R7731A
11
R7731A-10 August 2011
www.richtek.com
Protection
R7731A provides fruitful protection functions that intend
to protect system from being damaged. All the protection
functions can be listed as below :
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` Cycle-by-Cycle Current Limit
This is a basic but very useful function and it can be
implemented easily in current mode controller.
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` Over Load Protection
Long time cycle-by-cycle current limit will lead to system
thermal stress. To further protect system, system will
be shut down after about 4096 clock cycles. it's about
60ms delay in 67kHz operation. After shutdown, system
will resume and behave as hiccup. By proper start-up
resistor design, thermal will be averaged to an
acceptable level over the ON/OFF cycle of IC. This will
last until fault is removed.
#
It's highly recommended to
add a resistor in parallel with the opto-coupler. To provide
sufficient bias current to make TL-431 regulate properly,
1.2kΩ resistor is suggested.
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` Brownout Protection
During heavy load, this will trigger 60ms protection and
shut down the system. If it's in light load condition,
system will be shut down after V
DD
is running low and
triggers UVLO.
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` OVP
Output voltage can be roughly sensed by VDD pin.If the
sensed voltage reaches 27V threshold, system will be
shut down after 20μs deglitch delay.
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` Feedback Open and Opto-Coupler Short
This will trigger OVP or 60ms delay protection. It
depends on which one occurs first.
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` OTP
Internal OTP function will protect the controller itself
from suffering thermal stress and permanent damage. It
stops the system from switching until the temperature
is under threshold level. Meanwhile, if V
DD
reaches V
DD
turn off threshold voltage, system will hiccup till over
temperature condition is gone.
Figure 4. R-C Filter on CS Pin
Negative Voltage Spike on Each Pin
Negative voltage (< −0.3V) on each pin will cause substrate
injection. It leads to controller damage or circuit false
trigger. Generally, it happens at CS pin due to negative
spike because of improper layout or inductive current
sense resistor. Therefore, it is highly recommended to
add a R-C filter to avoid CS pin damage, as shown in
Figure 4. Proper layout and careful circuit design should
be done to guarantee yield rate in mass production.
VDD
GATE
CS
R7731A
AC Mains
(90V to 265V)
R-C Filter
