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PDF LTC1435 Data sheet ( Hoja de datos )
| Número de pieza | LTC1435 | |
| Descripción | High Efficiency Low Noise Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Dual N-Channel MOSFET Synchronous Drive
s Programmable Fixed Frequency
s Wide VIN Range: 3.5V to 36V Operation
s Ultrahigh Efficiency
s Very Low Dropout Operation: 99% Duty Cycle
s Low Standby Current
s Secondary Feedback Control
s Programmable Soft Start
s Remote Output Voltage Sense
s Logic Controlled Micropower Shutdown: IQ < 25µA
s Foldback Current Limiting (Optional)
s Current Mode Operation for Excellent Line and Load
Transient Response
s Output Voltages from 1.19V to 9V
s Available in 16-Lead Narrow SO and SSOP Packages
U
APPLICATIONS
s Notebook and Palmtop Computers, PDAs
s Cellular Telephones and Wireless Modems
s Portable Instruments
s Battery-Operated Devices
s DC Power Distribution Systems
LTC1435
High Efficiency Low Noise
Synchronous Step-Down
Switching Regulator
DESCRIPTION
The LTC®1435 is a synchronous step-down switching
regulator controller that drives external N-channel power
MOSFETs using a fixed frequency architecture. Burst
ModeTM operation provides high efficiency at low load
currents. A maximum duty cycle limit of 99% provides low
dropout operation which extends operating time in bat-
tery-operated systems.
The operating frequency is set by an external capacitor
allowing maximum flexibility in optimizing efficiency. A
secondary winding feedback control pin, SFB, guarantees
regulation regardless of load on the main output by
forcing continuous operation. Burst Mode operation is
inhibited when the SFB pin is pulled low which reduces
noise and RF interference.
Soft start is provided by an external capacitor which can
be used to properly sequence supplies. The operating
current level is user-programmable via an external current
sense resistor. Wide input supply range allows operation
from 3.5V to 30V (36V maximum).
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
COSC
68pF
CSS
0.1µF
COSC
RUN/SS
VIN
TG
CC
330pF
RC
10k
ITH
SW
LTC1435
INTVCC
100pF
SGND
VOSENSE
BOOST
BG
PGND
SENSE– SENSE+
1000pF
VIN
4.5V TO 28V
+
M1
Si4412DY
CIN
22µF
35V
×2
DB
CMDSH-3
CB
0.1µF
L1
10µH
RSENSE
0.033Ω
+
4.7µF
M2
Si4412DY
D1
MBRS140T3
VOUT
2.9V/3.5A
R1
32.4k
R2
22.1k
+
COUT
100µF
10V
×2
Figure 1. High Efficiency Step-Down Converter
1435 F01
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC1435
Burst Mode Operation
Soft Start: Load Current vs Time
VOUT
20mV/DIV
VITH
200mV/DIV
ILOAD = 50mA
1435 G16
RUN/SS
5V/DIV
INDUCTOR
CURRENT
1A/DIV
1435 G17
PIN FUNCTIONS
COSC (Pin 1): External capacitor COSC from this pin to
ground sets the operating frequency.
RUN/SS (Pin 2): Combination of Soft Start and Run
Control Inputs. A capacitor to ground at this pin sets the
ramp time to full current output. The time is approximately
0.5s/µF. Forcing this pin below 1.3V causes the device to
be shut down. In shutdown all functions are disabled.
ITH (Pin 3): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 2.5V.
SFB (Pin 4): Secondary Winding Feedback Input. Nor-
mally connected to a feedback resistive divider from the
secondary winding. This pin should be tied to: ground to
force continuous operation; INTVCC in applications that
don’t use a secondary winding; and a resistive divider from
the output in applications using a secondary winding.
SGND (Pin 5): Small-Signal Ground. Must be routed
separately from other grounds to the (–) terminal of COUT.
VOSENSE (Pin 6): Receives the feedback voltage from an
external resistive divider across the output.
SENSE – (Pin 7): The (–) Input to the Current Comparator.
SENSE + (Pin 8): The (+) Input to the Current Comparator.
