ICL828 Datasheet, PDF (5/7 Pages) Intersil Corporation – Switched-Capacitor Voltage Inverter

ICL828
load. The drop at B is I_OUT x t₁/C₂V. The peak-to-peak
ripple voltage is the sum of these voltage drops:
V
RIPPLE
≅


2^-----×^-----C^-----₂^----X^-1---f^--_P^-----_U^----_M^------_P^--
+
2
ESRC2
×
IO
U

T
Again, a low ESR capacitor will result in a higher
performance output.
Positive Voltage Doubling
The ICL828 may be employed to achieve positive voltage
doubling using the circuit shown in Figure 13. In this
application, the pump inverter switches of the ICL828 are
used to charge C₁ to a voltage level of V_IN -V_F where V_IN is
the supply voltage and V_F is the forward voltage on C₁ plus
the supply voltage (V_IN) is applied through diode D₂ to
capacitor C₂. The voltage thus created on C₂ becomes
(2V_IN) - (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.
Combined Negative Conversion and
Positive Supply Doubling
Figure 14 combines the functions shown on front page and
Figure 13 to provide negative voltage conversion and
positive voltage doubling simultaneously. This approach
would be, for example, suitable for generating +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.
Cascading Devices
The ICL828 may be cascaded to produce a larger
multiplication supply voltage (see Figure 15). The output
voltage is:
V_OUT = -n(V_IN),
where n is an integer representing the number of devices
cascaded.
The resulting output resistance would be approximately the
sum of the individual ICL828 R_OUT values.
t₁
0
V
- (V_IN)
B
A
FIGURE 12. OUTPUT RIPPLE
V+
1
2
3
D₁
5
-
C₁
+
D₂
4
V_OUT =
(2V_IN) - (2V_F)
+
- ^C2
NOTE: D₁ and D₂ can be any suitable diode.
FIGURE 13. POSITIVE VOLTAGE DOUBLER
V_IN
1
2
-³
+C¹
V_OUT = -V_IN
5
- ^C3
+
D₁
4
D₂
-+
C₂
V_OUT = (2V_IN) -
(V_FD1) - (V_FD2)
+
- ^C4
FIGURE 14. COMBINED NEGATIVE VOLTAGE AND POSITIVE
DOUBLER
1
OUT
5
C1+
C₂
+
1
OUT
5
C1+
+V_IN ² _IN
1
3 _C1-
4
GND
+
C₁
2
IN
3
C1 -
n
GND ⁴
C₄
+
C₃ ⁺
V_OUT
V_OUT = - nV_IN
FIGURE 15. CASCADING TO INCREASE OUTPUT VOLTAGE
5