SP6832 datasheet(10/15 Pages) SIPEX

10 / 15 page

SP6832DS/04

SP6832 High Speed, High Efficiency Voltage Inverter

Negative Voltage Converter

The typical operating circuit for the SP6832

devices is a negative voltage converter. Refer to

Figure 14. This circuit is used to obtain the

Typical Performance Characteristics found in

Figures 1 to 13 (unless otherwise noted).

Voltage Inverter with the Load from

OUT to VIN

A designer can find the most common application

for the SP6832 devices in Figure 15 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 18 for this

circuit connection.

APPLICATION INFORMATION

For the following applications, C1 = C2 = 0.1

µF

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 SP6832 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 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 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 SP6832

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 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:

RIPPLE = 2 x IOUT x ESRC2 +

OUT

OSC x C2

Input Bypass Capacitor

The bypass capacitor at the input pin will reduce

AC impedance and the impact of any of

the SP6832 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 SP6832 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 SP6832 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

if the supply has a high AC impedance.

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