LX1675
PRODUCTION DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 9
Copyright © 2004
Rev. 1.2a, 2006-02-16
WWW.Microsemi .COM
Multiple Output LoadSHARE™ PWM
TM
®
THEORY OF OPERATION
G
ENERAL DESCRIPTION
The LX1675 is a voltage-mode pulse-width modulation
controller integrated circuit. The internal ramp generator frequency
is set to 300kHz or 600kHz by the FS logic input. The device has
external compensation, for more flexibility of output current
magnitude.
U
NDER VOLTAGE LOCKOUT (UVLO)
At power up, the LX1675 monitors the supply voltage at the
VCCL pin. The VIN supply voltage has to be sufficient to produce
a voltage greater that 4.4 volts at the VCCL pin before the
controller will come out of the under-voltage lock-out state. The
soft-start (SS) pin is held low to prevent soft-start from beginning
and the oscillator is disabled and all MOSFETs are held off.
S
OFT-START
Once the VCCL output is 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
.
O
VER-CURRENT PROTECTION (OCP) AND HICCUP
The LX1675 uses the R
DS(ON)
of the lower MOSFET, together
with a resistor (R
SET
) to set the actual current limit point. The
current sense comparator senses the MOSFET current 50nS after
the lower 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
should be connected directly
at the lower MOSFET drain and the source needs a low impedance
return to get an accurate measurement across the low resistance
R
DS(ON)
.
When the sensed voltage across RDS
(ON)
plus the set resistor
voltage drop exceeds the 0.0Volt, V
TRIP
threshold, the OCP
comparator outputs a signal to reset the PWM latch on a cycle by
cycle basis until the current limit counter has reached a count of 4.
After a count of 4 the hiccup mode is started. The soft-start
capacitor (C
SS
) is discharged slowly (14 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. The
Shared Fault, SF logic input, allows all phases to be totally
independent if the SF pin is grounded. If the SF pin is tied to
VCCL then when one phase has a fault and goes into the hiccup
mode, all phases, including the LDO output will go into the hiccup
mode together.
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 current limit setting. 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 lower MOSFET, for
greater set-point accuracy.
OSCILLATOR FREQUENCY
An internal oscillator has a selectable switching frequency of
300kHz or 600kHz set by the FS logic input pin. Connect FS to
ground for 300kHz and to VCCL for 600kHz operation.
THEORY OF OPERATION FOR A BI-PHASE, LOADSHARE
CONFIGURATION
The basic principle used in LoadSHARE, 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 LX1675 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
S
S
