SP6830 datasheet(11/16 Pages) SIPEX

11 / 16 page

Rev. 9-22-00

SP6830/6831 Low Power Voltage Inverters With Shutdown

and P

OUT is the the power output. Both efficiencies

are provided to designers for comparison.

APPLICATION INFORMATION

For the following applications, C1 = C2 = C3 =

3.3

µF for the SP6830 and C1 = C2 = C3 = 1.0µF

for the SP6831.

Capacitor Selection

Low ESR capacitors are needed to obtain low

output resistance. Refer to Table 1 for some

suggested low ESR capacitors.

The output

resistance of the SP6830/6831 devices is a

function of the ESR of C1 and C2. This output

resistance can be determined by the equation

previously provided in the Efficiency section:

OUT ≈ 4 x (2 x RSWITCHES + ESRC1) +

ESR

C2 +

fOSC x C1

where R

OUT is the circuit's output resistance,

SWITCHES is the internal resistance of the MOSFET

switches, ESR

C1 and ESRC2 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.

Minimizing the ESR of C1 and C2 will minimize

the total output resistance and will improve the

efficiency.

Flying Capacitor

Decreasing flying capacitor, C1, values will

increase the output resistance of the SP6830/

6831 devices while increasing C1 will reduce the

output resistance.

There is a point where

increasing C1 will have a negligible effect on the

output resistance due to the the domination of the

output resistance by the internal MOSFET switch

resistance and the total capacitor ESR.

Output Capacitor

Increasing output capacitor, C2, values will

decrease the output ripple voltage. Reducing the

ESR of C2 will reduce both output ripple voltage

and output resistance. If higher output ripple can

be tolerated in designs, smaller capacitance values

for C2 should be used with light loads. The

following equation can be used to calculate the

peak-to-peak ripple voltage:

VRIPPLE = 2 x IOUT x ESRC2 +

IOUT

fOSC x C2 .

Input Bypass Capacitor

The bypass capacitor at the input voltage will

reduce AC impedance and the impact of any of

the SP6830/6831 device's switching noise. It is

recommended that for heavy loads a bypass

capacitor approximately equal to the flying

capacitor, C1, be used. For light loads, the value

of the bypass capacitor can be reduced.

Table 1. Suggested Low ESR Surface Mount Capacitors

Ω

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