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SP828

High Efficiency Voltage Inverters

Signal Processing Technologies 

DESCRIPTION

The SP828/829 devices are CMOS Charge Pump

Voltage Converters that can be used to invert a

+1.5V to +5.5V input voltage. These devices are

ideal for designs involving battery-powered and

board level voltage conversion applications.

The typical operating frequency of the SP828 is

12kHz. The typical operating frequency of the

SP829 is 35kHz. The SP828 has a typical

operating current of 50µA and the SP829

operates at 130µA. Both devices can output

25mA with a voltage drop of 500mV. These

devices are ideal for both battery-powered and

board level voltage inverter applications

combining a low quiescent current with high

efficiency (<95% over most of its load-current

range).

THEORY OF OPERATION

The SP828/829 devices should theoretically

produce an inverted input voltage. In real world

applications, there are small voltage drops at the

output that reduce efficiency. The circuit of an

ideal voltage inverter can be found in Figure 21.

The voltage inverters require two external

capacitors to store the charge. A description of

the two phases follows:

Phase 1

In the first phase of the clock cycle, switches S2

and S4 are opened and S1 and S3 closed. This

connects the flying capacitor, C1, from V to

IN

ground. C1 charges up to the input voltage applied

at VIN.

Phase 2

In the second phase of the clock cycle, switches

S2 and S4 are closed and S1 and S3 are opened.

This connects the flying capacitor, C1, in parallel

with the output capacitor, C2. The charge stored

in C1 is now transferred to C2. Simultaneously,

the negative side of C2 is connected to V and

OUT

the positive side is connected to ground. With

the voltage across C2 smaller than the voltage

across C1, the charge flows from C1 to C2 until

the voltage at the VOUT equals -VIN.

VOUT = -VIN

VIN

S1

S2

C1

C2

S3

S4

VOUT

Figure 21. Circuit for an Ideal Voltage Inverter

Charge-Pump Output

The output of the SP828/829 devices is not

regulated and therefore is dependent on the

output resistance and the amount of load current.

As the load current increases, losses may slightly

increase at the output and the voltage may become

slightly more positive. The loss at the negative

output, VLOSS, equals the current draw, IOUT, from

V times the negative converter's source

OUT

resistance, RS:

V =I xR.

LOSS OUT

S

The actual inverted output voltage at V will

OUT

equal the inverted voltage difference of VIN and

VLOSS:

VOUT = -(VIN - VLOSS).

Efficiency

Theoretically, the total power loss of a switched

capacitor voltage converter can be summed up as

follows:

∑PLOSS

=

P

INT

+

P

CAP

+

P,

CONV

where PLOSS is the total power loss, PINT is the total

internal loss in the IC including any losses in the

MOSFET switches, P is the resistive loss of

CAP

SP828DS/03

SP828/829 High Efficiency Voltage Inverters

9

© Copyright 1999 Sipex Corporation

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