lx1673 Microsemi Corporation, lx1673 Datasheet - Page 8

• Manufacturer Part Number: lx1673
• Description: High Frequency Pwm Regulator
• Manufacturer: Microsemi Corporation

Copyright © 2004
Rev 1.0, 3/18/2005
G
controller integrated circuit. The internal oscillator and ramp
generator frequency is fixed to 300KHz, 600KHz, or 900KHz.
The device has external compensation, for more flexibility of
output current magnitude.
U
VCC, VCCL, LDVCC and VC1 (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
capacitor begins to be charged by the reference through a 20k Ω
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
OFT
SET
DS(ON)
NDER
ENERAL
VER
The LX1673 is a voltage-mode pulse-width modulation
At power up, the LX1673 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 LX1673 uses the R
), whose magnitude is 50µA.
-S
-C
.
TART
V
TM
URRENT
OLTAGE
D
ESCRIPTION
P
L
SET
ROTECTION
OCKOUT
) to set the actual current limit point. The
The SS pin is connected to the error
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REF
DS(ON)
.
(UVLO)
(OCP)
of the upper MOSFET, together
AND
The set resistor R
H
ICCUP
T H E O R Y O F O P E R A T I O N
Integrated Products Division
SET
®
and VS
Microsemi
SET
is
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 are held off.
circuit to 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,.
resistor, instead of using the R
greater set-point accuracy.
O
600kHz, or 900kHz.
SCILLATOR
When the sensed voltage across R
Hiccup is disabled during the soft-start interval, allowing the
Over-current protection can also be implemented using a sense
An internal oscillator sets the switching frequency at 300kHz,
High Frequency PWM Regulator
F
P
REQUENCY
RODUCTION
SS
TRIP
) is discharged slowly (10 times slower
threshold, the OCP comparator outputs
DS(ON)
D
SS
ATA
). When the voltage on the SS
of the upper MOSFET, for
DS(ON)
S
HEET
plus the set resistor
LX1673
Page 8