TC7660 Datasheet, PDF (7/9 Pages) TelCom Semiconductor, Inc – CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER

CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
1
TC7660
V+
1
8
2
2
7
C1
TC7660
3
6
1
8
2
7
RL
4
"1"
5
C1
3 ^TC7660 6
4
"n"
5
3
+ ^C2
Figure 4. Paralleling Devices Lowers Output Impedance
Cascading Devices
The TC7660 may be cascaded as shown (Figure 6) to
produce larger negative multiplication of the initial supply
voltage. 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 ap-
proximately the weighted sum of the individual TC7660
R_OUT values.
Changing the TC7660 Oscillator Frequency
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 exter-
nal clock, as shown in Figure 6. In order to prevent possible
device latch-up, a 1kΩ resistor must be used in series with
4 ^{the clock output. In a situation where the designer has}
generated the external clock frequency using TTL logic, the
addition of a 10kΩ pull-up resistor to V⁺ 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.
It is also possible to increase the conversion efficiency
of the TC7660 at low load levels by lowering the oscillator
5 ^{frequency. This reduces the switching losses, and is achieved}
by connecting an additional capacitor, C_OSC, as shown in
Figure 7. Lowering the oscillator frequency will cause an
undesirable increase in the impedance of the pump (C₁) and
the reservoir (C₂) capacitors. To overcome this, increase the
values of C₁ and C₂ by the same factor that the frequency
has been reduced. For example, the addition of a 100pF
capacitor between pin 7 (OSC) and pin 8 (V⁺) will lower the
oscillator frequency to 1kHz from its nominal frequency of
10kHz (a multiple of 10), and necessitate a corresponding
increase in the values of C₁ and C₂ (from 10µF to 100µF).
6
+
10µF
V+
1
8
2
7
3 ^TC7660 6
4
"1"
5
+
10µF
1
8
2
7
3 ^TC7660 6
4
"n"
5
* NOTES:
1. VOUT = –n V+ for 1.5V ≤ V+ ≤ 10V
Figure 5. Increased Output Voltage by Cascading Devices
TELCOM SEMICONDUCTOR, INC.
7
VOUT*
+ ^10µF
8
4-57