LX1681/1682
TM
®
Voltage-Mode PWM Controllers
P
RODUCTION
D
ATA
S
HEET
DESCRIPTION
KEY FEATURES
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
WWW .
Microsemi
.C
OM
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 minimal external
components.
The LX1681 is a non-synchronous
controller;
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
DS(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. Pulling
the soft-start pin to ground can disable
the LX1681/82.
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.
APPLICATIONS
5V to 3.3V Or Less Buck
Regulators
FPGA Supplies
Microprocessor Chipset Supplies
(e.g. Camino, Whitney, etc.)
©
Rambus RIMM™ Supplies
Hard Disk Drives
Computer Add-on Cards
IMPORTANT:
For the most current data, consult
MICROSEMI’s
website:
http://www.microsemi.com
PRODUCT HIGHLIGHT
V
BO OST
12V
V
IN
5V
C
3
1µF
C
1
1500µF
x3
C
3
1µF
V
BOO ST
12V
V
IN
5V
C
1
V
FB
C
SS
0.1µF
V
C2
CS
V
FB
C
SS
V
C2
CS
1500µF
x3
SS
SS
LX1681
N.C.
GND
V
C1
TDRV
R
SET
D
2
Q
1
IRL3103S
L
1
5µH
C
2
1500µF
x3
R
1
0.1µF
V
OU T
LX1682
G ND
BDRV
V
C1
TDRV
R
SET
Q
1
IRL3103S
L
1
5µH
C
2
1500µF
x3
R
1
V
OU T
R
2
MBR2545
R
2
Q
2
IRL3103S
LX1681/1682
LX1681/1682
LX1681 Non-Synchronous Controller
LX1682 Synchronous Controller
PACKAGE ORDER INFO
T
A
(°C)
0 to 70
O
UTPUT
Non-Synchronous
Synchronous
DM
Plastic SOIC
8-PIN
LX1681CDM
LX1682CDM
RoHS Compliant / Pb-free
Transition D/C: 0440
Note: Available in Tape & Reel. Append the letters “TR” to the part number. (i.e. LX1681CDM-TR)
Copyright
©
2000
Rev. 1.1b,2005-03-09
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX1681/1682
TM
®
Voltage-Mode PWM Controllers
P
RODUCTION
D
ATA
S
HEET
ABSOLUTE MAXIMUM RATINGS (NOTE 1)
Supply Voltage (V
C1
).................................................................................................... 18V
Supply Voltage (V
CC
) ..................................................................................................... 7V
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
RoHS / Pb-free 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.
PACKAGE PIN OUT
V
FB
SS
N.C.
GND
1
2
3
4
8
7
6
5
V
CC
CS
V
C1
TDRV
WWW .
Microsemi
.C
OM
LX1681
DM P
ACKAGE
(Top View)
V
FB
SS
GND
BDRV
1
2
3
4
8
7
6
5
V
CC
CS
V
C1
TDRV
THERMAL DATA
LX1682
DM P
ACKAGE
(Top View)
DM
P
ACKAGE
165°C/W
RoHS / Pb-free 100% Matte Tin Lead Finish
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 20kΩ 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
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.
SS
GND
TDRV
BDRV
V
C1
CS
V
CC
P
ACKAGE
D
ATA
P
ACKAGE
D
ATA
Copyright
©
2000
Rev. 1.1b,2005-03-09
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX1681/1682
TM
®
Voltage-Mode PWM Controllers
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
=12V, T=25°C
Parameter
REFERENCE
WWW .
Microsemi
.C
OM
Symbol
Test Conditions
LX1681/1682
Min
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
170
190
1.25
230
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
Reference to V
CC
40
40
45
45
1.1
µA
µA
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
=12V
I
SINK
=20mA, V
C1
=12V
10
50
11
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
20
0.3
10
0.35
V
%
E
LECTRICALS
E
LECTRICALS
V
CC12
Dynamic Supply Current
Static Supply Current 12CV
5V
I
CD
I
VC1
I
VCC
Out Freq = 200kHz, C
L
=3000pF, Synch., V
SS
> 0.3V
V
SS
< 0.3V
V
SS
> 0.3V
24
5
10
28
7
12
mA
mA
mA
Copyright
©
2000
Rev. 1.1b,2005-03-09
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX1681/1682
TM
®
Voltage-Mode PWM Controllers
P
RODUCTION
D
ATA
S
HEET
THEORY OF OPERATION
GENERAL DESCRIPTION
OVER-CURRENT PROTECTION (OCP) and HICCUP
WWW .
Microsemi
.C
OM
The LX1681/82 are voltage-mode pulse-width modulation
controller integrated circuits. The internal oscillator and ramp
generator frequency is fixed at 200kHz. The devices have internal
compensation, so that no external compensation is required.
POWER UP and INITIALIZATION
At power up, the LX1681/82 monitors the supply voltage to
both the +5V and the +12V 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
.
The LX1681/1682 family uses the R
DS(ON)
of the upper
MOSFET, together with a resistor (R
SET
) to set the actual current
limit point. 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
delay after gate turn-on is built into the current sense comparator.
