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PDF ( 数据手册 , 数据表 ) LX1684CD

零件编号 LX1684CD
描述 Voltage-Mode PWM Controller
制造商 Microsemi Corporation
LOGO Microsemi Corporation LOGO 


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LX1684CD 数据手册, 描述, 功能
LINFINITY DIVISION
LX1684
Voltage-Mode PWM Controller
PRODUCTION
DESCRIPTION
The LX1684 is a monolithic, voltage-
mode pulse-width modulator controller.
It is designed to implement a flexible,
low cost buck (step-down) regulator
supply with a minimal of external
components.
The LX1684 has a synchronous
driver for higher efficiency and is
optimized to provide 12V to 3.3V or
12V to 2.5V regulation. It also can be
used to convert 5V or 3.3V to voltages
as low as 1.25V.
Switching frequency is fixed at
175kHz for optimal cost and space.
Short-circuit current limiting can be
implemented without expensive current
sense resistors.
Similar to the LX1682 in function but
with the positive side of the current
sense comparator brought out (CSP) to
allow it to be referenced to the topside
FET.
Current is sensed using the voltage
drop across the RDS(ON) of the MOSFET
this 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.
Under-voltage lockout and soft-start
are provided for optimal start-up
performance. Pulling the soft-start pin to
ground can disable the LX1684.
The small 14-pin SOIC packaging,
combined with low profile, low ESR
capacitors, and TO-252 packaged FETs
results in a high efficiency converter in a
small board area. In most cases PCB
copper can accomplish necessary heat
sinking and no bulky additional heat
sinks are required.
If a low profile design is not required
small electrolytic capacitors can be used
reducing the overall converter cost.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
KEY FEATURES
! Fixed 175kHz Switching
Frequency
! Constant Frequency Voltage-
Mode Control Requires No
External Compensation
! Hiccup-Mode Over-Current
Protection
! High Efficiency
! Output Voltage Set By
Resistor Divider
! Under-Voltage Lockout
! Soft-Start And Enable
! Synchronous Rectification
! Small, 14-pin Surface Mount
Package
APPLICATIONS
! 12V to 3.3V Or Less Buck
Regulators
! 3.3/5V to 2.5V Or Less Buck
Regulators
! Hard Disk Drives
! Computer Add-on Cards
PRODUCT HIGHLIGHT - TYPICAL 12V TO 3.3V APPLICATION
12V
5V 10%
5%
R1
12V 165 Ohm
LM78L05
or equivalent
5V
TO
VCC
Optional 12V only circuit
R2 Css
100 Ohm 0.1µF
1
FB
2
NC
3
SS
4
CSP
5
GND
6
PGND
7
BDR
C5
1µF
14
VCC
13
NC RSET
12 3.3K
CS
11
NC
10
VC1
9
NC
8
TDR
C1
0.1µF
D1
5817SMT
3x
820µF, C4
16V
Q1
SUB45N
05-20L
L1 is a powdered
iron toroid
Q2
SUB45N05-
20L
L1
10µH
4x
1500µF
6.3V
VOUT
3.3V
10A
C3
Copyright 2000
Rev. 1.0, 2001-09-19
PACKAGE ORDER INFO
TA (°C)
OUTPUT
Plastic SOIC
D 14-PIN
0 to 70
Synchronous
LX1684CD
Note: Available in Tape & Reel.
Append the letter “T” to the part number. (i.e. LX1684CDT)
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1







LX1684CD pdf, 数据表
LINFINITY DIVISION
LX1684
Voltage-Mode PWM Controller
PRODUCTION
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 = I 2 × RDS(ON) × 1 Duty Cycle = 3.26W
[SUB45N03-13L or 2.12W for the SUB70N03-09BP]
Non-Synchronous Operation - Schottky Diode
A typical Schottky diode, with a forward drop of 0.6V will
dissipate 0.6 * 15 * [1 – 3.3/12] = 6.5W (compared to the 2.1 to
4.2W 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.
Boost Operation
The LX1684 needs a secondary supply voltage (VC1) to provide
sufficient drive to the upper MOSFET. The top FET must be a
logic level power MOSFET such as SUB45N03-13L. It must be able
to turn on to a low RDS(ON) with VGS of 4.5V or higher. VC1 can be
generated using a bootstrap (charge pump) circuit, as shown in the
Product Highlight on page 1. The capacitor, (C1) is alternatively
charged up from 5V via the Schottky diode, (D1), and then boosted
up when the FET is turned on. Under any circumstance the voltage
at VC1 should not be more than 18V for more than 300nS and must
not be greater than 19V for more than 50nS. Lab evaluation and
module production test should be the final arbiter to verify the
proper operation. For application with a large MOSFET, the
maximum voltage at VC1 should be kept lower due to thermal
dissipation in the FET driver section. It is inherent in a higher
current power supply that the parasitic inductance and capacitance
on PCB board and Power MOSFET device induces ringing at the
gate drive. The extra thermal dissipation and the higher peak
voltage generated by gate ringing should be taken in account
during final design. The temperature rise due to gate drive thermal
dissipation can be reduced by extra heat sinking. A resistor in
series with the gate in order of 10ohm or snubber circuitry can
reduce the gate ringing. The voltage must provide sufficient gate
drive to the external MOSFET in order to get a low RDS(ON) and
MUST be lower than maximum voltage rating of 18V.
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..
+12V Input
LX1684
O utput
GND
FIGURE 2 — Key Power PCB Traces
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:
! Input power from 12V supply to drain of top MOSFET.
! Trace between top MOSFET and lower MOSFET or
Schottky diode.
! Trace between lower MOSFET or Schottky diode and
ground.
! 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.
Copyright 2000
Rev. 1.0, 2001-09-19
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 8














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