LINFINITY LX1681, LX1682 DATA SHEET
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LX1681/1682
VOLTAGE-MODE PWM CONTROLLERS
T HE I NFINITE P OWER OF I NNOVATION
DESCRIPTION KEY FEATURES
The LX1681/1682 are monolithic, pulsewidth modulator controller ICs. They are
designed to implement a flexible, low cost
buck (step-down) regulator supply with minimal 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 R
— sensing is delayed for 1µs to eliminate
of the MOSFET
DS(ON)
MOSFET ringing errors.
NOTE: For current data & package dimensions, visit our web site: http://www.linfinity.com.
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 softstart 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.
P RODUCTION DATA SHEET
■ Fixed 200kHz 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 (LX1682)
■ Non-Synchronous Rectification (LX1681)
■ Small, 8-pin Surface Mount Package
APPLICATIONS
■ 5V to 3.3V Or Less Buck Regulators
■ FPGA Supplies
■ Microprocessor Chipset Supplies
(e.g. Camino, Whitney, etc.)
■ Rambus® RIMMTM Supplies
■ Hard Disk Drives
■ Computer Add-on Cards
PRODUCT HIGHLIGHT
V
V
CS
V
TDRV
BOOST
12V
C
3
1µF
8
CC
7
6
C1
54
C
SS
0.1µF
V
BOOST
12V
C
3
1µF
1
V
FB
2
SS
LX1681
3
N.C.
GND
V
CS
V
TDRV
8
CC
7
6
C1
IRL3103S
54
R
SET
Q
1
V
IN
5V
C
1
1500µFx3
L
1
5µH
D
2
MBR2545
C
2
1500µF
x3
1
V
C
0.1µF
V
OUT
R
1
R
2
FB
SS
2
SS
LX1682
3
GND
BDRV
LX1681 NON-SYNCHRONOUS CONTROLLER LX1682 SYNCHRONOUS CONTROLLER
PACKAGE ORDER INFORMATION
T
(°C)
A
0 to 70
Output
Non-Synchronous LX1681CDM
Synchronous LX1682CDM
Note: All surface-mount packages are available in Tape & Reel,
append the letter "T" to part number. (i.e. LX1681CDMT)
Plastic SOIC
DM
8-pin
IRL3103S
R
SET
Q
1
V
IN
5V
C
1
1500µFx2
L
1
5µH
Q
2
IRL3103S
C
2
1500µF
x3
V
OUT
R
1
R
2
Copyright © 1999
Rev. 1.0 5/99
L INF INITY MICROELECTRONICS INC.
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841, 714-898-8121, FAX: 714-893-2570
1
LX1681/1682
PRODUCT DATABOOK 1996/1997
VOLTAGE-MODE PWM CONTROLLERS
P RODUCTION DATA SHEET
ABSOLUTE MAXIMUM RATINGS (Note 1)
PACKAGE PIN OUTS
Supply Voltage (VC1) ............................................................................................18V
Supply Voltage (VCC) .............................................................................................. 7V
Output Drive Peak Current Source (500ns)....................................................... 1.0A
Output Drive Peak Current Sink (500ns)........................................................... 1.0A
Input Voltage (SS/ENABLE Pin) ............................................................. -0.3V to 6V
Operating Junction Temperature .................................................................... 150°C
Storage Temperature Range ........................................................... -65°C to +150°C
Lead Temperature (Soldering, 10 Seconds).................................................... 300°C
Note 1. Exceeding these ratings could cause damage to the device. All voltages are with
respect to Ground. Currents are positive into, negative out of the specified
terminal.
N.C. / GND*
GND / BDRV*
* Pin 3 = N.C. for LX1681, GND for LX1682
Pin 4 = GND for LX1681, BDRV for LX1682
1 8
V
FB
27
SS
36
45
DM PACKAGE
(Top View)
V
CC
CS
V
C1
TDRV
THERMAL DATA
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θ
θθ
JA
165°C/W
Junction Temperature Calculation: TJ = TA + (PD x θJA).
The θJA numbers are guidelines for the thermal performance of the device/pc-board
system. All of the above assume no ambient airflow.
ELECTRICAL CHARACTERISTICS
(
Unless otherwise specified, 4.75V < VCC < 5.25V and 10.8V < VC1 < 13.2V, 0°C ≤ TA ≤ 70°C. Test conditions: VCC = 5V, VC1 = 12V, T = 25°C.
Parameter
Symbol
Test Conditions
LX1681/1682
Min. Typ. Max.
Reference
Reference Voltage V
V
FB
= VFB , TA = 25°C
OUT
V
= VFB , 0°C ≤ TA ≤ 70°C
OUT
1.237 1.25 1.262 V
1.231 1.269 V
Oscillator
Frequency F
Ramp Amplitude V
OSC
RAMP
170 190 230 kHz
1.25 V
Error Amplifier
Input Resistance R
V
OUT
= V
FB
IN
20 kΩ
Current Sense
Current Set I
V
TRIP
Current Sense Delayed T
SETVCS
CSD
= VCC - 0.4V
Reference to V
40 45 µA
CC
-460 -400 -340 mV
1.1 µsec
Output Drivers
Drive Rise Time, Fall Time T
Drive High V
Drive Low V
CL = 3000pF
RF
DHISOURCE
DLISINK
= 20mA, VC1 = 12V
= 20mA, VC1 = 12V
50 ns
10 11 V
0.1 0.2 V
UVLO and Soft-Start (SS)
V
Start-Up Threshold V
CC5
Hysteresis
SS Resistor R
SS Output Enable V
Hiccup Duty Cycle DC
ST
SS
EN
HICCSS
VC1 > 4.0V
= 0.1µF, F
= 100Hz
REQ
4.0 4.25 4.5 V
0.10 V
20 kΩ
0.25 0.3 0.35 V
10 %
Supply Current
V
Dynamic Supply Current I
CC12
Static Supply Current 12V I
5V I
Out Freq = 200kHz, CL = 3000pF, Synch., VSS > 0.3V
CD
VC1VSS
VCCVSS
< 0.3V
< 0.3V
24 28 mA
57mA
10 12 mA
)
Units
PP
2
Copyright © 1999
Rev. 1.0 5/99
PRODUCT DATABOOK 1996/1997
LX1681/1682
VOLTAGE-MODE PWM CONTROLLERS
P RODUCTION DATA SHEET
BLOCK DIAGRAM
I
SET
R
SET
CS
7
CS Comp
I
Error
Comp
RESET
PWM
R
Q
SQ
Set
V
RESET
V
TRIP
V
CC
I
SET
320k
20k
1
V
FB
Amplifier/
Compensation
6
5
4
3
+12V
V
C1
TDRV
BDRV
GND
VIN (5V)
C
IN
R
2
R
1
L
V
CORE
ESR
C
OUT
C
SS
V
REF
SS
2
SS/ENABLE
Hiccup
Hiccup
Ramp
UVLO
Oscillator
FIGURE 1 — Block Diagram
UVLO
+5VR
8
V
CC
Copyright © 1999
Rev. 1.0 5/99
3
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