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LX1684 Datasheet PDF : 12 Pages
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LX1684
TM
®
Voltage-Mode PWM Controller
PRODUCTION DATA SHEET
THEORY OF OPERATION
GENERAL DESCRIPTION
The LX1684 is a voltage-mode pulse-width modulation
controller integrated circuit. The internal oscillator and ramp
generator frequency is fixed at 175kHz. The devices have internal
compensation, so that no external compensation is required.
POWER UP AND INITIALIZATION
At power up, the LX1684 monitors the supply voltage to both
the +5V and the VC1 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 VREF .
OVER-CURRENT PROTECTION (OCP) AND HICCUP
The LX1684 uses the RDS(ON) of the upper MOSFET,
together with a resistor (RSET) to set the actual current limit point.
Unlike the LX1681/2 controllers the LX1684 includes the positive
current sense comparator input providing true Kelvin sensing
which is more accurate and offers more noise immunity. This
Kelvin sensing also simplifies the PCB layout for current sense.
The CSP pin has a useful common mode input range to about 14V.
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 blanking delay after gate turn-on
is built into the current sense comparator. This 1us delay reduces
the effectiveness of the current sense comparator when the output
pulse width is below 1us. This can be problem when the set
application output voltage is less than 3.0V with a 12V input.
Under this condition the output current limit protection will not
function properly. This is usually not true with a short circuit
current condition, which causes the on time to be greater than the
blanking delay. In this circumstance the current limit comparator
would turn off the top FET driver.
The comparator draws a current (ISET), whose magnitude is
45µA. The set resistor is selected to set the current limit for the
application. When the sensed voltage across the RDS(ON) plus the
set resistor exceeds the 400mV VTRIP threshold, the OCP
comparator outputs a signal to reset the PWM latch and to start
hiccup mode. The soft-start capacitor (CSS) is discharged slowly
(10 times slower than when being charged up by RSS). 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 RDS(ON)
of the upper MOSFET, for greater set-point accuracy. See
Application Information section.
OSCILLATOR FREQUENCY
An internal oscillator sets the switching frequency at about 175
kHz.
Copyright © 2000
Rev. 1.0c, 2005-03-08
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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