34717 datasheet(15/25 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34717

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 (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.

Output Undervoltage

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.

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

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.

• 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.

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 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

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.

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