Datasheet
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Typical Applications
Simple Negative Voltage Converter
1
2
3
4
8
7
6
5
+
TL7660
+
-
-
10 µF
V+
10 µF
V = –V+
OUT
TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
The majority of applications will undoubtedly utilize the TL7660 for generation of negative supply voltages.
Figure 3 shows typical connections to provide a negative supply negative (GND) for supply voltages below
3.5 V.
Figure 3. Simple Negative-Voltage Converter
The output characteristics of the circuit in Figure 3 can be approximated by an ideal voltage source in series with
a resistance. The voltage source has a value of –V
CC
. The output impedance (R
O
) is a function of the ON
resistance of the internal MOS switches (shown in Figure 2 ), the switching frequency, the value of C
1
and C
2
,
and the ESR (equivalent series resistance) of C
1
and C
2
. A good first order approximation for R
O
is:
R
O
≈ 2(R
SW1
+ R
SW3
+ ESR
C1
) + 2(R
SW2
+ R
SW4
+ ESR
C1
)
R
O
≈ 2(R
SW1
+ R
SW3
+ ESR
C1
) + 1/f
PUMP
C
1
+ ESR
C2
Where f
PUMP
= f
OSC
/2 , R
SWX
= MOSFET switch resistance.
Combining the four RSWX terms as RSW, we see that:
R
O
≈ 2 (R
SW
) + 1/f
PUMP
C
1
+ 4 (ESR
C1
) + ESR
C2
R
SW
, the total switch resistance, is a function of supply voltage and temperature (See the Output Source
Resistance graphs). Careful selection of C
1
and C
2
reduces the remaining terms, minimizing the output
impedance. High value capacitors reduce the 1/f
PUMP
C
1
component, and low ESR capacitors lower the ESR
term. Increasing the oscillator frequency reduces the 1/f
PUMP
C
1
term but may have the side effect of a net
increase in output impedance when C
1
> 10 µ F and there is no longer enough time to fully charge the capacitors
every cycle. In a typical application where f
OSC
= 10 kHz and C = C1 = C2 = 10 µ F:
R
O
≈ 2(23) + 1/(5 × 10
3
)(10
–5
) + 4(ESR
C1
) + ESR
C2
R
O
≈ 46 + 20 + 5 (ESR
C
)
Because the ESRs of the capacitors are reflected in the output impedance multiplied by a factor of 5, a high
value could potentially swamp out a low 1/f
PUMP
C
1
term, rendering an increase in switching frequency or filter
capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10 Ω .
9
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