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PDF LTC3522 Data sheet ( Hoja de datos )
| Número de pieza | LTC3522 | |
| Descripción | Synchronous 400mA Buck-Boost and 200mA Buck Converters | |
| Fabricantes | Linear Technology | |
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LTC3522
Synchronous 400mA
Buck-Boost and 200mA
Buck Converters
FEATURES
■ Dual High Efficiency DC/DC Converters:
Buck-Boost (VOUT: 2.2V to 5.25V, IOUT: 400mA
for VIN > 3V, VOUT = 3.3V)
Buck (VOUT: 0.6V to VIN, IOUT: 200mA)
■ 2.4V to 5.5V Input Voltage Range
■ Pin Selectable Burst Mode® Operation
■ 25μA Total Quiescent Current for Both Converters in
Burst Mode Operation
■ Independent Power Good Indicator Outputs
■ Integrated Soft-Start
■ Thermal and Overcurrent Protection
■ <1μA Quiescent Current in Shutdown
■ Small 0.75mm × 3mm × 3mm QFN Package
APPLICATIONS
■ Flash-Based MP3 Players
■ Medical Instruments
■ Digital Cameras
■ PDAs, Handheld PCs
■ Personal Navigation Devices
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 6404251 and 6166527.
DESCRIPTION
The LTC®3522 combines a 400mA buck-boost DC/DC con-
verter with a 200mA synchronous buck DC/DC converter in
a tiny 3mm × 3mm package. The 1MHz switching frequency
minimizes the solution footprint while maintaining high
efficiency. Both converters feature internal soft-start and
compensation, simplifying the design process.
The buck converter is current mode controlled and utilizes
an internal synchronous rectifier for high efficiency. The
buck converter supports 100% duty cycle operation to
extend battery life. If the PWM pin is held low, the buck
converter automatically transitions from Burst Mode opera-
tion to PWM mode. With the PWM pin held high, the buck
converter remains in low noise, 1MHz PWM mode.
The buck-boost converter provides continuous conduc-
tion operation to maximize efficiency and minimize noise.
At light loads, the buck-boost converter can be placed in
Burst Mode operation to improve efficiency and reduce
no-load standby current.
The LTC3522 provides a 1μA shutdown mode, overtem-
perature shutdown and current limit protection on both
converters. The LTC3522 is available in a 16-pin low profile
3mm × 3mm QFN package.
TYPICAL APPLICATION
VIN
2.4V TO 4.2V +
Li-Ion
VOUT2
1.8V
200mA
6.8μF
4.7μF
137k
L1
8.2μH
12pF
68.1k
ON
OFF
PWM
BURST
L1: COILCRAFT MSS6132-8.2μH
L2: COILCRAFT MSS6132-4.7μH
PVIN1 PVIN2
SW2 SW1A
L2
4.7μH
SW1B
LTC3522
FB2 VOUT1
SHDN2
1M
SHDN1
PWM
FB1
PGOOD2
PGOOD1
432k
PGND1 GND PGND2
3522 TA01a
VOUT1
3.3V
300mA
4.7μF
(400mA
VIN > 3V)
100
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
2.4
Efficiency vs VIN
BUCK-BOOST
IOUT = 100mA
VOUT = 3.3V
BUCK
IOUT = 100mA
VOUT = 1.8V
3.4 4.4
VIN (V)
5.4
3522 TA01b
3522f
1
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LTC3522
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Buck-Boost Feedback Voltage vs
Temperature
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
3522 G09
Buck-Boost Maximum Load
Current, PWM Mode
600
L = 4.7μH
500 VOUT = 3.3V
400
300
VOUT = 5V
200
100
0
2.4 2.9 3.4 3.9 4.4 4.9 5.4
VIN (V)
3522 G12
Buck-Boost Load Step,
0mA to 300mA
Buck Feedback Voltage vs
Temperature
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
3522 G09
Buck-Boost Maximum Load
Current, Burst Mode Operation
90
L = 4.7μH
80
70
VOUT = 3V
60
VOUT = 5V
50
40
30
20
10
0
2.4 2.9 3.4 3.9 4.4 4.9 5.4
VIN (V)
3522 G11
No Load Quiescent Current
vs VIN
50
BOTH CONVERTERS ENABLED
45
40
35
30
25
20
15
10
5
0
2.4 2.9 3.4 3.9 4.4 4.9 5.4
VIN (V)
3522 G13
Buck-Boost Burst to PWM
Transition
INDUCTOR
CURRENT
200mA/DIV
VOUT
100mV/DIV
VIN = 3.6V
VOUT = 3.3V
L = 4.7μH
COUT = 4.7μF
50μs/DIV
3522 G14
Buck Load Step, PWM Mode,
5mA to 200mA
Buck Load Step, Burst Mode
Operation, 5mA to 200mA
VOUT
100mV/DIV
VOUT
100mV/DIV
VOUT
100mV/DIV
INDUCTOR
CURRENT
200mA/DIV
VIN = 3.6V
VOUT = 3V
L = 4.7μH
COUT = 4.7μF
100μs/DIV
INDUCTOR
CURRENT
100mA/DIV
3522 G15
VIN = 3.6V
VOUT = 1.8V
L = 4.7μH
COUT = 4.7μF
100μs/DIV
INDUCTOR
CURRENT
100mA/DIV
3522 G16
VIN = 3.6V
VOUT = 1.8V
L = 4.7μH
