ICL7662_06 Datasheet, PDF (9/10 Pages) Intersil Corporation – CMOS Voltage Converter

ICL7662
+
10µF
-
1
8
2 _ICL7662 7
3
“1”
6
4
5
V+
+
10µF _-
1
8
2 _ICL7662 7
3
“N”
6
4
5
-
10µF
+
-
10µF
+
V_OUT
FIGURE 19. CASCADING DEVICES FOR INCREASED OUTPUT VOLTAGE
Changing the ICL7662 Oscillator Frequency
It may be desirable in some applications, due to noise or other
considerations, to increase the oscillator frequency. This is
achieved by overdriving the oscillator from an external clock, as
shown in Figure 20. In order to prevent possible device latchup,
a 1kΩ resistor must be used in series with the clock output. In
the situation where the designer has generated the external
clock frequency using TTL logic, the addition of a 10kΩ pullup
resistor to V+ supply is required. Note that the pump frequency
with external clocking, as with internal clocking, will be 1/2 of
the clock frequency. Output transitions occur on the positive-
going edge of the clock.
V⁺
V⁺
+
10µF
-
1
8
1kΩ
2
ICL7662
7
3
6
CMOS
GATE
4
5
-
V_OUT
+ ^10µF
FIGURE 20. EXTERNAL CLOCKING
It is also possible to increase the conversion efficiency of the
ICL7662 at low load levels by lowering the oscillator frequency.
This reduces the switching losses, and is achieved by
connecting an additional capacitor, COSC, as shown in Figure
21. However, lowering the oscillator frequency will cause an
undesirable increase in the impedance of the pump (C₁) and
reservoir (C₂) capacitors; this is overcome by increasing the
values of C₁ and C₂ by the same factor that the frequency has
been reduced. For example, the addition of a 100pF capacitor
between pin 7 (OSC) and V+ will lower the oscillator frequency
to 1kHz from its nominal frequency of 10kHz (a multiple of 10),
and thereby necessitate a corresponding increase in the value
of C₁ and C₂ (from 10mF to 100mF).
V⁺
C₁ ⁺
-
1
8
2
7
ICL7662
3
6
C_OSC
4
5
-
V_OUT
+ ^C2
FIGURE 21. LOWERING OSCILLATOR FREQUENCY
Positive Voltage Doubling
The ICL7662 may be employed to achieve positive voltage
doubling using the circuit shown in Figure 22. In this
application, the pump inverter switches of the ICL7662 are
used to charge C₁ to a voltage level of V+ -V_F (where V+ is
the supply voltage and V_F is the forward voltage drop of
diode D₁). On the transfer cycle, the voltage on C₁ plus the
supply voltage (V+) is applied through diode C₂ to capacitor
C₂. The voltage thus created on C₂ becomes (2V+) (2V_F) or
twice the supply voltage minus the combined forward
voltage drops of diodes D₁ and D₂.
The source impedance of the output (V_OUT) will depend on
the output current, but for V+ = 15V and an output current of
10mA it will be approximately 70Ω.
1
8
2
ICL7662
7
3
6
4
5
V+
D₁
D₂
+
- ^C1
V_OUT =
(2V+) - (2V_F)
+
- ^C2
NOTE: D₁ and D₂ can be any suitable diode.
FIGURE 22. POSITIVE VOLTAGE DOUBLER
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