FUNCTIONAL DEVICE OPERATION
PROTECTION AND DIAGNOSTIC FEATURES
PROTECTION AND DIAGNOSTIC FEATURES
The 34713 monitors the application for several fault
conditions to protect the load from overstress. The reaction of
the IC to these faults ranges from turning off the outputs to
just alerting the host that something is wrong. In the following
paragraphs, each fault condition is explained:
operation to limit the current, and a 10 ms over current limit
timer (tLIM) starts. The converter will stay in this mode of
operation until one of the following occurs:
• The current is reduced back to the normal level before
tLIM expires, and in this case normal operation is
regained.
• tLIM expires without regaining normal operation, at
which point the device turns off the output and the
power good output signal is pulled high. At the end of a
time-out period of 100 ms (tTIMEOUT), the device will
attempt another soft start cycle.
Output Overvoltage
An overvoltage condition occurs once the output voltage
goes higher than the rising overvoltage threshold (VOVR). In
this case, the power good output signal is pulled high, alerting
the host that a fault is present, but the output will stay active.
To avoid erroneous overvoltage conditions, a 20 µs filter is
implemented. The buck converter will use its feedback loop
to attempt to correct the fault. Once the output voltage falls
below the falling overvoltage threshold (VOVF), the fault is
cleared and the power good output signal is pulled low, the
device is back in normal operation.
• The device reaches the thermal shutdown limit (TSDFET
)
and turns off the output. The power good output signal
is pulled high.
• The output current keeps increasing until it reaches the
short circuit current limit (ISHORT). See below for more
details.
Output Undervoltage
Short Circuit Current Limit
An undervoltage condition occurs once the output voltage
falls below the falling undervoltage threshold (VUVF). In this
case, the power good output signal is pulled high, alerting the
host that a fault is present, but the output will stay active. To
avoid erroneous undervoltage conditions, a 20 µs filter is
implemented. The buck converter will use its feedback loop
to attempt to correct the fault. Once the output voltage rises
above the rising undervoltage threshold (VUVR), the fault is
cleared and the power good output signal is pulled low, the
device is back in normal operation.
This block uses the same current detection mechanism as
the over current limit detection block. If the load current
reaches the ISHORT value, the device reacts by shutting down
the output immediately. This is necessary to prevent damage
in case of a permanent short circuit. Then, at the end of a
time-out period of 100 ms (tTIMEOUT), the device will attempt
another soft start cycle.
Thermal Shutdown
Thermal limit detection block monitors the temperature of
the device and protects against excessive heating. If the
temperature reaches the thermal shutdown threshold
(TSDFET), the converter output switches off and the power
good output signal indicates a fault by pulling high. The
device will stay in this state until the temperature has
decreased by the hysteresis value and then After a time-out
period (TTIMEOUT) of 100 ms, the device will retry
Output Over Current
This block detects over current in the Power MOSFETs of
the buck converter. It is comprised of a sense MOSFET and
a comparator. The sense MOSFET acts as a current
detecting device by sampling a ratio of the load current. That
sample is compared via the comparator with an internal
reference to determine if the output is in over current or not.
If the peak current in the output inductor reaches the over
current limit (ILIM), the converter will start a cycle-by-cycle
automatically and the output will go through a soft start cycle.
If successful normal operation is regained, the power good
output signal is asserted low to indicate that.
34713
Analog Integrated Circuit Device Data
Freescale Semiconductor
13
FREESCALE [ Freescale ]
