MAX868EUB Просмотр технического описания (PDF) - Maxim Integrated

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MAX868EUB

Regulated, Adjustable -2x Inverting Charge Pump

Maxim Integrated 

Regulated, Adjustable -2x

Inverting Charge Pump

(a)

(a)

C2+

IN

IN

C2-

C1+

VOUT

C1-

COUT

(b)

(b)

C1+

C2+

IN

VOUT

COUT

GND

C1-

C2-

C2+

C2-

C1+

VOUT

C1-

COUT

C2+

C2-

C1+

C1-

VOUT

COUT

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 (COUT)

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

of operation.

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 (COUT). b) The equivalent circuit of the discharge

phase of operation.

__________________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:

R1 = R2 x | VOUT |

VREF

where VOUT is the desired output voltage, and VREF is

any available regulated positive voltage. When the

MAX868 is powered by a regulated voltage, VIN 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 VIN.

Alternatively, to configure the MAX868 as a simple,

unregulated doubler-inverter (VOUT = -2 x VIN), 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 (CC) 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.

6 _______________________________________________________________________________________

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