Built-in offsets between SENSE– and SENSE+ pins in
conjunction with RSENSE set the current trip thresholds.
EXTVCC (Pin 9): Input to the Internal Switch Connected to
INTVCC. This switch closes and supplies VCC power when-
ever EXTVCC is higher than 4.7V. See EXTVCC connection
in Applications Information section. Do not exceed 10V on
this pin. Connect to VOUT if VOUT ≥ 5V.
PGND (Pin 10): Driver Power Ground. Connects to source
of bottom N-channel MOSFET and the (–) terminal of CIN.
BG (Pin 11): High Current Gate Drive for Bottom
N-Channel MOSFET. Voltage swing at this pin is from
ground to INTVCC.
INTVCC (Pin 12): Output of the Internal 5V Regulator and
EXTVCC Switch. The driver and control circuits are pow-
ered from this voltage. Must be closely decoupled to power
ground with a minimum of 2.2µF tantalum or electrolytic
capacitor.
VIN (Pin 13): Main Supply Pin. Must be closely decoupled
to the IC’s signal ground pin.
SW (Pin 14): Switch Node Connection to Inductor. Volt-
age swing at this pin is from a Schottky diode (external)
voltage drop below ground to VIN.
BOOST (Pin 15): Supply to Topside Floating Driver. The
bootstrap capacitor is returned to this pin. Voltage swing
at this pin is from INTVCC to VIN + INTVCC.
TG (Pin 16): High Current Gate Drive for Top N-Channel
MOSFET. This is the output of a floating driver with a
voltage swing equal to INTVCC superimposed on the
switch node voltage SW.
5
5 Page 
LTC1435
APPLICATIONS INFORMATION
INTVCC Regulator
An internal P-channel low dropout regulator produces the
5V supply which powers the drivers and internal circuitry
within the LTC1435. The INTVCC pin can supply up to
15mA and must be bypassed to ground with a minimum
of 2.2µF tantalum or low ESR electrolytic. Good bypassing
is necessary to supply the high transient currents required
by the MOSFET gate drivers.
High input voltage applications, in which large MOSFETs
are being driven at high frequencies, may cause the
maximum junction temperature rating for the LTC1435 to
be exceeded. The IC supply current is dominated by the
gate charge supply current when not using an output
derived EXTVCC source. The gate charge is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1435 is
limited to less than 17mA from a 30V supply:
TJ = 70°C + (17mA)(30V)(100°C/W) = 126°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at maximum VIN.
EXTVCC Connection
The LTC1435 contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins.
The switch closes and supplies the INTVCC power when-
ever the EXTVCC pin is above 4.8V, and remains closed
until EXTVCC drops below 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.8V < VOUT < 9V) and from the
internal regulator when the output is out of regulation
(start-up, short circuit). Do not apply greater than 10V to
the EXTVCC pin and ensure that EXTVCC < VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN current resulting
from the driver and control currents will be scaled by a
factor of Duty Cycle/Efficiency. For 5V regulators this
supply means connecting the EXTVCC pin directly to VOUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTVCC power
from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5V regulator resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage which has been boosted to
greater than 4.8V. This can be done with either the
inductive boost winding as shown in Figure 4a or the
capacitive charge pump shown in Figure 4b. The charge
pump has the advantage of simple magnetics.
4. EXTVCC connected to an external supply. If an external
supply is available in the 5V to 10V range (EXTVCC ≤
VIN), it may be used to power EXTVCC providing it is
compatible with the MOSFET gate drive requirements.
When driving standard threshold MOSFETs, the exter-
nal supply must always be present during operation to
prevent MOSFET failure due to insufficient gate drive.
+
CIN
VIN
VIN
OPTIONAL
EXT VCC
CONNECTION
5V ≤ VSEC ≤ 9V
TG
EXTVCC
R6 LTC1435
SFB SW
R5
SGND
BG
PGND
N-CH
N-CH
1N4148
L1
1:N
VSEC
+
RSENSE
+
1µF
VOUT
COUT
LTC1435 • F04a
Figure 4a. Secondary Output Loop and EXTVCC Connection
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC1435.PDF ] | |
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