MAX1044/ICL7660
Switched-Capacitor Voltage Converters
+
1
C1
10µF
2
3
4
8
V+
C
BYPASS
V
OUT
= -(V+)
C2
10µF
+
1
2
10F
3
4
8
CONNECTION
FROM V+
TO BOOST
V+
MAX1044
ICL7660
7
6
5
MAX1044
7
6
5
C
OSC
V
OUT
= -(V+)
10F
*
*REQUIRED FOR V+ < 3.5V
Figure 5. Basic Negative Voltage Converter
Figure 6. Negative Voltage Converter with C
OSC
and BOOST
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.
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 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.
Figure 6 shows this connection. Higher frequency opera-
tion 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.
Output Voltage Considerations
Reducing the Oscillator Frequency Using C
OSC
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 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 sensi-
tive 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 output impedance can be reduced by
paralleling devices, increasing the capacitance of C1 and
C2, or connecting the MAX1044’s BOOST pin to V+ to
increase the oscillator frequency.
Maxim Integrated
│
8
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.
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