LX1684
TM
®
Voltage-Mode PWM Controller
P
RODUCTION
D
ATA
S
HEET
DESCRIPTION
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
WWW .
Microsemi
.C
OM
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 R
DS(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
PRODUCT HIGHLIGHT - TYPICAL 12V TO 3.3V APPLICATION
5V
5%
R1
165 Ohm
1
12V
10%
12V
C5
1µF
2
3
4
C1
0.1µF
D1
5817SMT
Q1
SUB45N
05-20L
3x
820µF,
16V
C4
FB
NC
SS
CSP
GND
PGND
BDR
VCC
NC
CS
NC
VC1
NC
TDR
14
13
LM78L05
or equivalent
5V
TO
VCC
R2
100 Ohm
Css
0.1µF
RSET
12
3.3K
11
10
9
8
5
6
7
Optional 12V only circuit
L1 is a powdered
iron toroid
Q2
SUB45N05-
20L
L1
10µH
4x
1500µF
6.3V
VOUT
3.3V
10A
C3
LX1684
LX1684
PACKAGE ORDER INFO
T
A
(°C)
0 to 70
Copyright
©
2000
Rev. 1.0c, 2005-03-08
O
UTPUT
Synchronous
D
Plastic SOIC
14-PIN
LX1684CD
RoHS Compliant / Pb-free Transition DC: 0440
Note: Available in Tape & Reel. Append the letters “TR” to the part number. (i.e. LX1684CD-TR)
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX1684
TM
®
Voltage-Mode PWM Controller
P
RODUCTION
D
ATA
S
HEET
ABSOLUTE MAXIMUM RATINGS (NOTE 1)
PACKAGE PIN OUT
FB
N/C
SS
VCSP
GND
PGND
BDRV
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
CC
N/C
CS
N/C
V
C1
N/C
TDRV
WWW .
Microsemi
.C
OM
Supply Voltage (V
C1
) ...................................................................................... 18V
Supply Voltage (V
CC
)........................................................................................ 7V
Input Voltage (CSP Pin) ................................................................................. 14V
Output Drive Peak Current Source (500ns) ................................................... 1.0A
Output Drive Peak Current Sink (500ns) ....................................................... 1.0A
Input Voltage (SS/ENABLE Pin) .......................................................... -0.3 to 6V
Operating Junction Temperature.................................................................. 150°C
Storage Temperature ................................................................... -65°C to +150°C
Peak Package Solder Reflow Temp. (40 second max. exposure) ...260°C (+0, -5)
Note:
Exceeding these ratings could cause damage to the device. All voltages are with respect to
Ground. Currents are positive into, negative out of specified terminal.
14-P
IN
SOIC
(Top View)
RoHS / Pb-free 100% Matte Tin Lead Finish
THERMAL DATA
D
P
ACKAGE
165°C/W
THERMAL RESISTANCE
-
JUNCTION TO AMBIENT
,
θ
JA
Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
θ
JA
).
The
θ
JA
numbers are guidelines for the thermal performance of the device/pc-board
system. All of the above assume no ambient airflow.
FUNCTIONAL PIN DESCRIPTION
P
IN
N
AME
V
FB
D
ESCRIPTION
Voltage Feedback. A 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 22kΩ resistor 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
Positive Over-Current Threshold Input
Analog ground for SS, FB, CS and VCC.
MOSFET driver power ground
Gate Drive For Upper MOSFET.
Gate Drive For Lower MOSFET.
Separate Supply For MOSFET Gate Drives. Connect to gate drive voltage.
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).
SS
VCSP
GND
PGND
TDRV
BDRV
V
C1
CS
V
CC
P
ACKAGE
D
ATA
P
ACKAGE
D
ATA
Copyright
©
2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX1684
TM
®
Voltage-Mode PWM Controller
P
RODUCTION
D
ATA
S
HEET
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, the following specifications apply over the operating ambient temperature 0°C
≤
T
A
≤
70°C except where
otherwise noted. Test conditions: V
CC
=5V, V
C1
=16V
Parameter
REFERENCE
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Microsemi
.C
OM
Symbol
Test Conditions
Min
LX1684
Typ
Max
Units
Reference Voltage
V
FB
V
OUT
=V
FB
, T
A
=25°C
V
OUT
=V
FB
, 0°C < T
A
< 70°C
1.237
1.231
1.25
1.262
1.269
V
V
OSCILLATOR
Frequency
Ramp Amplitude
ERROR AMPLIFIER
F
OSC
V
RAMP
135
175
1.25
200
kHz
VPP
Input Resistance
CURRENT SENSE
R
IN
V
OUT
= V
FB
20
kΩ
Current Set
V
TRIP
Current Sense Delayed
OUTPUT DRIVERS
I
SET
V
CS
= V
CC
–0.4V
40
300
45
380
1.0
420
µA
mV
µsec
T
CSD
Drive Rise Time, Fall Time
Drive High
Drive Low
UVLO AND SOFT-START (SS)
T
RF
V
DH
V
DL
C
L
= 3000pF
I
SOURCE
= 20mA, V
C1
=16V
I
SINK
= 20mA, V
C1
=16V
13
50
14
0.1
0.2
nS
V
V
V
CC5
Start-Up Threshold
Hysteresis
SS Resistor
SS Output Enable
Hiccup Duty Cycle
SUPPLY CURRENT
V
ST
V
C1
> 4.0V
4.0
4.25
0.10
4.5
V
V
kΩ
R
SS
V
EN
DC
HIC
C
SS
= 0.1µF, F
REQ
=100Hz
0.25
22
0.3
12
0.35
14
V
%
E
LECTRICALS
E
LECTRICALS
V
C1
Dynamic Supply Current
Static Supply Current V
C1
5V
I
CD
I
VC1
I
VCC
Out Freq = 175kHz, C
L
=3000pF, Synch., V
SS
> 0.3V
V
SS
< 0.3V ; outputs low (disable), V
C1
= 16V
V
SS
> 0.3V ; outputs low (disable)
30
7
9
40
9
12
mA
mA
mA
Copyright
©
2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX1684
TM
®
Voltage-Mode PWM Controller
P
RODUCTION
D
ATA
S
HEET
BLOCK DIAGRAM
WWW .
