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TPS54061 short circuit coil current

Ron Michallick
Guru**** 164496 points
Other Parts Discussed in Thread: TPS54061, TPS54622, TPS54618, TPS5430

What happens during a shorted output?
I see current limit on the high driver but no current sense on low driver. Because VOUT is zero and the higher driver can only see the current while driving high, the device will always have finite time high pulses regardless of the coil current. Won't the coil current keep increasing well beyond the current limit because very little coil current is lost (during catch time) with a VOUT of zero?

Does TI have any buck regulators that limit coil current during a shorted output?
I envision a current sense on catch transistor that prohibits high drive.

  • 0
    TI__Guru**** 191445 points

    TPS54061 uses high side pulse by pulse peak current limit as you noted.  There s always a minimum on time as the current limit is not in effect during the blanking (minimum on) time.  To help control inductor current, the TPS54061 uses frequency fold back.  As the current demand increases past the current limit, the output voltage will fall as you note.  As the fed back output voltage falls relative to the internal reference, the frequency folds back by divide by 2, 4, and 8.  This effectively increases the off time allowing the switch current to ramp down further than normal.  The other popular option is hiccup mode current limit.  In that case when a fault condition is encountered on the output, the device shuts down and restarts.  It will continue this hiccup operation until the fault is removed.  TPS54622 is an example of a device with hiccup mode current limit.  Devices in such as TPS5430 use both pulse by pulse and hiccup mode.  If a certain number (determined by internal counter) of consecutive switching cycles occur where the current limit is activated, hiccup mode is activated.  I'm sure there are many others with hiccup mode or with both high side and low side current limit (TPS54618 for example).