Switched-Capacitor Voltage Converters
MAX1044/ICL7660
CONNECTION
FROM V+
TO BOOST
1
C1
10µF
2
3
4
8
V+
C
BYPASS
V
OUT
=
-(V+)
C2
10µF
1
2
10µF
3
4
*REQUIRED FOR V+ < 3.5V
6
V
OUT
=
-(V+)
5
10µF
8
V+
MAX1044
ICL7660
7
6
5
MAX1044
7
C
OSC
*
Figure 5. Basic Negative Voltage Converter
Figure 6. Negative Voltage Converter with C
OSC
and BOOST
and allow operation down to 1.5V. For low-voltage
operation and compatibility with the industry-standard
LTC1044 and ICL7660, the LV pin should be connect-
ed to ground for supply voltages below 3.5V and left
open for supply voltages above 3.5V.
The MAX1044’s LV pin can be grounded for all operat-
ing conditions. The advantage is improved low-voltage
performance and increased oscillator frequency. The
disadvantage is increased quiescent current and
reduced efficiency at higher supply voltages. For
Maxim’s ICL7660, the LV pin must be left open for
supply voltages above 5V.
When operating at low supply voltages with LV open,
connections to the LV, BOOST, and OSC pins should
be short or shielded to prevent EMI from causing
oscillator jitter.
Figure 6 shows this connection. Higher frequency oper-
ation lowers output impedance, reduces output ripple,
allows the use of smaller capacitors, and shifts switch-
ing noise out of the audio band. When the oscillator is
driven externally, BOOST has no effect and should be
left open. The BOOST pin should also be left open for
normal operation.
Reducing the Oscillator Frequency Using C
OSC
An external capacitor can be connected to the OSC pin
to lower the oscillator frequency (Figure 6). Lower
frequency operation improves efficiency at low load
currents by reducing the IC’s quiescent supply current.
It also increases output ripple and output impedance.
This can be offset by using larger values for C1 and C2.
Connections to the OSC pin should be short to prevent
stray capacitance from reducing the oscillator frequency.
Overdriving the OSC Pin with an External Oscillator
Driving OSC with an external oscillator is useful when
the frequency must be synchronized, or when higher
frequencies are required to reduce audio interference.
The MAX1044/ICL7660 can be driven up to 400kHz.
The pump and output ripple frequencies are one-half
the external clock frequency. Driving the
MAX1044/ICL7660 at a higher frequency increases the
ripple frequency and allows the use of smaller
capacitors. It also increases the quiescent current.
The OSC input threshold is V+ - 2.5V when V+
≥
5V,
and is V+ / 2 for V+ < 5V. If the external clock does not
swing all the way to V+, use a 10kΩ pull-up resistor
(Figure 7).
Oscillator Frequency Considerations
For normal operation, leave the BOOST and OSC pins
of the MAX1044/ICL7660 open and use the nominal
oscillator frequency. Increasing the frequency reduces
audio interference, output resistance, voltage ripple,
and required capacitor sizes. Decreasing frequency
reduces quiescent current and improves efficiency.
Oscillator Frequency Specifications
The MAX1044/ICL7660 do not have a precise oscillator
frequency. Only minimum values of 1kHz and 5kHz for
the MAX1044 and a typical value of 10kHz for the
ICL7660 are specified. If a specific oscillator frequency
is required, use an external oscillator to drive the OSC
pin.
Increasing Oscillator Frequency
Using the BOOST Pin
For the MAX1044, connecting the BOOST pin to the V+
pin raises the oscillator frequency by a factor of about 6.
Output Voltage Considerations
The MAX1044/ICL7660 output voltage is not regulated.
The output voltages will vary under load according to
the output resistance. The output resistance is primarily
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