COUT = 4.7μF
100μs/DIV
3522 G17
3522f
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LTC3522
OPERATION
LTC3522
VOUT1
VOUT
+ 1V
–
R2
FB1
R1
GND
3522 F02
Figure 2. Buck-Boost Error Amplifier and Compensation
response of the compensation network. The ratio of R2 to
R1 must be set to program the desired output voltage but
this still allows the value of R2 to be adjusted to optimize
the transient response of the converter. Increasing the value
of R2 generally leads to greater stability at the expense of
reduced transient response speed. Increasing the value of
R2 can yield substantial transient response improvement in
cases where the phase margin has been reduced due to the
use of a small value output capacitor or a large inductance
(particularly with large boost step-up ratios). Conversely,
decreasing the value of R2 increases the loop bandwidth
which can improve the speed of the converter’s transient
response. This can be useful in improving the transient
response if a large valued output capacitor is utilized. In
this case, the increased bandwidth created by decreasing
R2 is used to counteract the reduced converter bandwidth
caused by the large output capacitor.
Current Limit Operation
The buck-boost converter has two current limit circuits.
The primary current limit is an average current limit circuit
which injects an amount of current into the feedback node
which is proportional to the extent that the switch A cur-
rent exceeds the current limit value. Due to the high gain
of this loop, the injected current forces the error amplifier
output to decrease until the average current through switch
A decreases approximately to the current limit value. The
average current limit utilizes the error amplifier in an ac-
tive state and thereby provides a smooth recovery with
little overshoot once the current limit fault condition is
removed. Since the current limit is based on the average
current through switch A, the peak inductor current in
current limit will have a dependency on the duty cycle
(i.e., on the input and output voltages in the overcurrent
condition).
The speed of the average current limit circuit is limited by
the dynamics of the error amplifier. On a hard output short,
it would be possible for the inductor current to increase
substantially beyond current limit before the average cur-
rent limit circuit would react. For this reason, there is a
second current limit circuit which turns off switch A if the
current ever exceeds approximately 165% of the average
current limit value. This provides additional protection in
the case of an instantaneous hard output short.
Reverse Current Limit
The reverse current comparator on switch D monitors
the inductor current entering VOUT1. When this current
exceeds 250mA (typical) switch D will be turned off for
the remainder of the switching cycle.
Burst Mode Operation
With the PWM pin held low, the buck-boost converter
operates utilizing a variable frequency switching algorithm
designed to improve efficiency at light load and reduce
the standby current at zero load. In Burst Mode operation,
the inductor is charged with fixed peak amplitude current
pulses. These current pulses are repeated as often as
necessary to maintain the output regulation voltage. The
typical output current which can be supplied in Burst Mode
operaton is dependent upon the input and output voltage
as given by the following formula:
( )IOUT(MAX),BURST
=
0.11• VIN
VIN + VOUT
A
In Burst Mode operation, the error amplifier is not used but
is instead placed in a low current standby mode to reduce
supply current and improve light load efficiency.
Soft-Start
The buck-boost converter has an internal voltage mode
soft-start circuit with a nominal duration of 600μs. The
converter remains in regulation during soft-start and will
therefore respond to output load transients that occur
during this time. In addition, the output voltage rise time
3522f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3522.PDF ] | |
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