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부품번호 LX1682 기능
기능 VOLTAGE - MODE PWM CONTROLLERS
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LX1682 데이터시트, 핀배열, 회로
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




LX1682 pdf, 반도체, 판매, 대치품
PRODUCT DATABOOK 1996/1997
LX1681/1682
VOLTAGE-MODE PWM CONTROLLERS
PRODUCTION DATA SHEET
Pin
Name
VFB
SS
GND
TDRV
BDRV
VC1
CS
VCC
FUNCTIONAL PIN DESCRIPTION
Description
Voltage feedback — 1.25V reference is connected to a resistor divider to set desired
output voltage.
Soft-start and hiccup capacitor pin. During start up the voltage of this pin controls the
output voltage. An internal 20kresistor and the external capacitor set the time constant
for soft-startup. Soft-start does not begin until the supply voltage exceeds the UVLO
threshold. When over-current occurs, this capacitor is used for timing hiccup.
The PWM can be disabled by pulling the SS pin below 0.3V.
Ground for IC
Gate drive for upper MOSFET
Gate drive for lower MOSFET
Separate supply for MOSFET gate drive. Connect to 12V
Over-current set. Connect resistor between CS pin and the source of the upper MOSFET to
set current-limit point.
IC supply voltage (nominal 5V) and high side drain sense voltage.
Pin Number
LX1681 LX1682
11
22
43
54
5
66
77
88
4 Copyright © 1999
Rev. 1.0 5/99

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LX1682 전자부품, 판매, 대치품
PRODUCT DATABOOK 1996/1997
VOLTAGE-MODE PWM CONTROLLERS
PRODUCTION DATA SHEET
LX1681/1682
APPLICATION INFORMATION
SOFT-START CAPACITOR (continued)
capacitor current plus the load current. The soft-start capacitor
should be selected so that the overall inductor current does not
exceed it maximum.
The capacitor current to follow the SS-pin voltage is:
ICout = COUT
dV
dt
=
COUT
CSS
* e-(t/RssCss )
where COUT is the output capacitance. The typical value of CSS
should be in the range of 0.1 to 0.2µF.
During the soft-start interval the load current from a micro-
processor is negligible; therefore, the capacitor current is ap-
proximately the required inductor current.
OVER-CURRENT PROTECTION
Current limiting occurs at current level ICL, when the voltage
detected by the current sense comparator is greater than the
current sense comparator threshold, VTRIP (400mV).
ICL * RDS(ON) + ISET * RSET = VTRIP
So, VTRIP - ICL * RDS(ON) 400mV - ICL * RDS(ON)
RSET = ISET =
45µA
Example:
For 10A current limit, using IRL3303 MOSFET (26mRDS(ON)):
RSET =
0.4 - 10 * 0.026
45 * 10-6
= 3.1k
Current Sensing Using Sense Resistor
The method of current sensing using the RDS(ON) of the upper
MOSFET is economical, but can have a large tolerance, since the
RDS(ON) can vary with temperature, etc. A more accurate alterna-
tive is to use an external sense resistor (RSENSE). Since one input
to the current sense comparator is the supply voltage to the IC
(VCC - pin 8), the sense resistor could be a PCB trace (for
construction details, see Application Note AN-10 or LX1668 data
sheet).
The over-current trip point is calculated as in the equations
above, replacing RDS(ON) with RSENSE.
Example:
For 10A current limit, using a 5msense resistor:
RSET
=
VTRIP
-
(ICL *
ISET
RSENSE )
0.4 - 10 * 0.005
=
45 x 10-6
= 7.8k
OUTPUT ENABLE
The LX1681/82 FET driver outputs are driven to ground by
pulling the soft-start pin below 0.3V.
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage is sensed by the feedback pin (VFB) which has
a 1.25V reference. The output voltage can be set to any voltage
above 1.25V (and lower than the input voltage) by means of a
resistor divider (see Product Highlight).
VOUT = VREF (1 + R1 /R2 )
Note: Keep R1 and R2 close to 100(order of magnitude).
FET SELECTION
To insure reliable operation, the operating junction temperature
of the FET switches must be kept below certain limits. The Intel
specification states that 115°C maximum junction temperature
should be maintained with an ambient of 50°C. This is achieved
by properly derating the part, and by adequate heat sinking. One
of the most critical parameters for FET selection is the RDS(ON)
resistance. This parameter directly contributes to the power
dissipation of the FET devices, and thus impacts heat sink design,
mechanical layout, and reliability. In general, the larger the
current handling capability of the FET, the lower the RDS(ON) will
be, since more die area is available.
TABLE 1 - FET Selection Guide
This table gives selection of suitable FETs from International Rectifier.
Device
IRL3803
RDS(ON) @
10V (m)
6
ID @
TC = 100°C
83
Max. Break-
down Voltage
30
IRL22203N
7
71 30
IRL3103
14
40
30
IRL3102
13
56
20
IRL3303
26
24
30
IRL2703
40
17
30
All devices in TO-220 package. For surface mount devices (TO-263 /
D2-Pak), add 'S' to part number, e.g. IRL3103S.
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
PD = (I2 * RDS(ON) * Duty Cycle) + (0.5 * I * VIN * tSW * fS )
Where tSW is switching transition line for body diode (~100ns)
and fS is the switching frequency.
For the IRL3102 (13mRDS(ON)), converting 5V to 2.0V at 15A
will result in typical heat dissipation of 1.92W.
Copyright © 1999
Rev. 1.0 5/99
7

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