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PDF LT3971-5 Data sheet ( Hoja de datos )
| Número de pieza | LT3971-5 | |
| Descripción | 2MHz Step-Down Regulator | |
| Fabricantes | Linear | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3971-5 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
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FEATURES
n Ultralow Quiescent Current:
2.8µA IQ Regulating 12VIN to 3.3VOUT
n Fixed Output Voltages: 3.3V, 5V,
2.1µA IQ Regulating 12VIN to 3.3VOUT
n Low Ripple Burst Mode® Operation:
Output Ripple < 15mVP-P
n Wide Input Voltage Range: 4.3V to 38V
n 1.2A Maximum Output Current
n Adjustable Switching Frequency: 200kHz to 2MHz
n Synchronizable Between 250kHz to 2MHz
n Fast Transient Response
n Accurate 1V Enable Pin Threshold
n Low Shutdown Current: IQ = 700nA
n Power Good Flag
n Soft-Start Capability
n Internal Compensation
n Output Voltage: 1.19V to 30V
n Small Thermally Enhanced 10-Lead MSOP, 16-Lead
MSOP and (3mm × 3mm) DFN Packages
APPLICATIONS
n Automotive Battery Regulation
n Power for Portable Products
n Industrial Supplies
LT3971/LT3971-3.3/LT3971-5
38V, 1.2A, 2MHz
Step-Down Regulator with
2.8µA Quiescent Current
DESCRIPTION
The LT®3971 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 38V. Low quiescent current design consumes only
2.8µA of supply current while regulating with no load. Low
ripple Burst Mode operation maintains high efficiency at
low output currents while keeping the output ripple below
15mV in a typical application. An internally compensated
current mode topology is used for fast transient response
and good loop stability. A high efficiency 0.33Ω switch
is included on the die along with a boost Schottky diode
and the necessary oscillator, control and logic circuitry.
An accurate 1V threshold enable pin can be used to shut
down the LT3971, reducing the input supply current to
700nA. A capacitor on the SS pin provides a controlled
inrush current (soft-start). A power good flag signals
when VOUT reaches 91% of the programmed output volt-
age. The LT3971 is available in small 10-lead MSOP and
3mm × 3mm DFN packages with exposed pads for low
thermal resistance. A 16-lead MSOP is also offered which
has enhanced pin-to-pin fault tolerance.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
VIN
4.5V TO 38V
3.3V Step Down Converter
OFF ON
EN
VIN BOOST
4.7µF
PG
SS LT3971-3.3 SW
RT
BD
49.9k
SYNC
f = 800kHz
GND VOUT
0.47µF 4.7µH
22µF
VOUT
3.3V
1.2A
3971 TA01
No Load Supply Current
3.0
OUTPUT IN REGULATION
2.5 LT3971-5
2.0
LT3971-3.3
1.5
1.0
5
10 15 20 25 30 35
INPUT VOLTAGE (V)
3971 TA01b
3971fd
1
1 page  
LT3971/LT3971-3.3/LT3971-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
LT3971 Feedback Voltage
1.205
1.200
1.195
1.190
1.185
1.180
1.175
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3971 G07
LT3971-3.3 Output Voltage
3.345
3.330
3.315
3.300
3.285
3.270
3.255
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3971 G28
LT3971-5 Output Voltage
5.06
5.04
5.02
5.00
4.98
4.96
4.94
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3971 G29
Maximum Load Current
3.0 VOUT = 3.3V
2.5
TYPICAL
2.0
MINIMUM
1.5
1.0
0.5
0
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3971 G08
Maximum Load Current
2.5 VOUT = 5V
2.0
TYPICAL
1.5 MINIMUM
1.0
0.5
0
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3971 G09
Load Regulation
0.30
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
REFERENCED FROM VOUT AT 0.5A LOAD
0 200 400 600 800 1000 1200
LOAD CURRENT (mA)
3971 G10
Switching Frequency
1000
950
900
850
800
750
700
650
600
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3971 G11
Switch Current Limit
3.0
2.5
2.0
1.5
1.0
0.5
0
0 20 40 60 80 100
DUTY CYCLE (%)
3971 G12
Switch Current Limit
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
DUTY CYCLE = 30%
1.5
–55 –25 5 35 65 95
TEMPERATURE (°C)
125 155
3971 G13
3971fd
5
5 Page 
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3971. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1µA of typical reverse leak-
age at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
VSW
5V/DIV
IL
500mA/DIV
VOUT
20mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
5µs/DIV
3971 F02
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple
will be inversely proportional to the output capacitance.
In a typical application with a 22μF output capacitor, the
output ripple is about 10mV, and with a 47μF output ca-
pacitor the output ripple is about 5mV. The output voltage
ripple can continue to be decreased by increasing the
output capacitance.
At higher output loads (above 92mA for the front page
application) the LT3971 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and low
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3971 will pulse skip at light loads. The qui-
escent current will significantly increase to 1.5mA in light
load situations when synchronized with an external clock.
Holding the SYNC pin high yields no advantages in terms
of output ripple or minimum load to full frequency, so is
not recommended.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=
R2
VOUT
1.19V
−
1
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from VOUT to FB.
The LT3971-3.3 and LT3971-5 contain an internal 10M FB
resistor divider as well as an internal phase lead capacitor.
Setting the Switching Frequency
The LT3971 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Table 1.
3971fd
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT3971-5.PDF ] | |
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