MAX1044/ICL7660
Switched-Capacitor Voltage Converters
V+
10kΩ
REQUIRED
FOR TTL
CMOS or
V+ TTL GATE
Efficiency, Output Ripple,
and Output Impedance
+
1
2
10µF
3
4
8
MAX1044
ICL7660
7
6
5
V
OUT
= -(V+)
10µF
The power efficiency of a switched-capacitor voltage
converter is affected by the internal losses in the con-
verter IC, resistive losses of the pump capacitors, and
conversion losses during charge transfer between the
capacitors. The total power loss is:
Σ P
LOSS
= P
INTERNAL
+
P
SWITCH
+
P
PUMP
LOSSES
LOSSES
CAPACITOR
LOSSES
+
P
CONVERSION
LOSSES
Figure 7. External Clocking
The internal losses are associated with the IC’s internal
functions such as driving the switches, oscillator, etc.
These losses are affected by operating conditions such as
input voltage, temperature, frequency, and connections to
the LV, BOOST, and OSC pins.
The next two losses are associated with the output resis-
tance of the voltage converter circuit. Switch losses occur
because of the on-resistances of the MOSFET switches
in the IC. Charge-pump capacitor losses occur because
of their ESR. The relationship between these losses and
the output resistance is as follows:
Inrush Current and EMI Considerations
During start-up, pump capacitors C1 and C2 must be
charged. Consequently, the MAX1044/ICL7660 develop
inrush currents during start-up. While operating, short
bursts of current are drawn from the supply to C1, and
then from C1 to C2 to replenish the charge drawn by the
load during each charge-pump cycle. If the voltage con-
verters are being powered by a high-impedance source,
the supply voltage may drop too low during the current
bursts for them to function properly. Furthermore, if the
supply or ground impedance is too high, or if the traces
between the converter IC and charge-pump capacitors
are long or have large loops, switching noise and EMI
may be generated. To reduce these effects:
1)
2)
Power the MAX1044/ICL7600 from a low-impedance
source.
Add a power-supply bypass capacitor with low
effective series resistance (ESR) close to the IC
between the V+ and ground pins.
Shorten traces between the IC and the charge-pump
capacitors.
Arrange the components to keep the ground pins of
the capacitors and the IC as close as possible.
Leave extra copper on the board around the voltage
converter as power and ground planes. This is easily
done on a double-sided PC board.
P
PUMP
CAPACITOR
LOSSES
+ P
SWITCH
= I
OUT
2
x R
OUT
LOSSES
where:
R
OUT
≅
1
+
(
f
OSC
/ 2
)
x C1
4( 2R
SWITCHES
+
E
SRC1
)
+
E
SRC2
and f
OSC
is the oscillator frequency.
The first term is the effective resistance from the switched-
capacitor circuit.
Conversion losses occur during the transfer of charge
between capacitors C1 and C2 when there is a voltage
difference between them. The power loss is:
1
2
2
2 C1 (V
+
)
−
V
OUT
+
P
CONV.LOSS
≅
1 C2 V
2
RIPPLE
−
2V
OUT
V
RIPPLE
2
3)
4)
5)
(
(
)
)
×
f
OSC
/2
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