SP6828 Datasheet, PDF (11/17 Pages) Sipex Corporation – +3V Low Power Voltage Inverters

the charge pump capacitors, and P_CONV is the total
conversion loss during charge transfer between
the flying and output capacitors. These are the
three theoretical factors that may effect the power
efficiency of the SP6828/6829 devices in designs.
Internal losses come from the power dissipated
in the IC's internal circuitry.
Losses in the charge pump capacitors will be
induced by the capacitors' ESR. The effects of
the ESR losses and the output resistance can be
found in the following equation:
I2
OUT
x
R_OUT
=
P_CAP
+
P_CONV
and
R
OUT
≈
4
x
(2
x
R
SWITCHES
+
ESR )
C1
+
ESR_C2 +
1
fOSC x C1
,
where I_OUT is the output current, R_OUT is the
circuit's output resistance, R_SWITCHES is the internal
resistance of the MOSFET switches, ESR and
C1
ESR are the ESR of their respective capacitors,
C2
and f is the oscillator frequency. This term
OSC
with f_OSC is derived from an ideal switched-
capacitor circuit as seen in Figure 22.
Conversion losses will happen during the charge
transfer between the flying capacitor, C1, and
the output capacitor, C2, when there is a voltage
difference between them. P_CONV can be determined
by the following equation:
P_CONV = f_OSC x [ ¹/₂ x C1 x (V_IN2 - V_OUT2) +
1_{/ x C2 x (V} 2 _{- 2 x V x V ) ].}
2
RIPPLE
OUT
RIPPLE
Actual Efficiency
To determine the actual efficiency of the SP6828/
6829 device operation, a designer can use the
following equation:
Efficiency (ACTUAL) =
P_OUT
P_IN
x 100% _,
where
P_OUT = V_OUT x I_OUT
and
P_IN = V_IN x I_IN
where P is the power output, V is the
OUT
OUT
output voltage, I_OUT is the output current, P_IN is
the power from the supply driving the SP6828/
6829 devices, V_IN is the supply input voltage, and
I_IN is the supply input current.
Ideal Efficiency
The ideal efficiency is not the true power
efficiency because it is not calculated relative to
the input power which includes the input current
losses in the charge pump. The ideal efficiency
can be determined with the following equation:
Efficiency (IDEAL) =
P_OUT
P_OUT (IDEAL)
x 100% _,
where
P_OUT (IDEAL)
=
-V_IN x
-V_IN
R_L
,
and P_OUT is the measured power output. Both
efficiencies are provided to designers for
comparison.
f
V+
V_OUT
C1
C2
R_L
Requivalent
V+
Requivalent
=
f
1
x C1
V_OUT
C2
R_L
SP6828DS/11
Figure 22. Equivalent Circuit for an Ideal Switched
Capacitor
SP6828/6829 +3V Low Power Voltage Inverter
11
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