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PDF LT3976 Data sheet ( Hoja de datos )
| Número de pieza | LT3976 | |
| Descripción | 2MHz Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3976 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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Electrical Specifications Subject to Change
LT3976
40V, 5A, 2MHz Step-Down
Switching Regulator with
3.3µA Quiescent Current
FEATURES
n Ultralow Quiescent Current:
3.3μA IQ at 12VIN to 3.3VOUT
n Low Ripple Burst Mode® Operation
Output Ripple < 15mVP-P
n Wide Input Range: Operation from 4.3V to 40V
n 5A Maximum Output Current
n Excellent Start-Up and Dropout Performance
n Adjustable Switching Frequency: 200kHz to 2MHz
n Synchronizable Between 250kHz to 2MHz
n Accurate Programmable Undervoltage Lockout
n Low Shutdown Current: IQ = 700nA
n Power Good Flag
n Soft-Start Capability
n Thermal Shutdown Protection
n Current Limit Foldback with SS Override
n Saturating Switch Design: 75mΩ On Resistance
n Small, Thermally Enhanced 16-Lead MSOP Package
APPLICATIONS
n Automotive Battery Regulation
n Power for Portable Products
n Industrial Supplies
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.
DESCRIPTION
The LT®3976 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 40V. Low quiescent current design consumes only
3.3μA 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. The LT3976 can supply up
to 5A of load current and has current limit foldback to
limit power dissipation during short circuit. A low dropout
voltage of 500mV is maintained when the input voltage
drops below the programmed output voltage, such as
during automotive cold crank.
An internally compensated current mode topology is used
for fast transient response and good loop stability. A high
efficiency 75mΩ switch is included on the die along with a
boost Schottky diode and the necessary oscillator, control,
andwww.DataSheet.net/ logic circuitry. An accurate 1.02V threshold enable pin
can be driven directly from a microcontroller or used as a
programmable undervoltage lockout. A capacitor on the
SS pin provides a controlled inrush current (soft-start).
A power good flag signals when VOUT reaches 91.6% of
the programmed output voltage. The LT3976 is available
in a small 16-lead MSOP package with exposed pad for
low thermal resistance.
TYPICAL APPLICATION
VIN
4.3V TO 42V
3.3V Step-Down Converter
OFF ON
10μF
VIN
EN BOOST
PG SW
LT3976
SS
RT
10nF SYNC
130k
OUT
FB
GND
f = 400kHz
0.47μF
3.3μH
PDS540
2Ω
470pF
1M
10pF
576k
VOUT
3.3V
5A
47μF
w2
3976 TA01a
No-Load Supply Current
8
IN REGULATION
7 VOUT = 3.3V
6
5
4
3
2
1
0
0 5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3976 G05
3976p
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Datasheet pdf - http://www.DataSheet4U.co.kr/
1 page  
LT3976
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Switch Current Limit
10.0
9.5
9.0
8.5
8.0
7.5
7.0
0 0.2 0.4 0.6 0.8 1.0
DUTY CYCLE
3976 G10
Soft-Start
10
3% DUTY CYCLE
9
8
VFB = 1V
VFB = 0V
7
6
5
4
3
2
1
0
0 0.5 1.0 1.5 2.0 2.5
SS PIN VOLTAGE (V)
3976 G13
Minimum On-Time
200
180
VSYNC = 0V
fSW = 2MHz
160
140
LOAD = 1A
120
100 LOAD = 2.5A
80
60
–55 –25
LOAD = 5A
5 35 65 95
TEMPERATURE (°C)
125 155
3976 G16
Switch Current Limit
12
30% DUTY CYCLE
11
10
9
8
7
6
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3976 G11
Switch VCESAT
350
300
250
200
150
100
www.DataSheet.net/
50
0
012345
SWITCH CURRENT (A)
3976 G14
Minimum Off-Time
250
225
VSYNC = 0V
fSW = 2MHz
200
175 LOAD = 5A
150 LOAD = 2.5A
125
100
–55 –25
LOAD = 1A
5 35 65 95
TEMPERATURE (°C)
125 155
3976 G17
Current Limit Foldback
100
30% DUTY CYCLE
98 VSS = 3V
96
94
92
90
88
86
84
82
80
0
0.2 0.4 0.6 0.8
FB PIN VOLTAGE (V)
1.0 1.2
3976 G12
BOOST Pin Current
90
80
70
60
50
40
30
20
10
0
012345
SWITCH CURRENT (A)
3976 G15
Switching Frequency
780
720
660
600
540
480
420
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3976 G18
3976p
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Datasheet pdf - http://www.DataSheet4U.co.kr/
5 Page 
LT3976
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3976 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current. In Burst Mode
operation the LT3976 delivers single pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. When in
sleep mode the LT3976 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3976
consumes several mA of input current in addition to the
switch current. Therefore, the total quiescent current will
be greater than 1.7μA when regulating.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3976 is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3976. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
900
800
700
600
500
400
300
200
100
0
0
VOUT = 5V
fSW = 800kHz
VOUT = 3.3V
fSW = 600kHz
20 40 60 80 100 120 140 160
LOAD CURRENT (mA)
3976 F01
Figure 1. Switching Frequency in Burst Mode Operation
diode should have less than a few μA of typical reverse
leakage 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
0.5A/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 20mA
COUT = 47μF
5μs/DIV
3976 F02
Figure 2. Burst Mode Operationwww.DataSheet.net/
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 capacitor
the output ripple is about 5mV. The output voltage ripple
can continue to be decreased by increasing the output
capacitance, though care must be taken to minimize the
effects of output capacitor ESR and ESL.
At higher output loads (above 150mA for the front page
application) the LT3976 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 is seamless, and 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 LT3976 will pulse skip at light loads. At very
3976p
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Datasheet pdf - http://www.DataSheet4U.co.kr/
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT3976.PDF ] | |
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