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| 型號： | LX1882-33CDU |
| 英文描述： | Charge Pump |
| 中文描述： | 電荷泵 |
| 文件頁數(shù)： | 6/13頁 |
| 文件大?。?/td> | 309K |
| 代理商： | LX1882-33CDU |
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
Copyright
2000
Rev. 1.0c, 2002-08-01
W
M
.
C
LX1882
Charge Pump Doubler with Linear Regulator
P
RODUCTION
D
ATA
S
HEET
I N T E G R A T E D P R O D U C T S
A P P L I C A T I O N I N F O R M A T I O N
O
PERATION
Using a switched-capacitor charge pump, the LX1882
generates a charge pump output (CPO) voltage approximately
twice the input voltage. The charge pump is a doubler
configuration using one external flying capacitor. A 2-phase
non-overlapping clock, generated from an internally trimmed
oscillator, controls the charge pump switches to ensure
maximum efficiency. CPO powers an internal low dropout
(LDO) linear regulator providing a regulated output at V
OUT.
CPO can also be used to drive an external load; refer to section
on CPO Loading. The CPO voltage and V
IN
are sensed via
internal comparators for power-up conditioning. At power-up,
the LDO is disabled and the load is removed from CPO. The
LDO is enabled when the CPO voltage reaches 1.75(V
IN
). When
the CPO voltage drops below 1.45(V
IN
), the LDO is disabled.
Sensing the LX1882 output current determines the charge pump
operating mode. If the output current is less than 100uA and the
CPO voltage is greater than 1.95(V
IN
), the charge pump operates
in Burst Mode. In Burst Mode, the clock is disabled for
increased
efficiency,
until
approximately 100mV at which time the clock is enabled. To
minimize noise at CPO, a low ESR capacitor of at least 2uF
should be used.
The internal LDO can supply up to 50mA of output current
and requires an output capacitor between V
OUT
and GND to
stabilize the internal control loop. A low ESR capacitor of at
least 2uF is recommended.
M
AXIMUM
V
OUT
AND
I
OUT
C
ALCULATIONS
(
CONT
’
D
)
The maximum available output voltage can be calculated given
a minimum input voltage and output current load using the
following equation:
V
OUT
(max)
=
2(V
IN
(min) – (I
OUT
)(R
CPO
) - V
DROPOUT
with the condition that (I
OUT
)(R
CPO
) < 0.55VIN .
Example:
V
IN
(max) = 3V
I
OUT
= 10mA
R
CPO
(max) = 20
Unloaded CPO Voltage (max) = 6V
Loaded CPO Voltage = 6V – (10mA)(20
) = 5.8V
V
DROPOUT
= 0.075V
V
OUT
(max) = (6V) – (0.2) – (0.075) = 5.73
V
OUT
< 5.5 and (I
OUT
)(R
CPO
) < 0.55V
IN
, 0.2V < 1.65V
For minimum noise applications, keep the LDO out of dropout to
prevent CPO noise from coupling into V
OUT
.
the
CPO
voltage
droops
A
DJUSTABLE
O
UTPUT
V
OLTAGE
The LX1882-00 develops a 1.175 reference voltage between
the FB pin and GND. The output voltage can be set using an
external resistor divider (see Figure 3).
7
8
4
1
2
3
6
V
1.8V to 4.4V
OFF ON
V
OUT
= 1.175 [1+(R
1
/R
2
)]
SHDN
FB
GND
C+
LX1882-00
V
IN
C-
CPO
5
C1
C2
C3
C4
V
OUT
R1
R2
Figure 3
. LX1882-00; Adjustable Version
The output voltage range is 2.5V to 5.5V.
M
AXIMUM
V
OUT
AND
I
OUT
C
ALCULATIONS
Using the open circuit CPO voltage (
≈
2(V
IN
)), the dropout
voltage of the LDO, and the effective charge pump output
resistance, the maximum available output voltage and current
can be calculated.
C
APACITOR
S
ELECTION
To reduce noise and ripple, it is recommended to use low ESR
capacitors for C2 (V
OUT
), C3 (CPO), and C4 (V
IN
); see Figure 1.
For stability purposes, C2 must be at least 2uF and C3 must be
equal to or greater than C2. It is also recommended that C4 be at
least 2uF. A 2.2
resistor may be required between V
IN
and the
supply to reduce ripple that may be induced onto the voltage rail
due to large instantaneous currents demanded by the charge
pump.
A 0.22uF, low ESR ceramic capacitor is recommended for C1
(charge pump flying capacitor). To improve V
OUT
ripple for light
loads or for high V
IN
, a smaller capacitor could be used for C1 to
reduce ripple on CPO.
To minimize the enable time, the CPO filter capacitor (C3)
should be at least 2x the V
OUT
filter capacitor (C2). At power-up,
the LDO is disabled and the load is removed from CPO. When
the CPO voltage reaches 1.75(V
IN
) the LDO is enabled and a
large amount of charge is dumped from the CPO capacitor into
the V
OUT
capacitor. If the amount of charge dumped causes the
CPO voltage to fall below 1.45(V
IN
), the LDO will be disabled
and the CPO voltage will have to charge up to 1.75(V
IN
) to
enable the LDO. This cycle results in an extended enable time.
For the LX1882-33, a 1nF filter capacitor should be connected
between the FILT pin and GND for optimum noise performance.
CPO
L
OADING
The CPO can drive an external load. Current draw by the
external load will reduce the maximum available current at V
OUT
(i.e. if CPO is driving a 5mA load, the available current from
V
OUT
will be reduced by 5mA).
A
P
P
L
I
C
A
T
I
O
N
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| LX1882-33CDU-TR | 制造商:MICROSEMI 制造商全稱:Microsemi Corporation 功能描述:Charge Pump Doubler with Linear Regulator |
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