-Capacitor Model

1

V+

C2

Figure 3b. Equivalent Circuit