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PDF ( 数据手册 , 数据表 ) LX1682CDM

零件编号 LX1682CDM
描述 VOLTAGE - MODE PWM CONTROLLERS
制造商 Microsemi Corporation
LOGO Microsemi Corporation LOGO 


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LX1682CDM 数据手册, 描述, 功能
T H E I N F I N I T E P O W E R O F I N N O VAT I O N
LX1681/1682
VOLTAGE-MODE PWM CONTROLLERS
PRODUCTION DATA SHEET
DESCRIPTION
The LX1681/1682 are monolithic, pulse-
width modulator controller ICs. They are
designed to implement a flexible, low cost
buck (step-down) regulator supply with mini-
mal external components.
The LX1681 is a non-synchronous con-
troller; the LX1682 has a synchronous driver
for higher efficiency.
The output voltage is adjustable by
means of a resistor divider to set the voltage
between 1.25V and 4.5V.
Short-circuit current limiting can be
implemented without expensive current
sense resistors. Current is sensed using the
voltage drop across the RDS(ON) of the MOSFET
— sensing is delayed for 1µs to eliminate
MOSFET ringing errors.
Hiccup-mode fault protection reduces
average power to the power elements during
short-circuit conditions.
Switching frequency is fixed at 200kHz
for optimal cost and space.
Under-voltage lockout and soft-start
for optimal start-up performance. The
LX1681/82 can be disabled by pulling the soft-
start pin to ground.
Small 8-pin SOIC packaging reduces
board space.
Optimized for 5V-to-3.3V or 5V-to-2.5V
conversion, the LX1681/82 can also be used
for converting 12V to 5V, 3.3V or other
voltages with high efficiency, eliminating the
need for bulky heat sinks.
NOTE: For current data & package dimensions, visit our web site: http://www.linfinity.com.
KEY FEATURES
s Fixed 200kHz Switching Frequency
s Constant Frequency Voltage-Mode Control
Requires NO External Compensation
s Hiccup-Mode Over-Current Protection
s High Efficiency
s Output Voltage Set By Resistor Divider
s Under-Voltage Lockout
s Soft-Start And Enable
s Synchronous Rectification (LX1682)
s Non-Synchronous Rectification (LX1681)
s Small, 8-pin Surface Mount Package
A P P L I C AT I O N S
s 5V to 3.3V Or Less Buck Regulators
s FPGA Supplies
s Microprocessor Chipset Supplies
(e.g. Camino, Whitney, etc.)
s Rambus® RIMMTM Supplies
s Hard Disk Drives
s Computer Add-on Cards
VBOOST
12V
PRODUCT HIGHLIGHT
VIN
5V
VBOOST
12V
VIN
5V
C3
1µF
1 VFB
CSS
2 SS
VCC 8
CS 7
0.1µF
LX1681
3 N.C.
VC1 6
4 GND
TDRV 5
C1
1500µFx3
Q1
IRL3103S
L1 C2
5µH 1500µF
x3
RSET
D2
MBR2545
VOUT
R1
R2
C3
1µF
1 VFB
CSS
2 SS
VCC 8
CS 7
0.1µF
LX1682
3 GND
VC1 6
4 BDRV
TDRV 5
C1
1500µFx2
Q1
IRL3103S
L1 C2
5µH 1500µF
x3
RSET
Q2
IRL3103S
VOUT
R1
R2
LX1681 NON-SYNCHRONOUS CONTROLLER
LX1682 SYNCHRONOUS CONTROLLER
Copyright © 1999
Rev. 1.0 5/99
PA C K A G E O R D E R I N F O R M AT I O N
TA (°C)
Output
DM
Plastic
8-pin
SOIC
0 to 70
Non-Synchronous
Synchronous
LX1681CDM
LX1682CDM
Note: All surface-mount packages are available in Tape & Reel,
append the letter "T" to part number. (i.e. LX1681CDMT)
LINFINITY MICROELECTRONICS INC.
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841, 714-898-8121, FAX: 714-893-2570
1







