Datasheet
2002-2012 Microchip Technology Inc. DS21453D-page 5
TC682
4.0 TYPICAL APPLICATIONS
4.1 Negative Doubling Converter
The most common application of the TC682 is as a
charge pump voltage converter which provides a
negative output of two times a positive input voltage
(Figure 4-1).
FIGURE 4-1: Inverting Voltage Doubler
4.2 Capacitor Selection
The output resistance of the TC682 is determined, in
part, by the ESR of the capacitors used. An expression
for R
OUT
is derived as shown below:
Assuming all switch resistances are approximately
equal:
R
OUT
is typically 140 at +25°C with V
IN
= +5V and 3.3
F low ESR capacitors. The fixed term (16R
SW
) is
about 80-90. It can be seen easily that increasing or
decreasing values of C1 and C2 will affect efficiency by
changing R
OUT
. However, be careful about ESR. This
term can quickly become dominant with large electro-
lytic capacitors. Table 4-1 shows R
OUT
for various
values of C1 and C2 (assume 0.5 ESR). C1 must be
rated at 6VDC or greater while C2 and C3 must be
rated at 12VDC or greater.
Output voltage ripple is affected by C3. Typically the
larger the value of C3 the less the ripple for a given load
current. The formula for
P-P
V
RIPPLE
is given below:
V
RIPPLE
= {1/[2(f
PUMP
x C3)] + 2(ESR
C3
)} (I
OUT
)
For a 10 F (0.5 ESR) capacitor for C3, f
PUMP
= 10
kHz and I
OUT
= 10 mA the peak-to-peak ripple voltage
at the output will be less then 60 mV. In most
applications (I
OUT
< = 10 mA) a 10-20 F capacitor and
1-5 F pump capacitors will suffice. Table 4-2 shows
V
RIPPLE
for different values of C3 (assume 1 ESR).
TABLE 4-1: OUTPUT RESISTANCE
VS. C1, C2
TABLE 4-2: V
RIPPLE
PEAK-TO-PEAK
VS. C3 (I
OUT
10mA)
GND
GND
TC682
22 μF
22 μF
22 μF
7
6
54
3
2
1
+
V
IN
C
1
+
C
2
+
C
1
–
C
2
–
V
–
OUT
C
1
C
2
V
–
OUT
+
+
V
IN
C
3
R
OUT
=2(R
SW1
+R
SW2
+ESR
C1
+R
SW3
+R
SW4
+ESR
C2
)
+2(R
SW1
+R
SW2
+ESR
C1
+R
SW3
+R
SW4
+ESR
C2
)
+1/(f
PUMP
x C1) +1/(f
PUMP
x C2)
+ESR
C3
R
OUT
= 16R
SW
+ 4ESR
C1
+ 4ESR
C2
+ ESR
C3
+1/(f
PUMP
x C1) +1/(f
PUMP
x C2)
C1, C2 (F) R
OUT
()
0.05 4085
0.10 2084
0.47 510
1.00 285
3.30 145
5.00 125
10.00 105
22.00 94
100.00 87
C3 (F) V
RIPPLE
(mV)
0.50 1020
1.00 520
3.30 172
5.00 120
10.00 70
22.00 43
100.00 25