SP829 datasheet(10/16 Pages) SIPEX

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SP828DS/03

SP828/829 High Efficiency Voltage Inverters

the charge pump capacitors, and P

CONV is the total

conversion loss during charge transfer between

the flying and output capacitors. These are the

three theoretical factors that may effect the power

efficiency of the SP828/829 devices in designs.

Internal losses come from the power dissipated

in the IC's internal circuitry.

Losses in the charge pump capacitors will be

induced by the capacitors' ESR. The effects of

the ESR losses and the output resistance can be

found in the following equation:

OUT

2 x R

OUT = PCAP + PCONV

and

OUT ≈ 4 x (2 x RSWITCHES + ESRC1) +

ESR

C2 +

fOSC x C1

where I

OUT is the output current, ROUT is the

circuit's output resistance, R

SWITCHES is the internal

resistance of the MOSFET switches, ESR

C1 and

ESR

C2 are the ESR of their respective capacitors,

and f

OSC is the oscillator frequency. This term

with f

OSC is derived from an ideal switched-

capacitor circuit as seen in Figure 22.

Conversion losses will happen during the charge

transfer between the flying capacitor, C1, and

the output capacitor, C2, when there is a voltage

difference between them. P

CONV can be determined

by the following equation:

CONV = fOSC x [

2 x C1 x (VIN

2 - V

OUT

2) +

2 x C2 x (VRIPPLE

2 - 2 x V

OUT x VRIPPLE) ].

Actual Efficiency

To determine the actual efficiency of the SP828/

829 device operation, a designer can use the

following equation:

Efficiency (ACTUAL) =

x 100%

POUT

where

OUT = VOUT x IOUT

and

IN = VIN x IIN

where P

OUT is the power output, VOUT is the

output voltage, I

OUT is the output current, PIN is

the power from the supply driving the SP828/

829 devices, V

IN is the supply input voltage, and

IN is the supply input current.

Ideal Efficiency

The ideal efficiency is not the true power

efficiency because it is not calculated relative to

the input power which includes the input current

losses in the charge pump. The ideal efficiency

can be determined with the following equation:

Efficiency (IDEAL) =

x 100%

POUT

POUT(IDEAL)

where

POUT(IDEAL) = -VIN x

-VIN

RL ,

and P

OUT is the measured power output. Both

efficiencies are provided to designers for

comparison.

Figure 22. Equivalent Circuit for an Ideal Switched

Capacitor

V+

VOUT

V+

VOUT

Requivalent =

f x C1

Requivalent

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