lx1672 Microsemi Corporation, lx1672 Datasheet - Page 10

• Manufacturer Part Number: lx1672
• Description: Multiple Output Loadshare Pwm
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0, 2005-08-10
G
controller integrated circuit.
frequency is fixed to 300kHz.
compensation, for more flexibility of output current magnitude.
VCC, VCCL, and VCX (there is no requirement for sequencing
the supplies). Before all 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 is disabled and all
MOSFETs are held off. There is an internal delay that will filter
out transients less that 1.5µSec.
capacitor begins to be charged by the reference through a 20kΩ
internal resistor. The capacitor voltage at the SS pin rises as a
simple RC circuit.
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.
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. A comparator monitors the SS
pin voltage and indicates the end of soft-start when SS pin
voltage reaches 95% of V
O
with a resistor (R
current sense comparator senses the MOSFET current 350nS
after the top MOSFET is switched on in order to reduce
inaccuracies due to ringing. A current source supplies a current
(I
selected to set the current limit for the application. R
should be connected directly at the upper MOSFET drain and
source to get an accurate measurement across the low resistance
R
exceeds the 300mV, V
a signal to reset the PWM latch and to start hiccup mode. The
soft-start capacitor (C
than when being charged up by R
pin reaches a 0.1V threshold, hiccup finishes and the circuit soft-
starts again. During hiccup both MOSFETs for that phase are
held off.
U
S
OFT
SET
DS(ON)
ENERAL
NDER
VER
The LX1672 is a voltage-mode pulse-width modulation
At power up, the LX1672 monitors the supply voltage for
Once the supplies are above the UVLO threshold, the soft-start
The simple RC soft-start allows the output to rise faster at the
The LX1672 uses the R
When the sensed voltage across R
), whose magnitude is 50µA.
-S
-C
.
V
TART
TM
URRENT
OLTAGE
D
ESCRIPTION
P
SET
L
ROTECTION
OCKOUT
) to set the actual current limit point. The
SS
TRIP
The SS pin is connected to the error
) is discharged slowly (10 times slower
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
REF
threshold, the OCP comparator outputs
DS(ON)
.
(UVLO)
(OCP)
of the upper MOSFET, together
SS
The internal ramp generator
). When the voltage on the SS
The device has external
AND
DS(ON)
The set resistor R
H
ICCUP
plus the set resistor
T H E O R Y O F O P E R A T I O N
Integrated Products Division
SET
and VSX
®
Microsemi
SET
is
up with maximum current. If the rate of rise of the output voltage
is too fast, the required charging current to the output capacitor
may be higher than the limit-current.
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. When the
output has a short circuit the hiccup circuit
average heat generation in both MOSFETs and the average current
is much less than in normal operation.
resistor, instead of using the R
greater set-point accuracy.
O
300KHz, 500KHz, or 600KHz.
phase buck converter topology, is that if multiple, identical,
inductors have the same identical voltage impressed across their
leads, they must then have the same identical current passing
through them. The current that we would like to balance between
inductors is mainly the DC component along with as much as
possible the transient current. All inductors in a multiphase buck
converter topology have their output side tied together at the
output filter capacitors. Therefore, this side of all the inductors
have the same identical voltage.
equivalent DC potential on this lead, then they will have the same
DC current flowing. To achieve this requirement, phase 1 will be
the control phase that sets the output operating voltage, under
normal PWM operation. To force the current of phase 2 to be
equal to the current of phase 1, a second feedback loop is used.
Phase 2 has a low pass filter connected from the input side of each
inductor. This side of the inductors has a square wave signal that
is proportional to its duty cycle. The output of each LPF is a DC
(+ some AC) signal that is proportional to the magnitude and duty
cycle of its respective inductor signal. The second feedback loop
will use the output of the phase 1 LPF as a reference signal for an
error amplifier that will compare this reference to the output of the
phase 2 LPF. This error signal will be amplified and used to
control the PWM circuit of phase 2. Therefore, the duty cycle of
phase 2 will be set so that the equivalent voltage potential will be
forced across the phase 2 inductor as compared to the phase 1
inductor. This will force the current in the phase 2 inductor to
follow and be equal to the current in the phase 1 inductor.
LoadSHARE feature of the LX1672 in the Bi-Phase mode of
operation.
T
CONFIGURATION
SCILLATOR
HEORY
Multiple Output LoadSHARE™ PWM
Hiccup is disabled during the soft-start interval, allowing start
Over-current protection can also be implemented using a sense
An internal oscillator sets the PWM switching frequency at
The basic principle used in LoadSHARING™, in a multiple
If the input side of the inductors can be forced to have the same
There are four methods that can be used to implement the
OF
F
P
REQUENCY
O
RODUCTION
PERATION
FOR
D
DS(ON)
ATA
A
of the upper MOSFET, for
S
B
I
HEET
-P
In this case, the peak
HASE
, L
ensures that the
OAD
LX1672
SHARE
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