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2002 Microchip Technology Inc.
DS21548B-page 5
TC2682/TC2683/TC2684
4.0
APPLICATIONS INFORMATION
4.1
Output Voltage Considerations
The
voltage conversions but do not provide any type of
regulation. The output voltage will droop in a linear
manner with respect to the output load current. The
value of the equivalent output resistance is
approximately 120
nominal at +25
°
C and V
IN
= +5V.
In this particular case, the output is approximately -10V
at very light loads and will droop according to the
equation below:
V
DROOP
= I
OUT
x R
OUT
TC2682/TC2683/TC2684
perform
inverting
4.2
Capacitor Selection
In order to maintain the lowest output resistance and
output ripple voltage, it is recommended that low ESR
capacitors be used. Additionally, larger values of C1
and C2 will lower the output resistance and larger
values of C
OUT
will reduce output ripple.
Note:
For proper charge pump operation, C1
must have a voltage rating greater than or
equal to V
IN
, while C2 and C
OUT
must have
a voltage rating greater than or equal to
2V
IN
.
Table 4-1 shows various values of C1/C2 and the
corresponding output resistance values for V
IN
= 5V
@ +25
°
C.
Table 4-2 shows the output voltage ripple for various
values of C
OUT
(again assuming V
IN
= 5V @ +25
°
C).
The V
RIPPLE
values assume a 1mA output load current
and a 0.1
ESR
COUT
.
TABLE 4-1:
OUTPUT RESISTANCE
VS. C1/C2 (ESR = 0.1
)
TABLE 4-2:
OUTPUT VOLTAGE RIPPLE
VS. C
OUT
2
(ESR = 0.1
)
I
OUT
= 1mA
4.3
Input Supply Bypassing
The V
IN
input should be capacitively bypassed to
reduce AC impedance and minimize noise effects due
to the switching internal to the device. It is recom-
mended that a large value capacitor (at least equal to
C1) be connected from V
IN
to GND for optimal circuit
performance.
4.4
Inverting Voltage Doubler
The most common application for the TC2682/TC2683/
TC2684 devices is the inverting voltage doubler
(Figure 4-1). This application uses three external
capacitors: C1, C2 and C
OUT
.
Note:
A power supply bypass capacitor is
recommended.
The output is equal to -2V
IN
plus any voltage drops due
to loading. Refer to Table 4-1 and Table 4-2 for
capacitor selection guidelines.
FIGURE 4-1:
DUAL VOLTAGE
INVERTER TEST CIRCUIT
4.5
Layout Considerations
As with any switching power supply circuit, good layout
practice is recommended. Mount components as close
together as possible to minimize stray inductance and
capacitance. Also use a large ground plane to minimize
noise leakage into other circuitry.
C1, C2
(
μ
F)
TC2682
R
OUT
(
)
633
262
120
TC2683
R
OUT
(
)
184
120
95
TC2684
R
OUT
(
)
120
102
84
0.33
1
3.3
C
OUT
(
μ
TC2682
V
(mV)
TC2683
V
(mV)
TC2684
V
(mV)
0.33
1
3.3
192
63
17
60
21
8
27
16
7
1
2
N
N
UT
UT
C2682
C2683
C2684
1
2
1
2
ND
evice
TC2682 3.3
μ
F 3.3
μ
F 3.3
μ
F 3.3
μ
F
TC2683 1
μ
F
1
μ
F 1
μ
F
TC2684 0.33
μ
F 0.33
μ
F 0.33
μ
F 0.33
μ
F
1
2
UT
1
μ
F
UT