__________________Design Procedure

Setting the Output Voltage

Set the output voltage using two external resistors, R1

and R2, as shown in Figure 4. Since the input bias cur-

rent at FB has a 50nA maximum, large resistor values in

the feedback loop do not significantly degrade accura-

cy. Begin by selecting R2 in the 100k

Ω to 500kΩ range,

and calculate R1 using the following equation:

any available regulated positive voltage. When the

used as the reference for setting the output voltage.

When the MAX868 is powered by an unregulated sup-

ply, such as when operating directly from a battery, use

any available positive reference voltage in the system.

Note that due to the MAX868’s doubling and inverting

charge-pump action, the output voltage is limited to

Alternatively, to configure the MAX868 as a simple,

nect FB to IN. In this configuration, the MAX868 runs at

its maximum oscillator frequency, operating as a con-

ventional, open-loop charge pump.

If multiple oscillator cycles are required to regulate the

output, reduce the values for R1 and R2, or parallel a

feedback loop and ensure stability. Choose the lowest

capacitor value that ensures stability; values up to 47pF

are adequate for most applications.

R

R

x

V

V

OUT

REF

1

2

|

|

=

Regulated, Adjustable -2x

Inverting Charge Pump

6

_______________________________________________________________________________________

C2-

C1+

C1-

C2+

IN

IN

GND

(a)

(b)

C2-

C2+

C1-

C1+

Figure 2. a) In charge phase, the left-hand switches are

closed and the right-hand switches are open, charging the fly-

services the load. b) The equivalent circuit of the charge phase

of operation.

C2-

C1+

C1-

C2+

IN

(a)

(b)

C2+

C2-

C1+

C1-

Figure 3. a) In discharge phase, the left-hand switches are

open and the right-hand switches are closed, transferring

energy from the flying capacitors (C1 and C2) to the output

phase of operation.