LX1682CDM pdf, 数据表
PRODUCT DATABOOK 1996/1997
LX1681/1682
VOLTAGE-MODE PWM CONTROLLERS
PRODUCTION DATA SHEET
APPLICATION INFORMATION
FET SELECTION (continued)
Synchronous Rectification – Lower MOSFET
The lower pass element can be either a MOSFET or a Schottky
diode. The use of a MOSFET (synchronous rectification) will result
in higher efficiency, but at higher cost than using a Schottky diode
(non-synchronous).
Power dissipated in the bottom MOSFET will be:
PD = I2 * RDS(ON) * [1 - Duty Cycle] = 3.51W
[IRL3303 or 1.76W for the IRL3102]
Non-Synchronous Operation - Schottky Diode
A typical Schottky diode, with a forward drop of 0.6V will dissipate
0.6 * 15 * [1 – 2/5] = 5.4W (compared to the 1.8 to 3.5W dissipated
by a MOSFET under the same conditions).
This power loss becomes much more significant at lower duty
cycles. The use of a dual Schottky diode in a single TO-220
package (e.g. the MBR2535) helps improve thermal dissipation.
Operation From A Single Power Supply
The LX1681/1682 needs a secondary supply voltage (VC1) to
provide sufficient drive to the upper MOSFET. In many applica-
tions with a 5V (VCC) and a 12V (VC1) supply are present. In
situations where only 5V is present, VC1 can be generated using
a bootstrap (charge pump) circuit, as shown in Figure 4 (Typical
Applications section).
The capacitor (C4) is alternatively charged up from VCC via the
Schottky diode (D2), and then boosted up when the FET is turned
on. This scheme provedes a VC1 voltage equal to 2* VCC - VDS(D2),
or approximately 9.5V with VCC = 5V. This voltage will provide
sufficient gate drive to the external MOSFET in order to get a low
RDS(ON). Note that using the bootstrap circuit in synchronous
rectification mode is likely to result in faster turn-on than in non-
synchronous mode.
LAYOUT GUIDELINES - THERMAL DESIGN
A great deal of time and effort were spent optimizing the thermal
design of the demonstration boards. Any user who intends to
implement an embedded motherboard would be well advised to
carefully read and follow these guidelines. If the FET switches
have been carefully selected, external heatsinking is generally not
required. However, this means that copper trace on the PC board
must now be used. This is a potential trouble spot; as much
copper area as possible must be dedicated to heatsinking the FET
switches, and the diode as well if a non-synchronous solution is
used.
In our VRM module, heatsink area was taken from internal
ground and VCC planes which were actually split and connected
with VIAS to the power device tabs. The TO-220 and TO-263
cases are well suited for this application, and are the preferred
packages. Remember to remove any conformal coating from all
exposed PC traces which are involved in heatsinking.
5V Input
LX168x
GND
Output
FIGURE 2 — Enabling Linear Regulator
General Notes
As always, be sure to provide local capacitive decoupling close to
the chip. Be sure use ground plane construction for all high-
frequency work. Use low ESR capacitors where justified, but be
alert for damping and ringing problems. High-frequency designs
demand careful routing and layout, and may require several
iterations to achieve desired performance levels.
Power Traces
To reduce power losses due to ohmic resistance, careful consid-
eration should be given to the layout of traces that carry high
currents. The main paths to consider are:
s Input power from 5V supply to drain of top MOSFET.
s Trace between top MOSFET and lower MOSFET or Schottky
diode.
s Trace between lower MOSFET or Schottky diode and ground.
s Trace between source of top MOSFET and inductor and load.
All of these traces should be made as wide and thick as possible,
in order to minimize resistance and hence power losses. It is also
recommended that, whenever possible, the ground, input and
output power signals should be on separate planes (PCB layers).
See Figure 2 – bold traces are power traces.
Layout Assistance
Please contact Linfinity’s Applications Engineers for assistance
with any layout or component selection issues. A Gerber file with
layout for the most popular devices is available upon request.
Evaluation boards are also available upon request. Please
check Linfinity's web site for further application notes.
8 Copyright © 1999
Rev. 1.0 5/99














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