TC682 Просмотр технического описания (PDF) - Microchip Technology
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TC682 Datasheet PDF : 14 Pages
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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).
+
C1
22 μF
+
C2
22 μF
1 C1–
2 C2+
C1+ 7
TC682
3 C2–
VIN 6
4 V–OUT
5
GND
+ C3
22 μF
VIN
GND
V–OUT
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 ROUT is derived as shown below:
ROUT = 2(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2)
+2(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2)
+1/(fPUMP x C1) +1/(fPUMP x C2)
+ESRC3
Assuming all switch resistances are approximately
equal:
ROUT = 16RSW + 4ESRC1 + 4ESRC2 + ESRC3
+1/(fPUMP x C1) +1/(fPUMP x C2)
ROUT is typically 140 at +25°C with VIN = +5V and 3.3
F low ESR capacitors. The fixed term (16RSW) is
about 80-90. 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 electro-
lytic capacitors. Table 4-1 shows ROUT 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.
TC682
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 VRIPPLE is given below:
VRIPPLE = {1/[2(fPUMP x C3)] + 2(ESRC3)} (IOUT)
For a 10 F (0.5 ESR) capacitor for C3, fPUMP = 10
kHz and IOUT = 10 mA the peak-to-peak ripple voltage
at the output will be less then 60 mV. In most
applications (IOUT < = 10 mA) a 10-20 F capacitor and
1-5 F pump capacitors will suffice. Table 4-2 shows
VRIPPLE for different values of C3 (assume 1 ESR).
TABLE 4-1: OUTPUT RESISTANCE
VS. C1, C2
C1, C2 (F)
0.05
0.10
0.47
1.00
3.30
5.00
10.00
22.00
100.00
ROUT()
4085
2084
510
285
145
125
105
94
87
TABLE 4-2:
VRIPPLE PEAK-TO-PEAK
VS. C3 (IOUT 10mA)
C3 (F)
VRIPPLE (mV)
0.50
1020
1.00
520
3.30
172
5.00
120
10.00
70
22.00
43
100.00
25
2002-2012 Microchip Technology Inc.
DS21453D-page 5
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