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MAX828SNTR

Switched Capacitor Voltage Converters

ON Semiconductor 

MAX828 MAX829

DETAILED OPERATING DESCRIPTION

The MAX828/829 charge pump converters invert the

voltage applied to the VIN pin. Conversion consists of a

two–phase operation (Figure 1). During the first phase,

switches S2 and S4 are open and S1 and S3 are closed.

During this time, C1 charges to the voltage on VIN and load

current is supplied from C2. During the second phase, S2

and S4 are closed, and S1 and S3 are open. This action

connects C1 across C2, restoring charge to C2.

S1

S2

IN

C1

MAX828/829

C2

S3

S4

Vout = –(Vin)

Figure 1. Ideal Switched Capacitor Charge Pump

APPLICATIONS INFORMATION

Output Voltage Considerations

The MAX828/829 perform voltage conversion but do not

provide regulation. The output voltage will drop in a linear

manner with respect to load current. The value of this

W equivalent output resistance is approximately 25 nominal

at +25°C and VIN = +5V. VOUT is approximately - 5V at light

loads, and droops according to the equation below:

(4) Losses that occur during charge transfer (from the

commutation capacitor to the output capacitor)

when a voltage difference between the two

capacitors exists.

Most of the conversion losses are due to factors (2), (3)

and (4) above. These losses are given by Equation 1.

+ ^ PLOSS(2,3,4) IOUT 2 ROUT IOUT 2

1

ƪ ) ) ) ƫ (fOSC)C1

8RSWITCH

4ESRC1

ESRC2

Equation 1.

The 1/(fOSC)(C1) term in Equation 1 is the effective output

resistance of an ideal switched capacitor circuit (Figures 2a,

2b).

The losses in the circuit due to factor (4) above are also

shown in Equation 2. The output voltage ripple is given by

Equation 3.

+ ƪ * ) PLOSS(4) (0.5)(C1)(VIN2 VOUT 2) (0.5)(C2)

ƪ * ƫ (VRIPPLE 2 2VOUT VRIPPLE) fOSC

Equation 2.

+ ) VRIPPLE

IOUT

(fOSC)(C2)

2(IOUT)(ESRC2)

Equation 3.

f

V+

Vout

C1

C2

RL

VDROP = IOUT x ROUT

VOUT = – (VIN – VDROP)

Charge Pump Efficiency

The overall power efficiency of the charge pump is

affected by four factors:

(1) Losses from power consumed by the internal

oscillator, switch drive, etc. (which vary with input

voltage, temperature and oscillator frequency).

(2) I2R losses due to the on–resistance of the MOSFET

switches on–board the charge pump.

(3) Charge pump capacitor losses due to effective

series resistance (ESR).

Figure 2a. Ideal Switched Capacitor Model

REQUIV

V+

+ REQUIV

1

f C1

Vout

C2

RL

Figure 2b. Equivalent Output Resistance

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