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TC7662A Datasheet PDF : 16 Pages
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TC7662A
4.5 Changing the TC7662A Oscillator
Frequency
It is possible to increase the conversion efficiency of
the TC7662A at low load levels by lowering the
oscillator frequency. This reduces the switching losses,
and is shown in Figure 4-5. However, lowering the
oscillator frequency will cause an undesirable increase
in the impedance of the pump (CP) and reservoir (CR)
capacitors; this is overcome by increasing the values of
CP and CR by the same factor that the frequency has
been reduced. For example, the addition of a 100pF
capacitor between pin 7 (OSC) and VDD will lower the
oscillator frequency to 2kHz from its nominal frequency
of 12kHz (multiple of 6), and thereby necessitate a
corresponding increase in the value of CP and CR (from
10F to 68F).
FIGURE 4-5:
LOWERING OSCILLATOR
FREQUENCY
VDD
+
10μF
1
8
2
7
3 TC7662A 6
4
5
COSC
VOUT
10μF
+
4.6 Positive Voltage Doubling
The TC7662A may be employed to achieve positive
voltage doubling using the circuit shown in Figure 4-6.
In this application, the pump inverter switches of the
TC7662A are used to charge CP to a voltage level of
VDD – VF (where VDD is the supply voltage and VF is
the forward voltage on CP plus the supply voltage (VDD)
applied through diode D2 to capacitor CR). The voltage
thus created on CR becomes (2 VDD) – (2 VF), or twice
the supply voltage minus the combined forward voltage
drops of diodes D1 and D2.
The source impedance of the output (VOUT) will depend
on the output current, but for VDD = 5V and an output
current of 10 mA, it will be approximately 60.
FIGURE 4-6:
POSITIVE VOLTAGE
MULTIPLIER
VDD
1
8
2
7
TC7662A
3
6
4
5
D1
VOUT =
D2
(2 VDD) – (2 VF)
+
CP
+
CR
4.7 Combined Negative Voltage
Conversion and Positive Supply
Multiplication
Figure 4-7 combines the functions shown in Figure 4-1
and Figure 4-6 to provide negative voltage conversion
and positive voltage doubling simultaneously. This
approach would be, for example, suitable for generat-
ing +9V and -5V from an existing +5V supply. In this
instance, capacitors C1 and C3 perform the pump and
reservoir functions, respectively, for the generation of
the negative voltage, while capacitors C2 and C4 are
pump and reservoir, respectively, for the doubled
positive voltage. There is a penalty in this configuration
which combines both functions, however, in that the
source impedances of the generated supplies will be
somewhat higher due to the finite impedance of the
common charge pump driver at pin 2 of the device.
FIGURE 4-7:
COMBINED NEGATIVE
CONVERTER AND
POSITIVE DOUBLER
VDD
1
8
2
7
TC7662A
3
6
+
4
C1
5
+
C2
VOUT =
-(VDD – VF)
D1
+
C3
VOUT =
D2 (2 VDD) – (2 VF)
+
C4
4.8 Voltage Splitting
The same bidirectional characteristics can be used to
split a higher supply in half, as shown in Figure 4-8.
The combined load will be evenly shared between the
two sides. Because the switches share the load in
parallel, the output impedance is much lower than in
the standard circuits, and higher currents can be drawn
from the device. By using this circuit, and then the
circuit of Figure 4-4, +15V can be converted (via +7.5V
and -7.5V) to a nominal -15V, though with rather high
series resistance (~250).
FIGURE 4-8:
SPLITTING A SUPPLY IN
HALF
VDD
+
RL1
50μF
–
1
8
VOUT =
VDD – V – 50
+
2
μF –
RL2
2
7
TC7662A
3
6
4
5
+
50μF –
V–
DS21468B-page 8
2001-2012 Microchip Technology Inc.
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