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PDF LT3975IMSETRPBF Data sheet ( Hoja de datos )
| Número de pieza | LT3975IMSETRPBF | |
| Descripción | 2MHz Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n Ultralow Quiescent Current:
2.7µA 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 42V
n 2.5A 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 Soft-Start Override
n Saturating Switch Design: 75mΩ On Resistance
n Small, Thermally Enhanced 16-Lead MSOP Package
APPLICATIONS
n Automotive Battery Regulation
n 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.
LT3975
42V, 2.5A, 2MHz Step-Down
Switching Regulator with
2.7µA Quiescent Current
DESCRIPTION
The LT®3975 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 42V. Low quiescent current design consumes only
2.7µA 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 LT3975 can supply up
to 2.5A 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,
and 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 LT3975 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
LT3975
SS
RT
10nF SYNC
78.7k
OUT
FB
GND
f = 600kHz
0.47µF 3.3µH
PDS560
1M
10pF
576k
3975 TA01a
VOUT
3.3V
2.5A
47µF
1210
No-Load Supply Current
4.5
IN REGULATION
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
3975 TA01b
3975f
1
1 page  
LT3975
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Thermal Derating
3.0 VIN = 12V
VOUT = 5V
2.5
2.0
I-GRADE
H-GRADE
1.5
LIMITED BY MAXIMUM
1.0 JUNCTION TEMPERATURE
θJA = 40°C/W
0.5
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3975 G10
Current Limit Foldback
6 30% DUTY CYCLE
5 VSS = 3V
4
3
2
1
0
0 0.2 0.4 0.6 0.8 1.0 1.2
FB PIN VOLTAGE (V)
3975 G13
BOOST Pin Current
50
45
40
35
30
25
20
15
10
5
0
0 0.5 1 1.5 2
SWITCH CURRENT (A)
2.5 3
3975 G16
Switch Current Limit
7
6
5
4
3
2
1
0
0 0.2 0.4 0.6 0.8 1
DUTY CYCLE
3975 G11
Soft-Start
6 30% DUTY CYCLE
5
4
VFB = 1V
VFB = 0V
3
2
1
0
0 0.5 1 1.5 2
SS PIN VOLTAGE (V)
2.5
3975 G14
Minimum On-Time
180
170
160
VSYNC = 0V
fSW = 2MHz
150
140
130
120 LOAD = 1A
110
100
90 LOAD = 2.5A
80
70
60
–55 –25
5 35 65 95 125
TEMPERATURE (°C)
155
3975 G17
Switch Current Limit
7.0
30% DUTY CYCLE
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3975 G12
Switch VCESAT
250
200
150
100
50
0
0 0.5 1 1.5 2.0 2.5 3.0
SWITCH CURRENT (A)
3975 G15
Minimum Off-Time
200
190
VSYNC = 0V
fSW = 2MHz
180
170
160
150 LOAD = 2.5A
140
130 LOAD = 1A
120
110
100
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
3975 G18
3975f
5
5 Page 
LT3975
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3975 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 LT3975 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 LT3975 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3975
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 LT3975 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 LT3975. 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)
3975 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
3975 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 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 LT3975 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 LT3975 will pulse skip at light loads. At very
3975f
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| PDF Descargar | [ Datasheet LT3975IMSETRPBF.PDF ] | |
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