TL7660_14 Datasheet, PDF (12/23 Pages) Texas Instruments – CMOS VOLTAGE CONVERTER

TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
www.ti.com
APPLICATION INFORMATION (continued)
It is also possible to increase the conversion efficiency of the TL7660 at low load levels by lowering the oscillator
frequency (see Figure 8). This reduces the switching losses. However, lowering the oscillator frequency causes
an undesirable increase in the impedance of the pump (C₁) and reservoir (C₂) capacitors; this is overcome by
increasing the values of C₁ and C₂ by the same factor that the frequency has been reduced. For example, the
addition of a 100-pF capacitor between terminal 7 (OSC) and V_CC lowers the oscillator frequency to 1 kHz from
its nominal frequency of 10 kHz (a multiple of 10), and thereby necessitate a corresponding increase in the
value of C₁ and C₂ (from 10 µF to 100 µF).
V+
++
C_{1 --}
1
8
2
7
TL7660
3
6
C_OSC
4
5
V_OUT
C₂
--
++
Figure 8. Lowering Oscillator Frequency
Positive Voltage Doubling
The TL7660 may be used to achieve positive voltage doubling using the circuit shown in Figure 9. In this
application, the pump inverter switches of the TL7660 are used to charge C₁ to a voltage level of V_CC – V_F
(where V_CC is the supply voltage and V_F is the forward voltage drop of diode D1). On the transfer cycle, the
voltage on C₁ plus the supply voltage (V_CC) is applied through diode D₂ to capacitor C₂. The voltage thus
created on C₂ becomes (2V_CC) – (2V_F) or twice the supply voltage minus the combined forward voltage drops of
diodes D₁ and D₂.
The source impedance of the output (V_OUT) depends on the output current.
V+
1
8
2
7
TL7660
3
6
4
5
D₁
D₂
C₂
C₁
V_OUT = (2V+) – (2VF)
Figure 9. Positive-Voltage Doubler
12
Submit Documentation Feedback