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| 型號(hào)： | ICL828IH-T |
| 英文描述： | Analog IC |
| 中文描述： | 模擬IC |
| 文件頁(yè)數(shù)： | 4/7頁(yè) |
| 文件大小： | 103K |
| 代理商： | ICL828IH-T |
4
Test Circuit
Description
The ICL828 contains all the necessary circuitry to complete
a negative converter, utilizing two external inexpensive 10
μ
F
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 C
1
is charged to a voltage, V
IN
, for the half cycle
when switches S
1
and S
3
are closed (Note: switches S
2
and
S
4
are open during this half cycle). During the second half
cycle of operation, switches S
2
and S
4
are closed, with S
1
and S
2
open, thereby shifting capacitor C
1
negatively by V
IN
Volts. Charge is then transferred from C
1
to C
2
such that the
voltage on C
2
is exactly V
IN
, assuming ideal switches and
no load on C
2
.
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. Theimpedancesofthepumpandreservoircapacitorsare
negligible at the pump frequency.
4. The losses due to the 1/f
C
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:
Where V
1
and V
2
are the voltages on C
1
during the pump
and transfer cycles. If the impedances of C
1
and C
2
are
relatively high at the pump frequency (refer to Figure 10)
compared to the value of R
L
, there will be a substantial
difference in the voltages V
1
and V
2
. Therefore it is not only
desirable to make C
2
as large as possible to eliminate output
voltage ripple, but also to employ a correspondingly large
value for C
1
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
-(V
IN
). The output impedance (R
O
) is a function of the ON
resistance of the internal MOS switches (shown in Figure
10), the switching frequency, the value of C
1
and C
2
, and the
ESR (equivalent series resistance) of C
1
and C
2
. A good
first order approximation for R
O
is:
R
sw
, 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.
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
C
2
at the instant it goes from being charged by C
1
(current
flowing into C
2
) to being discharged through the load
(current flowing out of C
2
). The magnitude of this current
change is 2 x I
OUT
, hence the total drop is 2 x I
OUT
x
ESR
C2
V. Segment B is the voltage change across C
2
during
time t
1
, the half of the cycle when C
2
supplies current the
1
2
3
5
4
C
1
-
IN
OUT
C
1
+
C
2
10
μ
F
GND
+
C
1
10
μ
F
+
V
OUT
V
IN
+
10
μ
F
C
3
R
L
NOTE: V
IN
= +5V, C
1
= C
2
= C
3
, T
A
= 25
o
C, unless otherwise noted.
FIGURE 9. TEST CIRCUIT
C
2
IN
C
1
S
3
S
4
S
1
S
2
2
5
3
4
1
FIGURE 10. IDEALIZED NEGATIVE VOLTAGE CONVERTER
OUT
V
OUT
= -V
IN
E
1
2
--
C
1
V
1
2
V
2
2
–
(
)
=
R
O
+
2 R
sw1
R
sw3
+
ESRC
1
)
+
+
)
2 R
sw2
R
sw4
ESRC
1
+
1
fpump
(
)
C1
(
)
ESRC
2
+
+
=
-
+
R
O
V
OUT
V
IN
FIGURE 11. EQUIVALENT CIRCUIT
ICL828
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