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ICL828 Datasheet, PDF (4/7 Pages) Intersil Corporation – Switched-Capacitor Voltage Inverter
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ICL828
Test Circuit
1
OUT
2
IN
+ C3
10µF
3
C1-
C 1+ 5
VIN
RL
VOUT
C2 + C1
+ 10µF 10µF
4
GND
NOTE: VIN = +5V, C1 = C2 = C3, TA = 25oC, unless otherwise noted.
FIGURE 9. TEST CIRCUIT
2
S1
IN
4
5
S2
C1
S3
S4
3
C2
1
OUT
VOUT = -VIN
FIGURE 10. IDEALIZED NEGATIVE VOLTAGE CONVERTER
Description
The ICL828 contains all the necessary circuitry to complete
a negative converter, utilizing two external inexpensive 10µF
polarized electrolytic capacitor. The mode of operation of the
device may be understood by considering Figure 10 which
shows an idealized negative voltage converter.
Capacitor C1 is charged to a voltage, VIN, for the half cycle
when switches S1 and S3 are closed (Note: switches S2 and
S4 are open during this half cycle). During the second half
cycle of operation, switches S2 and S4 are closed, with S1
and S2 open, thereby shifting capacitor C1 negatively by VIN
Volts. Charge is then transferred from C1 to C2 such that the
voltage on C2 is exactly VIN, assuming ideal switches and
no load on C2.
Theoretical Power Efficiency
Considerations
In theory a voltage converter can approach 100% efficiency
if certain conditions are met:
1. The driver circuitry consumes minimal power.
2. The output switches have extremely low ON resistance
and virtually no offset.
3. The impedances of the pump and reservoir capacitors are
negligible at the pump frequency.
4. The losses due to the 1/fC terms is small.
Energy is lost only in the transfer of charge between
capacitors if a change in voltage occurs.
The energy lost is defined by:
E = 12-- C1(V12 – V22)
Where V1 and V2 are the voltages on C1 during the pump
and transfer cycles. If the impedances of C1 and C2 are
relatively high at the pump frequency (refer to Figure 10)
compared to the value of RL, there will be a substantial
difference in the voltages V1 and V2. Therefore it is not only
desirable to make C2 as large as possible to eliminate output
voltage ripple, but also to employ a correspondingly large
value for C1 in order to achieve maximum efficiency of
operation.
Negative Voltage Converter
The output characteristics of the circuit on the first page can
be approximated by an ideal voltage source in series with a
resistance (Figure 11). The voltage source has a value of
-(VIN). The output impedance (RO) is a function of the ON
resistance of the internal MOS switches (shown in Figure
10), the switching frequency, the value of C1 and C2, and the
ESR (equivalent series resistance) of C1 and C2. A good
first order approximation for RO is:
RO = 2(Rsw1 + Rsw3 + ESRC1)
+ 2(Rsw2 + Rsw4 + ESRC1) + 1 ⁄ (fpump)(C1) + ESRC2
Rsw, the switch resistance, is a function of supply voltage
and temperature (see Figure 3). Careful selection of
capacitors will minimize the output resistance, and low
capacitor ESR will lower the ESR term.
-
VIN
+
VOUT
RO
FIGURE 11. EQUIVALENT CIRCUIT
Output Ripple
ESR also affects the ripple voltage seen at the output. The
total ripple is determined by 2 voltages, A and B, as shown in
Figure 12. Segment A is the voltage drop across the ESR of
C2 at the instant it goes from being charged by C1 (current
flowing into C2) to being discharged through the load
(current flowing out of C2). The magnitude of this current
change is 2 x I OUT, hence the total drop is 2 x IOUT x
ESRC2V. Segment B is the voltage change across C2 during
time t1, the half of the cycle when C2 supplies current the
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