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PDF LT3990-5 Data sheet ( Hoja de datos )
| Número de pieza | LT3990-5 | |
| Descripción | 350mA Step-Down Regulator | |
| Fabricantes | Linear | |
| 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 62V 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
2µA IQ at 12VIN
n Accurate Programmable Undervoltage Lockout
n FMEA Fault Tolerant (MSOP Package)
Output Stays at or Below Regulation Voltage During
Adjacent Pin Short or When a Pin is Left Floating
n Low Shutdown Current: IQ = 0.7µA
n Internal Sense Limits Catch Diode Current
n Power Good Flag
n Small, Thermally Enhanced 16-Pin MSOP
and (3mm × 3mm) DFN Packages
APPLICATIONS
n Automotive Battery Regulation
n Power for Portable Products
n Industrial Supplies
LT3990/LT3990-3.3/LT3990-5
62V, 350mA Step-Down
Regulator with 2.5µA
Quiescent Current and
Integrated Diodes
DESCRIPTION
The LT®3990 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage
range up to 62V, 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 accurate programmable
undervoltage lockout feature 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
LT3990 is available in small, thermally enhanced 16-pin
MSOP and 3mm × 3mm 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
6.5V TO 62V
OFF ON
VIN BOOST
LT3990-5
EN/UVLO SW
PG BD
0.22µF
33µH
VOUT
5V
350mA
2.2µF
RT
374k
GND VOUT
f = 400kHz
22µF
3990 TA01a
1000
VIN = 12V
Power Loss
100
10
1
0.1
0.01
0.001
0.01 0.1 1 10
LOAD CURRENT (mA)
100
3990 TA01b
3990fa
1
1 page  
LT3990/LT3990-3.3/LT3990-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Maximum Load Current
600
H-GRADE
500 LIMITED BY CURRENT LIMIT
400
LIMITED BY MAXIMUM
300 JUNCTION TEMPERATURE
θJA = 45°C/W
200
100
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3990 G10
Load Regulation
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
–0.15 FRONT PAGE APPLICATION
–0.20 REFERENCED FROM VOUT AT 100mA LOAD
0 50 100 150 200 250 300 350
LOAD CURRENT (mA)
3990 G11
Switch Current Limit
900
800
SWITCH PEAK CURRENT LIMIT
700
600
500
CATCH DIODE VALLEY CURRENT LIMIT
400
300
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3990 G13
vSswTitecmh pVeCrEaStAuTre(ISW = 200mA)
300
250
200
150
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3990 G16
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)
3990 G14
Switch VCESAT
600
500
400
300
200
100
0
0 100 200 300 400 500
SWITCH CURRENT (mA)
3990 G17
Switch Current Limit
800
700 SWITCH PEAK
CURRENT LIMIT
600
500
400 CATCH DIODE VALLEY CURRENT LIMIT
300
200
0
20 40 60 80
DUTY CYCLE (%)
100
3990 G12
Minimum
Switch On-Time/Switch Off-Time
250 LOAD CURRENT = 175mA
225
200
175
MINIMUM ON-TIME
150
125
100
75 MINIMUM OFF-TIME
50
25
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3990 G15
BOOST Pin Current
21
18
15
12
9
6
3
0
0 100 200 300 400 500
SWITCH CURRENT (mA)
3990 G18
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5 Page 
LT3990/LT3990-3.3/LT3990-5
APPLICATIONS INFORMATION
If a lower dropout voltage is desired, a lower switching
frequency should be used.
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.
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 800mA. 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 LT3990 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 LT3990 and will easily handle
the ripple current. Note that larger input capacitance
is required when a lower switching frequency is used
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11
11 Page | ||
| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet LT3990-5.PDF ] | |
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