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PDF LT3740 Data sheet ( Hoja de datos )
| Número de pieza | LT3740 | |
| Descripción | Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3740 (archivo pdf) en la parte inferior de esta página. Total 20 Páginas | ||
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LT3740
FEATURES
Wide Operating Range,
Valley Mode, No RSENSE™
Synchronous Step-Down Controller
DESCRIPTION
n Wide VIN Range: 2.2V to 22V
n Internal Boost Provides 6V Gate Drive
For VIN Down to 2.2V
n No Sensing Resistor Required
n Dual N-Channel MOSFET Synchronous Drive
n Valley Current Mode Control
n Optimized for High Step-Down Ratio
n Power Good Output Voltage Monitor
n 0.8V Reference
n Three Pin-Selected Current Limit Levels
n Constant Switching Frequency: 300kHz
n Programmable Soft-Start
n Output Voltage Tracking
n Available in 16-Pin 5mm × 3mm DFN
APPLICATIONS
n Notebook and Palmtop Computers, PDA
n Portable Instruments
n Distributed Power Systems
The LT®3740 is a synchronous step-down switching
regulator controller that drives N-channel power MOSFET
stages. The controller uses valley current mode architecture
to achieve very low duty cycles with excellent transient
response without requiring a sense resistor.
The LT3740 includes an internal step-up converter to
provide a bias 7.8V higher than the input voltage for the
drive. This enables the part to work from an input voltage
as low as 2.2V.
The XREF pin is an external reference input that allows the
user to override the internal 0.8V feedback reference with
any lower value, allowing full control of the output voltage
during operation, output voltage tracking or soft-start.
The LT3740 has three current limit levels that can be
chosen by connecting the RANGE pin to ground, open,
and input respectively.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
wwwT.DYataPShIeCetA4UL.comAPPLICATION
High Efficiency Step-Down Converter
22μH
15k
0.22μF
LT3740
SWB
BGDP
1μF
VIN
SHDN
XREF
BIAS
TGATE
SW
SN+
BGATE
1Ω
1Ω
1Ω
M1
HAT2168H
M2
HAT2165H
15k
20k
VC
SN–
PGND
1nF RANGE GND FB
22pF
VIN
3V to 12V
10μF
0.9μH
D1
B320A
39pF
VOUT
1.8V
10A
100μF
s3
105k
80.6k
3740 TA01a
Efficiency vs Load Current
94
92
90 VIN = 3V
VIN = 5V
88
86 VIN = 12V
84
82
80
0
VOUT = 1.8V
2468
LOAD CURRENT (A)
10
3740 TA01b
3740fc
1
1 page  
LT3740
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, unless otherwise noted)
Error Amplifier Transconductance
vs Temperature
400
390
380
370
360
350
340
330
320
310
300
–50 –25 0 25 50 75 100
TEMPERATURE (°C)
3740 G12
BIAS-VIN to Enable Controller
vs Temperature
7.50
7.45
7.40
7.35
7.30
7.25
7.20
7.15
7.10
7.05
7.00
–50 –25 0 25 50 75 100
TEMPERATURE (°C)
3740 G13
Undervoltage Lockout Threshold
vs Temperature
3.0
2.5
2.0
1.5
1.0
0.5
0
–50 –25 0 25 50
TEMPERATURE (°C)
75 100
3740 G14
PIN FUNCTIONS
SN– (Pin 1): Negative Current Sensing Pin. Connect this
pin to the source of the bottom MOSFET for No RSENSE
or to a current sense resistor.
PGND (Pin 2): Power Ground. Connect this pin closely to
the source of the bottom N-channel MOSFET.
BGATE (Pin 3): Bottom Gate Drive. Drives the gate of the
wwwb.DoatttaoSmheeNt4-Uch.caonmnel MOSFET.
BGDP (Pin 4): Bottom Gate Drive Power Supply. Connect
this pin to a voltage source higher than 7V (VIN or BIAS).
SN+ (Pin 5): Positive Current Sensing Pin. Connect this
pin to the drain of the bottom MOSFET for No RSENSE or
to a current sense resistor.
SW (Pin 6): Switch Node. Connect this pin to the source
of the top N-channel MOSFET and the drain of the bottom
N-channel MOSFET.
TGATE (Pin 7): Top Gate Drive. Drives the gate of the top
N-channel MOSFET to BIAS.
BIAS (Pin 8): Top Gate Drive Power Supply. Connect a
capacitor between this pin and VIN.
SWB (Pin 9): Switch Pin of the Internal Boost. Connect
the boost inductor here.
VIN (Pin 10): Input Supply Pin. Must be locally bypassed
with a capacitor.
RANGE (Pin 11): Current Limit Range Select Pin. Ground
this pin for 50mV current sense voltage limit. Leave this
pin open for 80mV current sense voltage limit. Connect
this pin to VIN for current sense voltage limit of 105mV.
PGOOD (Pin 12): Power Good Output. Open collector
logic output that is pulled low when the FB voltage lower
than 720mV.
SHDN (Pin 13): Shutdown Pin. Connect to 2.5V or higher to
enable device; 0.5V or less to disable device. Also, this pin
functions as soft-start when a voltage ramp is applied.
XREF (Pin 14): External Reference Pin. This pin sets the
FB voltage externally between 0V and 0.8V. It can be used
to slave the output voltage during normal operation or the
output start-up behavior to an external signal source. Tie
this pin to 1V or higher to use the internal 0.8V reference.
FB (Pin 15): Feedback Pin. Pin voltage is regulated to 0.8V
if internal reference is used or to the XREF pin if voltage
is between 0V and 0.8V. Connect the feedback resistor
divider to this pin.
3740fc
5
5 Page 
LT3740
APPLICATIONS INFORMATION
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores;
instead use ferrite, molypermalloy or Kool Mμ® cores. A
variety of inductors designed for high current, low voltage
applications are available from manufacturers such as
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 4 conducts dur-
ing the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
of the bottom MOSFET from turning on and storing charge
during the dead time, which can cause a modest (about
1%) efficiency loss. The diode can be rated for about one
half of the full load current since it is on for only a fraction
of the duty cycle. In order for the diode to be effective, the
inductance between it and the bottom MOSFET must be
as small as possible, mandating that these components
be placed adjacently. Another important benefit of the
Schottky diode is that it reduces the SW node ringing at
switching edges, which reduces the noise in the circuit
and also makes the MOSFETs more reliable.
CIN and COUT Selection
The input capacitance CIN is required to filter the square
wwww.DaavtaeSchueerrt4eUn.tcoamt the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
IRMS
≈ IOUT(MAX )
•
VOUT
VIN
•
VIN − 1
VOUT
This formula has a maximum at VIN = 2VOUT, where:
IRMS
=
1
2
•
IOUT(MAX
)
This simple worst-case condition is commonly used for
design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based on only 2000 hours of life
which makes it advisable to derate the capacitor.
The selection of COUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ΔVOUT is approximately bounded by:
ΔVOUT
<
ΔlL
•
⎛
⎝⎜
ESR
+
8
• FS
1
• COUT
⎞
⎠⎟
Since ΔIL increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-sensitive applications providing that
consideration is given to ripple current ratings and long
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing. When used as input capacitors, care must be taken
to ensure that ringing from inrush currents and switching
does not pose an overvoltage hazard to the power switches
and controller. To dampen input voltage transients, add
a small 5μF to 50μF aluminum electrolytic capacitor with
an ESR in the range of 0.5Ω to 2Ω.
3740fc
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT3740.PDF ] | |
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