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Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
Copyright
2000
Rev. 1.0, 2001-09-19
WWW
.M
ic
ro
sem
i
.COM
LX1684
Voltage-Mode PWM Controller
PRODUCTION
L I N F IN IT Y
D I V I S I O N
DESCRI PTION
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 1s 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 FEATU R ES
! 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
APP L ICAT IONS
! 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 H I GHL IG HT
PRODUCT H I GHL IG HT ---- TYP I CAL 1 2 V TO 3 . 3V
TYP I CAL 1 2 V TO 3 . 3V
TYP I CAL 1 2 V TO 3 . 3V APP L ICA T I O N
APP L ICA T I O N
FB
NC
SS
CSP
GND
PGND
BDR
TDR
NC
VC1
NC
CS
NC
VCC
Css
0.1F
R2
100 Ohm
R1
165 Ohm
C5
1F
5V
5%
RSET
3.3K
L1
10H
VOUT
Q2
SUB45N05-
20L
4x
1500F
6.3V
3x
820F,
16V
Q1
SUB45N
05-20L
12V
10%
C1
0.1F
3.3V
10A
C3
C4
L1 is a powdered
iron toroid
1
2
3
4
5
6
7
14
13
12
11
10
9
8
D1
5817SMT
Optional 12V only circuit
5V
12V
TO
VCC
LM78L05
or equivalent
PACKAGE OR DER I N FO
TA (°
°°°C)
OUTPUT
D
Plastic SOIC
14-PIN
0 to 70
Synchronous
LX1684CD
Note: Available in Tape & Reel.
Append the letter “T” to the part number. (i.e. LX1684CDT)
LL
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