S w it c h e d -Ca p a c it o r Vo lt a g e Co n ve rt e rs
switching noise and EMI may be generated. To reduce
these effects:
10kΩ
REQUIRED
FOR TTL
V+
1) Power the MAX1044/ICL7600 from a low-impedance
source.
2) Ad d a p owe r-s up p ly b yp a s s c a p a c itor with low
effective series resistance (ESR) close to the IC
between the V+ and ground pins.
CMOS or
TTL GATE
1
2
3
4
8
7
6
5
V+
MAX1044
ICL7660
3) Shorten traces between the IC and the charge-pump
capacitors.
10µF
4) Arrange the components to keep the ground pins of
the capacitors and the IC as close as possible.
V
= -(V+)
OUT
10µF
5) Leave extra copper on the board around the voltage
c onve rte r a s p owe r a nd g round p la ne s . This is
easily done on a double-sided PC board.
Figure 7. External Clocking
Effic ie n c y, Ou t p u t Rip p le ,
a n d Ou t p u t Im p e d a n c e
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:
MAX14/ICL760
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.
∑ P
= PINTERNAL +P
+P
PUMP
+P
CONVERSION
LOSSES
LOSS
SWITCH
LOSSES
Minor supply voltage variations that are inconsequential
to d ig ita l c irc uits c a n a ffe c t s ome a na log c irc uits .
The re fore , whe n us ing the MAX1044/ICL7660 for
powering sensitive analog circuits, the power-supply
rejection ratio of those circuits must be considered.
The outp ut rip p le a nd outp ut d rop inc re a s e und e r
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.
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
s witc he s in the IC. Cha rg e -p ump c a p a c itor los s e s
occur because of their ESR. The relationship between
these losses and the output resistance is as follows:
In ru s h Cu rre n t a n d EMI Co n s id e ra t io n s
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
volta g e c onve rte rs a re b e ing p owe re d b y a hig h-
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,
2
PPUMP
CAPACITOR
LOSSES
+ PSWITCH = I
LOSSES
x R
OUT
OUT
where:
1
R
+
OUT
(f
/ 2) x C1
OSC
4 2R
(
+ ESR
+ ESR
)
C1 C2
SWITCHES
and f
is the oscillator frequency.
OSC
8
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
