SP6830 datasheet(10/16 Pages) SIPEX

10 / 16 page

Rev. 9-22-00

SP6830/6831 Low Power Voltage Inverters With Shutdown

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 SP6830/6831 devices in designs.

Any internal losses come from the IC's on board

circuitry. Losses in the IC can be induced by the

input voltage, the frequency of the oscillator, and

the ambient temperature. The most influential

internal loss in the IC may be found in the power-

on resistance of the internal MOSFET switches.

Any of the losses with the charge pump capacitors

will be induced by the capacitor's ESR. The

affects 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 it 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 19.

Any losses due to the conversion process 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 - 2V

OUTVRIPPLE) ].

Actual Efficiency

To determine the actual efficiency of the SP6830/

6831 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 SP6830/

6831 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 does not involve 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 ,

Figure 19. Equivalent Circuit for an Ideal Switched

Capacitor

V+

VOUT

V+

VOUT

Requivalent =

f x C1

Requivalent

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