TC682CPA View Datasheet(PDF) - TelCom Semiconductor Inc => Microchip
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TC682CPA Datasheet PDF : 6 Pages
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INVERTING VOLTAGE DOUBLER
TC682
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 140Ω at +25°C with VIN = +5V and
3.3µF low ESR capacitors. The fixed term (16RSW) is about
80-90Ω. 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 capacitors. Table 1
shows ROUT for various values of C1 and C2 (assume 0.5Ω
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 VRIPPLE is given below:
VRIPPLE = {1/[2(fPUMP x C3)] + 2(ESRC3)} (IOUT)
For a 10µF (0.5Ω ESR) capacitor for C3, fPUMP = 10kHz
and IOUT = 10mA the peak-to-peak ripple voltage at the
output will be less then 60mV. In most applications (IOUT <
= 10mA) a 10-20µF capacitor and 1-5µF pump capacitors
will suffice. Table 2 shows VRIPPLE for different values of C3
(assume 1Ω ESR).
Table 1. ROUT vs. C1, C2
C1, C2 (µF)
0.05
0.10
0.47
1.00
3.30
5.00
10.00
22.00
100.00
ROUT (Ω)
4085
2084
510
285
145
125
105
94
87
Table 2. VRIPPLE Peak- to-Peak vs. C3 (IOUT = 10mA)
C3 (µF)
VRIPPLE (mV)
0.50
1020
1.00
520
3.30
172
5.00
120
10.00
70
22.00
43
100.00
25
Paralleling Devices
Paralleling multiple TC682s reduces the output resis-
tance of the converter. The effective output resistance is the
output resistance of a single device divided by the number
of devices. As illustrated in Figure 5, each requires separate
pump capacitors C1 and C2, but all can share a single
reservoir capacitor.
–5V Regulated Supply From A Single
3V Battery
Figure 6 shows a – 5V power supply using one 3V
battery. The TC682 provides – 6V at VO– UT, which is regu-
lated to – 5V by the negative LDO. The input to the TC682
can vary from 3V to 5.5V without affecting regulation appre-
ciably. A TC54 device is connected to the battery to detect
undervoltage. This unit is set to detect at 2.7V. With higher
input voltage, more current can be drawn from the outputs
of the TC682. With 5V at VIN, 10mA can be drawn from the
regulated output. Assuming 150Ω source resistance for the
converter, with IL–= 10mA, the charge pump will droop 1.5V.
4-24
TELCOM SEMICONDUCTOR, INC.
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