_______________Detailed Description

The MAX828/MAX829 capacitive charge pumps invert the

voltage applied to their input. For highest performance,

use low equivalent series resistance (ESR) capacitors.

During the first half-cycle, switches S2 and S4 open,

switches S1 and S3 close, and capacitor C1 charges to

the voltage at IN (Figure 2). During the second half-

cycle, S1 and S3 open, S2 and S4 close, and C1 is level

allel with the reservoir capacitor C2. If the voltage across

C2 is smaller than the voltage across C1, then charge

flows from C1 to C2 until the voltage across C2 reaches -

load drains charge from C2.

Charge-Pump Output

The MAX828/MAX829 are not voltage regulators: the

charge pump’s output source resistance is approxi-

mately 20

droop toward GND as load current increases. The

er’s source resistance (RS-):

The negative output voltage will be:

Efficiency Considerations

The power efficiency of a switched-capacitor voltage

converter is affected by three factors: the internal loss-

es in the converter IC, the resistive losses of the pump

capacitors, and the conversion losses during charge

transfer between the capacitors. The total power loss is:

The internal losses are associated with the IC’s internal

functions, such as driving the switches, oscillator, etc.

These losses are affected by operating conditions such

as input voltage, temperature, and frequency.

The next two losses are associated with the voltage

converter circuit’s output resistance. Switch losses

occur because of the on-resistance of the MOSFET

switches in the IC. Charge-pump capacitor losses

occur because of their ESR. The relationship between

these losses and the output resistance is as follows:

the effective resistance from an ideal switched-

capacitor circuit. See Figures 3a and 3b.

ΣP

= P

+ P

+ P

+ P

LOSS

INTERNAL LOSSES

SWITCH LOSSES

PUMP CAPACITOR LOSSES

CONVERSION LOSSES

Switched-Capacitor Voltage Inverters

_______________________________________________________________________________________

5

S1

IN

S2

S3

S4

C1

C2

Figure 2. Ideal Voltage Inverter

V+

C1

f

C2

Figure 3a. Switched-Capacitor Model

1

V+

f

× C1

C2

Figure 3b. Equivalent Circuit

P

+ P

= I

x R

PUMP CAPACITOR LOSSES

CONVERSION LOSSES

OUT

2

OUT

R

f

x C

R

ESR

ESR

OUT

OSC

SWITCHES

C

C

+

+

(

1

1

4 2

1

2