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PDF LT3970-3.3 Data sheet ( Hoja de datos )
| Número de pieza | LT3970-3.3 | |
| Descripción | 350mA Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n Low Ripple Burst Mode® Operation
2.5μA IQ at 12VIN to 3.3VOUT
Output Ripple < 5mVP-P
n Wide Input Voltage Range: 4.2V to 40V Operating
n Adjustable Switching Frequency: 200kHz to 2.2MHz
n Integrated Boost and Catch Diodes
n 350mA Output Current
n Fixed Output Voltages: 3.3V, 5V
1.8μA IQ at 12VIN
n Accurate 1V Enable Pin Threshold
n Low Shutdown Current: IQ = 0.7μA
n Internal Sense Limits Catch Diode Current
n Power Good Flag
n Output Voltage: 1.21V to 25V
n Internal Compensation
n Small 10-Pin MSOP and (3mm × 2mm) DFN Packages
APPLICATIONS
n Automotive Battery Regulation
n Power for Portable Products
n Industrial Supplies
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LT3970/LT3970-3.3/LT3970-5
40V, 350mA Step-Down
Regulator with 2.5µA
Quiescent Current and
Integrated Diodes
DESCRIPTION
The LT®3970 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage
range up to 40V, and consumes only 2.5μA of quiescent
current. A high efficiency switch is included on the die
along with the catch diode, boost diode, and the neces-
sary oscillator, control and logic circuitry. Low ripple Burst
Mode operation maintains high efficiency at low output
currents while keeping the output ripple below 5mV in a
typical application. Current mode topology is used for fast
transient response and good loop stability. A catch diode
current limit provides protection against shorted outputs
and overvoltage conditions. An enable pin with accurate
threshold is available, producing a low shutdown current
of 0.7μA. A power good flag signals when VOUT reaches
90% of the programmed output voltage. The LT3970 is
available in small 10-pin MSOP and 3mm × 2mm DFN
packages.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
5V Step-Down Converter
VIN
6V TO 40V
OFF ON
VIN BOOST
LT3970
EN SW
PG BD
2.2μF
RT FB
226k
GND
f = 600kHz
0.22μF
22μH
22pF
1M
316k
VOUT
5V
350mA
22μF
3490 TA01a
90
VIN = 12V
80
Efficiency
1000
100
70 10
60 1
50 0.1
40
0.01
0.1 1 10
LOAD CURRENT (mA)
0.01
100
3970 TA01b
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LT3970/LT3970-3.3/LT3970-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
No-Load Supply Current
15
FRONT PAGE APPLICATION
VIN = 12V
12
VOUT = 3.3V
R1 = 1M
R2 = 576k
9
6
3
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3970 G05
Maximum Load Current
550
FRONT PAGE APPLICATION
VOUT = 3.3V
TYPICAL
500
450 MINIMUM
400
350
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3870 G06
Maximum Load Current
600
FRONT PAGE APPLICATION
VOUT = 5V
550 TYPICAL
500
MINIMUM
450
400
350
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3870 G07
Maximum Load Current
600
LIMITED BY CURRENT LIMIT H GRADE
500
400
LIMITED BY MAXIMUM
300 JUNCTION TEMPERATURE;
θJA = 76°C/W
200
100 FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
0
–50 –25 0 25 50 75
100
TEMPERATURE (°C)
125 150
3970 G08
Load Regulation
0.20
0.15
0.10
0.05
0
–0.05
–0.10
–0.15 FRONT PAGE APPLICATION
REFERENCED FROM VOUT AT 100mA LOAD
–0.20
0 50 100 150 200 250 300 350
LOAD CURRENT (mA)
3970 G09
Switch Current Limit
800
SWITCH PEAK CURRENT LIMIT
700
600
500
CATCH DIODE VALLEY CURRENT LIMIT
400
300
200
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (oC)
3970 G11
Switching Frequency
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3970 G12
Switch Current Limit
800
700 SWITCH PEAK
CURRENT LIMIT
600
500
CATCH DIODE VALLEY CURRENT LIMIT
400
300
200
0
20 40 60 80
DUTY CYCLE (%)
100
3970 G10
Minimum
Switch On-Time/Switch Off-Time
200
LOAD CURRENT = 175mA
180
160
140
120 MINIMUM OFF-TIME
100
80
MINIMUM ON-TIME
60
40
20
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3970 G13
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LT3970/LT3970-3.3/LT3970-5
APPLICATIONS INFORMATION
The highest allowed VIN during normal operation
(VIN(OP-MAX)) is limited by minimum duty cycle and can
be calculated by the following equation:
VIN(OP-MAX)
=
VOUT + VD
fSW • tON(MIN)
–
VD
+
VSW
where tON(MIN) is the minimum switch on-time (~150ns).
However, the circuit will tolerate inputs up to the absolute
maximum ratings of the VIN and BOOST pins, regardless of
chosen switching frequency. During such transients where
VIN is higher than VIN(OP-MAX), the switching frequency will
be reduced below the programmed frequency to prevent
damage to the part. The output voltage ripple and inductor
current ripple may also be higher than in typical operation,
however the output will still be in regulation.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher VIN or VOUT and
decreases with higher inductance and faster switching
frequency. A good starting point for selecting the induc-
tor value is:
L
=
3
VOUT +
fSW
VD
Table 2. Inductor Vendors
VENDOR
Coilcraft
Sumida
Toko
Würth Elektronik
Coiltronics
Murata
URL
www.coilcraft.com
www.sumida.com
www.tokoam.com
www.we-online.com
www.cooperet.com
www.murata.com
where VD is the voltage drop of the catch diode (~0.7V),
L is in μH and fSW is in MHz. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be about 30% higher.
For robust operation in fault conditions (start-up or short
circuit) and high input voltage (>30V), the saturation
current should be above 500mA. To keep the efficiency
high, the series resistance (DCR) should be less than
0.1Ω, and the core material should be intended for high
frequency applications. Table 2 lists several vendors and
suitable types.
This simple design guide will not always result in the
optimum inductor selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the ap-
plication requires less than 350mA load current, then a
lesser inductor value may be acceptable. This allows use
of a physically smaller inductor, or one with a lower DCR
resulting in higher efficiency. There are several graphs in
the Typical Performance Characteristics section of this data
sheet that show the maximum load current as a function
of input voltage for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu-
ous mode operation, see Linear Technology Application
Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See Application Note 19.
Input Capacitor
Bypass the input of the LT3970 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 1μF to 4.7μF ceramic capacitor
is adequate to bypass the LT3970 and will easily handle
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| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet LT3970-3.3.PDF ] | |
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