PRODUCTION D
TA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
Copyright © 2000
Rev. 1.0, 2005-08-10
WWW.Microsemi .COM
LX1672
Multiple Output LoadSHARE™ PWM
TM
®
THEORY OF OPERATION
G
ENERAL DESCRIPTION
The LX1672 is a voltage-mode pulse-width modulation
controller integrated circuit. The internal ramp generator
frequency is fixed to 300kHz. The device has external
compensation, for more flexibility of output current magnitude.
U
NDER VOLTAGE LOCKOUT (UVLO)
At power up, the LX1672 monitors the supply voltage for
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.
S
OFT-START
Once the supplies are above the UVLO threshold, the soft-start
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. The SS pin is connected to the error
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. 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 LX1672 uses the R
DS(ON)
of the upper MOSFET, together
with a resistor (R
SET
) to set the actual current limit point. The
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
SET
), whose magnitude is 50µA. The set resistor R
SET
is
selected to set the current limit for the application. R
SET
and VSX
should be connected directly at the upper MOSFET drain and
source to get an accurate measurement across the low resistance
R
DS(ON)
.
When the sensed voltage across R
DS(ON)
plus the set resistor
exceeds the 300mV, 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
pin reaches a 0.1V threshold, hiccup finishes and the circuit soft-
starts again. During hiccup both MOSFETs for that phase are
held off.
Hiccup is disabled during the soft-start interval, allowing start
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. 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. When the
output has a short circuit the hiccup circuit ensures that the
average heat generation in both MOSFETs and the average current
is much less than in normal operation.
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.
O
SCILLATOR FREQUENCY
An internal oscillator sets the PWM switching frequency at
300KHz, 500KHz, or 600KHz.
T
HEORY OF OPERATION FOR A BI-PHASE, LOADSHARE
CONFIGURATION
The basic principle used in LoadSHARING™, in a multiple
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.
If the input side of the inductors can be forced to have the same
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.
There are four methods that can be used to implement the
LoadSHARE feature of the LX1672 in the Bi-Phase mode of
operation.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
