TL7660_14 Datasheet, PDF (13/23 Pages) Texas Instruments – CMOS VOLTAGE CONVERTER

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TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
Combined Negative Voltage Conversion and Positive Supply Doubling
Figure 10 combines the functions shown in Figure 3 and Figure 9 to provide negative voltage conversion and
positive voltage doubling simultaneously. This approach would be, for example, suitable for generating +9 V and
–5 V from an existing 5-V supply. In this instance, capacitors C₁ and C₃ perform the pump and reservoir
functions, respectively, for the generation of the negative voltage, while capacitors C₂ and C₄ are pump and
reservoir, respectively, for the doubled positive voltage. There is a penalty in this configuration that combines
both functions, however, in that the source impedances of the generated supplies are somewhat higher, due to
the finite impedance of the common charge pump driver at terminal 2 of the device.
+
C_{1 -}
V+
1
8
2
7
TL7660
3
6
4
5
-+
C₂
-
+
C₃
V_OUT = –(nV_IN – V_FDX)
D₁
D₂
V_OUT = (2V+) – (V_FD1) – (V_FD2)
+
- ^C4
Figure 10. Combined Negative-Voltage Converter and Positive-Voltage Doubler
Voltage Splitting
The bidirectional characteristics can also be used to split a higher supply in half (see Figure 11. The combined
load is evenly shared between the two sides. Because the switches share the load in parallel, the output
impedance is much lower than in the standard circuits, and higher currents can be drawn from the device. By
using this circuit and then the circuit of Figure 6, 15 V can be converted (via 7.5 V, and –7.5 V) to a nominal
–15 V, although with rather high series output resistance (~250 Ω).
V+
+
R_L1
50 µF _-
V = ^{V+ V}
OUT
2
+
50 µF _-
R_L2
50 µF ⁺
-
1
8
2
7
TL7660
3
6
4
5
V–
Figure 11. Splitting a Supply in Half
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