Microsemi
.C
OM
R
SET
I
SET
12
VCSP
CS
+17.5V
CS Com p
-
I
RESET
PW M
V
IN
(+12V)
R
2
C
IN
R
1
10
V
C1
4
V
TRIP
320k
20k
+
I
SET
R
S
Q
Q
8
TDRV
L
1
V
FB
-
+
Am plifier/
Com pensation
V
CO RE
ESR
C
O UT
Error Com p
Set
+
-
V
RESET
7
BDRV
V
REF
R
SS
Hiccup
Hiccup
6
PG ND
5
G ND
UVLO
+5V
UVLO
Ramp
O scillator
14
V
CC
3
SS/ENABLE
C
SS
B
LOCK
D
IAGRAM
B
LOCK
D
IAGRAM
Copyright
©
2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
LX1684
TM
®
Voltage-Mode PWM Controller
P
RODUCTION
D
ATA
S
HEET
THEORY OF OPERATION
GENERAL DESCRIPTION
The LX1684 is a voltage-mode pulse-width modulation
controller integrated circuit. The internal oscillator and ramp
generator frequency is fixed at 175kHz. The devices have internal
compensation, so that no external compensation is required.
POWER UP AND INITIALIZATION
At power up, the LX1684 monitors the supply voltage to both
the +5V and the V
C1
pins (there is no special requirement for the
sequence of the two supplies). Before both supplies reach their
under-voltage lock-out (UVLO) thresholds, the soft-start (SS) pin
is held low to prevent soft-start from beginning; the oscillator
control is disabled and the top MOSFET is kept OFF.
SOFT-START
Once the supplies are above the UVLO threshold, the soft-start
capacitor begins to be charged up by the reference through a 20k
internal resistor. The capacitor voltage at the SS pin rises as a
simple RC circuit. The SS pin is connected to the amplifier's non-
inverting input that controls the output voltage. The output voltage
will follow the SS pin voltage if sufficient charging current is
provided to the output capacitor. The simple RC soft-start allows
the output to rise faster at the beginning and slower at the end of
the soft-start interval. Thus, the required charging current into the
output capacitor is less at the end of the soft-start interval so
decreasing the possibility of an over-current. A comparator
monitors the SS pin voltage and indicates the end of soft-start
when SS pin voltage reaches 95% of V
REF
.
OVER-CURRENT PROTECTION (OCP) AND HICCUP
The LX1684 uses the RDS(ON) of the upper MOSFET,
together with a resistor (RSET) to set the actual current limit point.
Unlike the LX1681/2 controllers the LX1684 includes the positive
current sense comparator input providing true Kelvin sensing
which is more accurate and offers more noise immunity.
This
Kelvin sensing also simplifies the PCB layout for current sense.
The CSP pin has a useful common mode input range to about 14V.
The comparator senses the current 1µs after the top MOSFET is
switched on.
Experiments have shown that the MOSFET drain voltage will
ring for 200-500ns after the gate is turned on. In order to reduce
inaccuracies due to ringing, a 1µs blanking delay after gate turn-on
is built into the current sense comparator. This 1us delay reduces
the effectiveness of the current sense comparator when the output
pulse width is below 1us. This can be problem when the set
application output voltage is less than 3.0V with a 12V input.
Under this condition the output current limit protection will not
function properly. This is usually not true with a short circuit
current condition, which causes the on time to be greater than the
blanking delay. In this circumstance the current limit comparator
would turn off the top FET driver.
The comparator draws a current (ISET), whose magnitude is
45µA. The set resistor is selected to set the current limit for the
application. When the sensed voltage across the RDS(ON) plus the
set resistor exceeds the 400mV VTRIP threshold, the OCP
comparator outputs a signal to reset the PWM latch and to start
hiccup mode. The soft-start capacitor (CSS) is discharged slowly
(10 times slower than when being charged up by RSS). When the
voltage on the SS/ENABLE pin reaches a 0.3V threshold, hiccup
finishes and the circuit soft-starts again. During hiccup, the top
MOSFET is OFF and the bottom MOSFET remains ON. Hiccup is
disabled during the soft-start interval, allowing the circuit to start
up with the maximum current. If the rise speed of the output
voltage is too fast, the required charging current to the output
capacitor may be higher than the limit-current. In this case, the
peak MOSFET current is regulated to the limit-current by the
current-sense comparator. If the MOSFET current still reaches its
limit after the soft-start finishes, the hiccup is triggered again. The
hiccup ensures the average heat generation on both MOSFET’s and
the average current to be much less than that in normal operation,
if the output has a short circuit. Over-current protection can also be
implemented using a sense resistor, instead of using the RDS(ON)
of the upper MOSFET, for greater set-point accuracy. See
Application Information section.
OSCILLATOR FREQUENCY
An internal oscillator sets the switching frequency at about 175
kHz.
WWW .
Microsemi
.C
OM
DESCRIPTION
DESCRIPTION
Copyright
©
2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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