The comparator draws a current (I
SET
), whose magnitude is 45µA.
The set resistor is selected to set the current limit for the
application. When the sensed voltage across the R
DS(ON)
plus the
set resistor exceeds the 400mV V TRIP threshold, the OCP
comparator outputs a signal to reset the PWM latch and to start
hiccup mode. The soft-start capacitor (C
SS
) is discharged slowly
(10 times slower than when being charged up by R
SS
). 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 R
DS(ON)
of
the upper MOSFET, for greater set-point accuracy. See
Application Information section.
OSCILLATOR FREQUENCY
An internal oscillator sets the switching frequency at 200 kHz.
D
ESCRIPTION
D
ESCRIPTION
Copyright
©
2000
Rev. 1.1b,2005-03-09
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
LX1681/1682
TM
®
Voltage-Mode PWM Controllers
P
RODUCTION
D
ATA
S
HEET
APPLICATION INFORMATION
OUTPUT INDUCTOR
OUTPUT CAPACITOR
(continued)
WWW .
Microsemi
.C
OM
The output inductor should be selected to meet the requirements
of the output voltage ripple in steady-state operation and the
inductor current slew-rate during transient. The peak-to-peak
output voltage ripple is:
V
RIPPLE
=
ESR
×
I
RIPPLE
where
I
RIPPLE
=
(
V
IN
- V
OUT
)
×
V
OUT
f
SW
×
L
V
IN
I
RIPPLE
is the inductor ripple current, L is the output inductor
value and ESR is the Effective Series Resistance of the output
capacitor.
I
RIPPLE
should typically be in the range of 20% to 40% of the
maximum output current. Higher inductance results in lower
output voltage ripple, allowing slightly higher ESR to satisfy the
transient specification. Higher inductance also slows the inductor
current slew rate in response to the load-current step change,
∆I,
resulting in more output-capacitor voltage droop. The inductor-
current rise and fall times are:
Electrolytic capacitors can be used for the output capacitor, but
are less stable with age than tantalum capacitors. As they age, their
ESR degrades, reducing the system performance and increasing the
risk of failure. It is recommended that multiple parallel capacitors
be used, so that, as ESR increases with age, overall performance
will still meet the processor’s requirements.
There is frequently strong pressure to use the least expensive
components possible, however, this could lead to degraded long-
term reliability, especially in the case of filter capacitors.
Linfinity’s demonstration boards use Sanyo MV-GX filter
capacitors, which are aluminum electrolytic, and have
demonstrated reliability. The Oscon series from Sanyo generally
provides the very best performance in terms of long term ESR
stability and general reliability, but at a substantial cost penalty.
The MV-GX series provides excellent ESR performance at a
reasonable cost. Beware of off-brand, very low-cost filter
capacitors, which have been shown to degrade in both ESR and
general electrolytic characteristics over time.
INPUT CAPACITOR
T
RISE
=
and
L
× ∆
I
(
V
IN
−
V
OUT
)
The input capacitor and the input inductor are to filter the
pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 5V rail. In
addition, the input capacitor provides local de-coupling the buck
converter. The capacitor should be rated to handle the RMS current
requirement. The RMS current is:
T
FALL
=
L
× ∆
I
V
OUT
I
RMS
=
I
L
d
(1
−
d
)
where
I
L
is the inductor current and the d is the duty cycle. The
maximum value, when d = 50%,
I
RMS
= 0.5I
L
. For 5V input and
output in the range of 2 to 3V, the required RMS current is very
close to 0.5I
L
.
SOFT-START CAPACITOR
When using electrolytic capacitors, the capacitor voltage droop
is usually negligible, due to the large capacitance.
OUTPUT CAPACITOR
The output capacitor is sized to meet ripple and transient
performance specifications. Effective Series Resistance (ESR) is a
critical parameter. When a step load current occurs, the output
voltage will have a step that equals the product of the ESR and the
current step,
∆I.
In an advanced microprocessor power supply, the
output capacitor is usually selected for ESR instead of capacitance
or RMS current capability. A capacitor that satisfies the ESR
requirement usually has a larger capacitance and current capability
than strictly needed. The allowed ESR can be found by:
The value of the soft-start capacitor determines how fast the
output voltage rises and how large the inductor current is required
to charge the output capacitor. The output voltage will follow the
voltage at SS pin if the required inductor current does not exceed
the maximum current in the inductor. The SS pin voltage can be
expressed as:
A
PPLICATION
A
PPLICATION
V
SS
=
V
SET
(1
−
e
−
t
/
R
SS
C
SS
)
where
V
SET
is the reference voltage.
R
SS
and
C
SS
are soft start
resistor and capacitor. The required inductor current for the output
capacitor to follow the SS-pin voltage equals the required capacitor
current plus the load current. The soft-start capacitor should be
selected so that the overall inductor current does not exceed it
maximum.
ESR
×
(
I
RIPPLE
+ ∆
I
)
<
V
EX
where I
RIPPLE
is the inductor ripple current,
∆I
is the maximum
load current step change, and
V
EX
is the allowed output voltage
excursion in the transient.
Copyright
©
2000
Rev. 1.1b,2005-03-09
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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