34717 Datasheet, PDF (15/25 Pages) Freescale Semiconductor, Inc – 5.0 A 1.0 MHz Fully Integrated Dual Switch-Mode Dual Switch-Mode Power Supply

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
current that can result in an undesired voltage spike once the
capacitor is fully charged. The soft start is active each time
the IC goes out of standby or shutdown mode, power is
recycled, or after a fault retry.
After a successful start-up cycle where the device is
enabled, no faults have occurred, and the output voltage has
reached its regulation point, the 34717 pulls the power good
output signal low after a 10 ms reset delay, to indicate to the
host that the device is in normal operation.
PROTECTION FUNCTIONS
The 34717 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:
Output Overvoltage
An overvoltage condition occurs once the output voltage
goes higher than the rising overvoltage threshold (V_OVR). 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 (V_OVF), the fault is
cleared and the power good output signal is pulled low, the
device is back in normal operation.
Output Undervoltage
An undervoltage condition occurs once the output voltage
falls below the falling undervoltage threshold (V_UVF). 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 (V_UVR), the fault is
cleared and the power good output signal is pulled low, the
device is back in normal operation.
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 (I_LIM), the converter will start a cycle-by-cycle
operation to limit the current, and a 10 ms over current limit
timer (t_LIM) 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
t_LIM expires, and in this case normal operation is
regained.
• t_LIM 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 (t_TIMEOUT), the device will
attempt another soft start cycle.
• The device reaches the thermal shutdown limit (T_SDFET)
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 (I_SHORT). See below for more
details.
Short Circuit Current Limit
This block uses the same current detection mechanism as
the over current limit detection block. If the load current
reaches the I_SHORT 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 (t_TIMEOUT), 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
(T_SDFET), 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 (t_TIMEOUT) of 100 ms, the device will retry
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.
Analog Integrated Circuit Device Data
Freescale Semiconductor
34717
15