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
MAX14/ICL760
The firs t te rm is the e ffe c tive re s is ta nc e from the
switched-capacitor circuit.
oscillator to 400Hz by connecting a 100pF capacitor to
OSC reduces the quiescent current to about 15µA.
Maintaining 20mA output current capability requires
inc re a s ing the b uc ke t a nd re s e rvoir c a p a c itors to
100µF.
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:
Note that lower capacitor values can be used for lower
output currents. For example, setting the oscillator to
40Hz by connecting a 1000pF capacitor to OSC pro-
vides the highest efficiency possible. Leaving the bucket
and reservoir capacitors at 100µF gives a maximum
1
2
2
2
P
=
C1 (V+) − V
+
CONV.LOSS
OUT
1
2
2
C2
V
− 2V
V
x f
/ 2
RIPPLE
OUT RIPPLE
OSC
I
of 2mA, a no-load quiescent current of 10µA, and
OUT
a power conversion efficiency of 98%.
Increasing Efficiency
Efficiency can be improved by lowering output voltage
ripple and output impedance. Both output voltage rip-
ple and output impedance can be reduced by using
large capacitors with low ESR.
Ge n e ra l P re c a u t io n s
1) Connecting any input terminal to voltages greater
than V+ or less than ground may cause latchup. Do
not apply any input sources operating from external
supplies before device power-up.
The output voltage ripple can be calculated by noting
that the output current is supplied solely from capacitor
C2 during one-half of the charge-pump cycle.
2) Never exceed maximum supply voltage ratings.
3) Do not connect C1 and C2 with the wrong polarity.
1
4) Do not short V+ to ground for extended periods with
supply voltages above 5.5V present on other pins.
V
+ 2 x ESR
I
OUT
RIPPLE
C2
2 x f
x C2
OSC
5) Ensure that V
(pin 5) does not go more positive
OUT
Slowing the oscillator frequency reduces quiescent cur-
rent. The oscillator frequency can be reduced by con-
necting a capacitor to the OSC pin.
than GND (pin 3). Adding a diode in parallel with
C2, with the anode connected to V and cathode
to LV, will prevent this condition.
OUT
Reducing the oscillator frequency increases the ripple
volta g e in the MAX1044/ICL7660. Comp e nsa te b y
inc re a s ing the va lue s of the b uc ke t a nd re s e rvoir
capacitors. For example, in a negative voltage converter,
the pump frequency is around 4kHz or 5kHz. With the
recommended 10µF bucket and reservoir capacitors,
the circuit consumes about 70µA of quiescent current
while providing 20mA of output current. Setting the
________________Ap p lic a t io n Circ u it s
Ne g a t ive Vo lt a g e Co n ve rt e r
Figure 8 shows a negative voltage converter, the most
popular application of the MAX1044/ICL7660. Only two
external capacitors are needed. A third power-supply
bypass capacitor is recommended (0.1µF to 10µF)
V+
1
2
3
4
8
7
6
5
1
2
3
4
8
7
6
5
V+
BOOST
MAX1044
ICL7660
C
BYPASS
0.1µF
V
= 2(V+) - 2V
D
OUT
MAX1044
ICL7660
C1
10µF
C1
C2
LV
V
OUT
= -(V+)
C2
10µF
Figure 9. Voltage Doubler
Figure 8. Negative Voltage Converter with BOOST and LV
Connections
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
