Switched-Capacitor Voltage Converters
MAX1044/ICL7660
switching noise and EMI may be generated. To reduce
these effects:
1) Power the MAX1044/ICL7600 from a low-impedance
source.
2) Add a power-supply bypass capacitor with low
effective series resistance (ESR) close to the IC
between the V+ and ground pins.
3) Shorten traces between the IC and the charge-pump
capacitors.
4) Arrange the components to keep the ground pins of
the capacitors and the IC as close as possible.
5) 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.
Figure 7. External Clocking
V+
10kΩ
REQUIRED
FOR TTL
CMOS or
V+ TTL GATE
1
2
10µF
3
4
8
MAX1044
ICL7660
7
6
5
V
OUT
=
-(V+)
10µF
a function of oscillator frequency and the capacitor
value. Oscillator frequency, in turn, is influenced by
temperature and supply voltage. For example, with a
5V input voltage and 10µF charge-pump capacitors,
the output resistance is typically 50Ω. Thus, the output
voltage is about -5V under light loads, and decreases
to about -4.5V with a 10mA load current.
Minor supply voltage variations that are inconsequential
to digital circuits can affect some analog circuits.
Therefore, when using the MAX1044/ICL7660 for
powering sensitive analog circuits, the power-supply
rejection ratio of those circuits must be considered.
The output ripple and output drop increase under
heavy loads. If necessary, the MAX1044/ICL7660 out-
put impedance can be reduced by paralleling devices,
increasing the capacitance of C1 and C2, or connect-
ing the MAX1044’s BOOST pin to V+ to increase the
oscillator frequency.
Efficiency, Output Ripple,
and Output Impedance
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:
∑
P
LOSS
= P
INTERNAL
+P
SWITCH
+P
PUMP
LOSSES
LOSSES
+P
CONVERSION
LOSSES
CAPACITOR
LOSSES
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 connec-
tions to the LV, BOOST, and OSC pins.
The next two losses are associated with the output
resistance 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:
P
PUMP
CAPACITOR
LOSSES
Inrush Current and EMI Considerations
During start-up, pump capacitors C1 and C2 must be
charged. Consequently, the MAX1044/ICL7660 devel-
op 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 converters are being powered by a high-
impedance source, the supply voltage may drop too
low during the current bursts for them to function prop-
erly. 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,
+
P
SWITCH
=
I
OUT
x R
OUT
LOSSES
2
where:
R
OUT
≅
1
+
(f
OSC
/ 2) x C1
4 2R
SWITCHES
+
ESR
C1
+
ESR
C2
(
)
and f
OSC
is the oscillator frequency.
8
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