SP6830 Datasheet, PDF (10/16 Pages) Sipex Corporation – Low Power Voltage Inverters With Shutdown

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 SP6830/6831 devices in designs.
Any internal losses come from the IC's on board
circuitry. Losses in the IC can be induced by the
input voltage, the frequency of the oscillator, and
the ambient temperature. The most influential
internal loss in the IC may be found in the power-
on resistance of the internal MOSFET switches.
Any of the losses with the charge pump capacitors
will be induced by the capacitor's ESR. The
affects 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 it the output current, R is the
OUT
OUT
circuit's output resistance, R
is the internal
SWITCHES
resistance of the MOSFET switches, ESR_C1 and
ESR_C2 are the ESR of their respective capacitors,
and f is the oscillator frequency. This term
OSC
with f_OSC is derived from an ideal switched-
capacitor circuit as seen in Figure 19.
Any losses due to the conversion process 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_{/2 x C2 x (VRIPPLE2 - 2VOUTVRIPPLE) ].}
Actual Efficiency
To determine the actual efficiency of the SP6830/
6831 device operation, a designer can use the
following equation:
Efficiency (actual) =
POUT
PIN
x 100% _,
where
P =V xI
OUT
OUT OUT
and
P_IN = V_IN x I_IN
where P_OUT is the power output, V_OUT is the
output voltage, I_OUT is the output current, P_IN is
the power from the supply driving the SP6830/
6831 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 does not involve 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) =
POUT
POUT(IDEAL)
x 100% _,
where
POUT(IDEAL)
=
-VIN x
-VIN
RL
,
f
V+
C1
VOUT
C2 RL
Requivalent
V+
Requivalent
=
f
1
x C1
VOUT
C2 RL
Figure 19. Equivalent Circuit for an Ideal Switched
Capacitor
Rev. 9-22-00
SP6830/6831 Low Power Voltage Inverters With Shutdown
10
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