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Cascading Devices
1
2
3
4
8
7
6
5
TL7660
"1"
1
2
3
4
8
7
6
5
10 µF
+
+
-
+
+
-
V+
+
-
10 µF
V = –nV+
OUT
10 µF
TL7660
"n"
+
-
10 µF
Changing the TL7660 Oscillator Frequency
1
2
3
4
8
7
6
5
TL7660
10 µF
+
-
CMOS
Gate
+
+
–
10 µF
V
OUT
V+
V+
–
TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
The TL7660 may be cascaded as shown to produced larger negative multiplication of the initial supply voltage
(see Figure 6 ). However, due to the finite efficiency of each device, the practical limit is 10 devices for light
loads. The output voltage is defined by:
V
OUT
= –n (V
IN
)
Where n is an integer representing the number of devices cascaded. The resulting output resistance would be
approximately the weighted sum of the individual TL7660 R
OUT
values.
Figure 6. Cascading Devices for Increased Output Voltage
It may be desirable in some applications, due to noise or other considerations, to increase the oscillator
frequency. This is achieved by overdriving the oscillator from an external clock, as shown in Figure 7 . To prevent
possible device latchup, a 1-k Ω resistor must be used in series with the clock output. When the external clock
frequency is generated using TTL logic, the addition of a 10-k Ω pullup resistor to V
CC
supply is required. Note
that the pump frequency with external clocking, as with internal clocking, will be 1/2 of the clock frequency.
Output transitions occur on the positive-going edge of the clock.
Figure 7. External Clocking
11
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