ADM660/ADM8660
Inverting Negative Voltage Generator
Table II. ADM8660 Charge-Pump Frequency Selection
Figures 19 and 20 show the ADM660/ADM8660 configured to
generate a negative output voltage. Input supply voltages from
1.5 V up to 7 V are allowable. For supply voltage less than 3 V,
LV must be connected to GND. This bypasses the internal
regulator circuitry and gives best performance in low voltage
applications. With supply voltages greater than 3 V, LV may
be either connected to GND or left open. Leaving it open facili-
tates direct substitution for the ICL7660.
+1.5V TO +7V
INPUT
FC
CAP+
C1
10µF
GND
CAP–
FC
GND
V+
GND or V+
GND
OSC
Open
Open
Ext Cap
Ext CLK
Charge Pump
C1, C2
25 kHz
10
µF
120 kHz
2.2
µF
See Typical Characteristics
Ext CLK Frequency/2
+1.5V TO +7V
INPUT
CLK OSC
ADM660
V+
OSC
LV
OUT
C2
10µF
INVERTED
NEGATIVE
OUTPUT
C1
FC
CAP+
GND
CAP–
ADM660
ADM8660
V+
OSC
LV
OUT
CMOS GATE
C2
INVERTED
NEGATIVE
OUTPUT
Figure 19. ADM660 Voltage Inverter Configuration
+1.5V TO +7V
INPUT
FC
CAP+
C1
10µF
SHUTDOWN
CONTROL
GND
CAP–
SD
LV
OUT
C2
10µF
INVERTED
NEGATIVE
OUTPUT
V+
Figure 21. ADM660/ADM8660 External Oscillator
Voltage Doubling Configuration
ADM8660
Figure 22 shows the ADM660 configured to generate increased
output voltages. As in the inverting mode, only two external ca-
pacitors are required. The doubling function is achieved by re-
versing some connections to the device. The input voltage is
applied to the GND pin and V+ is used as the output. Input
voltages from 2.5 V to 7 V are allowable. In this configuration,
pins LV, OUT must be connected to GND.
The unloaded output voltage in this configuration is 2 (V
IN
).
Output resistance and ripple are similar to the voltage inverting
configuration.
Note that the ADM8660 cannot be used in the voltage
doubling configuration.
FC
CAP+
+2.5V
TO +7V
INPUT
10µF
GND
CAP–
LV
OUT
Figure 20. ADM8660 Voltage Inverter Configuration
OSCILLATOR FREQUENCY
The internal charge-pump frequency may be selected to be
either 25 kHz or 120 kHz using the Frequency Control (FC)
input. With FC unconnected (ADM660) or connected to GND
(ADM8660), the internal charge pump runs at 25 kHz while, if
FC is connected to V+, the frequency is increased by a factor of
five. Increasing the frequency allows smaller capacitors to be
used for equivalent performance or, if the capacitor size is un-
changed, it results in lower output impedance and ripple.
If a charge-pump frequency other than the two fixed values is
desired, this is made possible by the OSC input, which can ei-
ther have a capacitor connected to it or be overdriven by an
external clock. Please refer to the Typical Performance Charac-
teristics, which shows the variation in charge-pump frequency
versus capacitor size. The charge-pump frequency is one-half
the oscillator frequency applied to the OSC pin.
If an external clock is used to overdrive the oscillator, its levels
should swing to within 100 mV of V+ and GND. A CMOS
driver is, therefore, suitable. When OSC is overdriven, FC has
no effect but LV must be grounded.
Note that overdriving is permitted only in the voltage
inverter configuration.
Table I. ADM660 Charge-Pump Frequency Selection
ADM660
V+
OSC
10µF
DOUBLED
POSITIVE
OUTPUT
Figure 22. Voltage Doubler Configuration
Shutdown Input
The ADM8660 contains a shutdown input that can be used to
disable the device and hence reduce the power consumption. A
logic high level on the SD input shuts the device down reducing
the quiescent current to 0.3
µA.
During shutdown the output
voltage goes to 0 V. Therefore, ground referenced loads are
not powered during this state. When exiting shutdown it takes
several cycles (approximately 500
µs)
for the charge pump to
reach its final value. If the shutdown function is not being used,
then SD should be hardwired to GND.
Capacitor Selection
FC
Open
V+
Open or V+
Open
REV. A
OSC
Open
Open
Ext Cap
Ext CLK
Charge Pump
C1, C2
25 kHz
10
µF
120 kHz
2.2
µF
See Typical Characteristics
Ext CLK Frequency/2
–7–
The optimum capacitor value selection depends the charge-
pump frequency. With 25 kHz selected, 10
µF
capacitors are
recommended, while with 120 kHz selected, 2.2
µF
capacitors
may be used. Other frequencies allow other capacitor values to
be used. For maximum efficiency in all cases, it is recommended
that capacitors with low ESR are used for the charge pump.
Low ESR capacitors give both the lowest output resistance and
lowest ripple voltage. High output resistance degrades the overall
power efficiency and causes voltage drops, especially at high
AD [ ANALOG DEVICES ]
