+5V TO ±10V VOLTAGE CONVERTER
TCM680
Capacitor Selection
EFFICIENCY CONSIDERATIONS
The TCM680 requires only 4 external capacitors for
operation. These can be inexpensive polarized aluminum
electrolytic types. For the circuit in Figure 6 the output
characteristics are largely determined by the external
capacitors.AnexpressionforROUT canbederivedasshown
below:
Theoretically a charge pump can approach 100% effi-
ciency under the following conditions:
• The charge Pump switches have virtually no offset
and extremely low on resistance
• Minimal power is consumed by the drive circuitry
• The impedances of the reservoir and pump capaci-
tors are negligible
R+OUT = 4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2
)
For the TCM680, efficiency is as shown below:
+4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2
+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC4
)
Efficiency V+ = VDD /(2VIN)
VDD = 2VIN – V+DROP
R–OUT
=
4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2
+4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4+ ESRC2
+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC3
)
)
V+DROP = (I+OUT)(R+OUT
)
Efficiency V– = VSS /(– 2VIN)
VSS = 2VIN – V–DROP
V–DROP = (I–OUT)(R–OUT
Assuming all switch resistances are approximately
equal...
)
Power Loss = (V+DROP)(I+OUT) + (V–DROP)(I–OUT
)
R+OUT = 32RSW + 8ESRC1 + 8ESRC2 + ESRC4
+1/(fPUMP x C1) + 1/(fPUMP x C2)
There will be a substantial voltage difference between
(V+OUT – VIN) and VIN for the positive pump and between
V+OUT andVO– UTiftheimpedancesofthepumpcapacitorsC1
and C2 are high with respect to the output loads.
R–OUT = 32RSW + 8ESRC1 + 8ESRC2 + ESRC3
+1/(fPUMP x C1) + 1/(fPUMP x C2)
Larger values of reservoir capacitors C3 and C4 will
reduce output ripple. Larger values of both pump and
reservoir capacitors improve the efficiency. See "Capacitor
Selection" in Applications Section.
ROUT is typically 140Ω at +25°C with VIN = +5V and C1
and C2 as 4.7µF low ESR capacitors. The fixed term
(32RSW) is about 130Ω. It can be seen easily that increasing
or decreasing values of C1 and C2 will affect efficiency by
changing ROUT. However, be careful about ESR. This term
can quickly become dominant with large electrolytic capaci-
tors. Table 1 shows ROUT for various values of C1 and C2
(assume 0.5Ω ESR). C1 and C4 must be rated at 6VDC or
greater while C2 and C3 must be rated at 12VDC or greater.
OutputvoltagerippleisaffectedbyC3andC4. Typically
the larger the value of C3 and C4 the less the ripple for a
given load current. The formula for VRIPPLE(p-p) is given
below:
APPLICATIONS
Positive and negative Converter
The most common application of the TCM680 is as a
dualchargepumpvoltageconverterwhichprovidespositive
and negative outputs of two times a positive input voltage.
The simple circuit of Figure 6 performs this same function
using the TCM680 and external capacitors, C1, C2, C3 and C4.
V+RIPPLE(p-p) = {1/[2(fPUMP /3) x C4] + 2(ESRC4)}(I+OUT
V–RIPPLE(p-p) = {1/[2(fPUMP /3) x C3] + 2(ESRC3)}(I–OUT
)
C1
22µF
22µF
)
+
+
8
7
1
2
–
VOUT
VOUT
C1
For a 10µF (0.5Ω ESR) capacitor for C3, C4,
fPUMP = 21kHz and IOUT = 10mA the peak-to-peak ripple
voltage at the output will be less than 100mV. In most
applications(IOUT <=10mA)10-20µFoutputcapacitorsand
1-5µF pump capacitors will suffice. Table 2 shows VRIPPLE
for different values of C3 and C4 (assume 1Ω ESR).
+
+
C4
C2
C1
22µF
C2
TCM680
6
5
–
3
4
VIN
VIN
C2
–
VOUT
GND
GND
C3
22µF
–
VOUT
Figure 6. Positive and Negative Converter
4-16
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
