SP829 datasheet(11/16 Pages) SIPEX

Part #	SP829

Description	High Efficiency Voltage Inverters

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Manufacturer	SIPEX [Sipex Corporation]

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11

SP828DS/03

SP828/829 High Efficiency Voltage Inverters

Negative Voltage Converter

The typical operating circuit for the SP828/829

devices is a negative voltage converter. Refer to

Figure 19. This circuit is used to obtain the

Typical Performance Characteristics found in

Figures 1 to 18 (unless otherwise noted).

Voltage Inverter with the Load from

V

OUT to VIN

A designer can find the most common application

for the SP828/829 devices in Figure 20 as a

voltage inverter. The only external components

needed are 3 capacitors: the flying capacitor, C1,

the output capacitor, C2, and the bypass capacitor,

C3 (if necessary).

Driving Excessive Loads

The output should never be pulled above ground.

A designer should implement a Schottky diode

(1N5817) from OUT to GND when driving

heavy loads where a higher supply is sourcing

current into OUT. Refer to Figure 23 for this

circuit connection.

APPLICATION INFORMATION

For the following applications, C1 = C2 = 10

µF

for the SP828 and C1 = C2 = 3.3

µF for the

SP829.

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 SP828/829 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:

R

OUT ≈ 4 x (2 x RSWITCHES + ESRC1) +

ESR

C2 +

1

fOSC x C1

,

where R

OUT is the circuit output resistance,

R

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 21.

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 SP828/829

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 pin will reduce

AC impedance and the impact of any of the

SP828/829 devices' 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.

When loading the SP828/829 devices from IN to

OUT, the input current remains constant

(disregarding any spikes due to internal

switching). Implementing a 0.1

µF bypass

capacitor should be sufficient.

When loading the SP828/829 devices from OUT

to GND, the current from the supply will flow

into the input for half of the cycle and will be zero

for the other half of the cycle. Designers should

implement a large bypass capacitor (C3 = C1)

if the supply has a high AC impedance.

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