SP6828 Datasheet, PDF (12/17 Pages) Sipex Corporation – +3V Low Power Voltage Inverters

APPLICATION INFORMATION
For the following applications, C1 = C2 = 10µF
for the SP6828 and C1 = C2 = 3.3µF for the
SP6829.
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 SP6828/6829 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
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 is the oscillator
OSC
frequency. This term with f is derived from an
OSC
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 SP6828/
6829 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:
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
SP6828/6829 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 SP6828/6829 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 SP6828/6829 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.
Negative Voltage Converter
The typical operating circuit for the SP6828/
6829 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 V_IN
A designer can find the most common application
for the SP6828/6829 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.
SP6828DS/11
SP6828/6829 +3V Low Power Voltage Inverter
12
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