UCC28910: UCC28881 and UCC28910 Application Question

Cao

In both cases, it's not so easy to implement external shutdown, but it can be done.

For UCC28910 - you can only cause a shutdown by triggering one of the faults. Since all faults are non-latching, the IC will allow the VDD to cycle and attempt to restart - if the fault persists, the start attempt will be aborted, and after a 1-2 s delay, restart will be attempted again. So it would not be complete shutdown, the IC would attempt to restart every couple of seconds (depending on size of VDD cap). It would also mean that the response time to a release of the shutdown would be delayed until the next VDD cycle and restart attempt.

The easiest fault to trip is probably the brownout/line UV. This can be done by externally shorting the VS pin to GND with a MOSFET that is driven by the shutdown command. This will bypass the Ivsl current out of the VS pin, and it will look like very low input voltage. After 3 successive PWM cycles with low Ivsl current, the fault will be triggered, the VDD will decay to the reset level, then the internal current source will recharge VDD through the DRAIN pin. When VDD charges up, the IC will generate 3 PWM cycles to attempt to start, Ivsl will be low due to the ext MOSFET across VS-GND, then the fault will re-trigger. The cycle will repeat until the external short on VS is opened.

One thing to note, when the power stage is running and already enabled, by shorting VS pin to GND to cause a shutdown, this also shorts out the lower divider resistor for the output voltage feedback. So Vout will increase slightly as a result - but this will only happen for 3 PWM cycles, until the Ivsl(stop) fault is triggered, so the increase in Vout should be limited.

An alternative method to shutdown the UCC28910 is short the VDD pin to GND. If Vdd < 0.3 V, the startup charging current is limited to 0.3 mA max, so this will not cause excessive power dissipation in the IC, even at high line [ this means the VDD pull-down impedance must be < 1k-ohm max].

This method also ensures complete shutdown, since VDD is held low, there will no PWM cycles or restart attempts. However, depending on the size of the VDD cap, significant energy could be stored, so the short-term dissipation in the external discharge MOSFET could be quite large while it is discharging the VDD cap.


For the UCC28880, external shutdown is more tricky and depends on the topology being used. It's not recommended in this case to short VDD to GND, since the available charging current is not limited at low VDD, and could be as high as 3.8 mA.

The easiest method would be to short FB to VDD to inhibit switching, but this would not necessarily put the IC into a low power standby mode. You would also need to ensure that FB pin < abs max limit of 6 V. This could be tricky to implement with high-side buck configuration, since the IC GND is tied to the switch node and is moving up and down at the switching freq.

Low-side buck configuration would be easier to implement this shutdown.


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Thanks,
Bernard

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