Rainbow-electronics MAX868 Manual de usuario descargar pdf (Pagina 6)

MAX868

__________________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 100kΩ to 500kΩ range,

and calculate R1 using the following equation:

where V

OUT

is the desired output voltage, and V

REF

any available regulated positive voltage. When the

MAX868 is powered by a regulated voltage, V

can be

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

-2 x V

Alternatively, to configure the MAX868 as a simple,

unregulated doubler-inverter (V

OUT

= -2 x V

), con-

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

small capacitor (C

) across R1 to compensate the

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

OUT

REF

1 2

| |

Regulated, Adjustable -2x

Inverting Charge Pump

6 _______________________________________________________________________________________

OUT

C2-

C1+

C1-

C2+

GND

(a)

(b)

OUT

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-

ing capacitors (C1 and C2) while the output capacitor (C

OUT

)

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

of operation.

C2-

C1+

C1-

C2+

(a)

(b)

OUT

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

capacitor (C

OUT

). b) The equivalent circuit of the discharge

phase of operation.

1 2 3 4 5 6 7 8 9 10 11 12

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Rainbow-electronics MAX868 Manual de usuario Pagina 6

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