SP6832 Datasheet, PDF (10/15 Pages) Sipex Corporation – High Speed, High Efficiency Voltage Inverter

APPLICATION INFORMATION
For the following applications, C1 = C2 = 0.1µF
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:
R_OUT ≈ 4 x (2 x R_SWITCHES + ESR_C1) +
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 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 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:
V_RIPPLE = 2 x I_OUT x ESR_C2 +
I_OUT
f_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 0.1µF 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.
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
V_OUT to V_IN
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.
SP6832DS/04
SP6832 High Speed, High Efficiency Voltage Inverter
10
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