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TPS40055 SS/SD Pin Behavior

Tim Finks
Prodigy 155 points
Other Parts Discussed in Thread: TPS40057

+40 V to +5 V buck converter:  Circuit slow start works as expected when the device is first enabled by de-assering an open collector switch at the SS/SD pin. If I then pull SS/SD low, the device is disabled, also as expected. However, if I subsequently re-enable the device by turning off the OC switch at the SS/SD pin, the converter does not restart. Is this expected behavior?

  • 0
    TI__Guru 56925 points

    Tim,

     

    No, this is not expected behavior of the TPS40057 controller.

     

    Have you monitored the SS/SD pin voltage following the release of the Open-Collector switch?

    This pin should be charged by the internal soft-start current source to 3.4-3.6V after the open collector has been released.

     

    If the SS/SD voltage is rising and falling in a ramp (average voltage about 0.5V) the controller is entering into a fault condition on restart and shutting down.

    Do you have a load on the 5V output while conducting this test?  With 5V on the ouptut, the pre-bias circuitry of the TPS40057 controller can limit the charging of the bootstrap capacitor (capacitor from SW to BOOST) that powers the high-side driver.  This could result in less than 5V available to drive the high-side driver and errant triggering of OCP due to higher high-side Rdson.

    Alternately, if the output voltage maintains 5V on the ouptut, the conveter could be waiting in pre-bias for the ouptut voltage to drop and the converter to start running.

     

    If SS/SD is high (3.6V) Feedback is low (less than 680mV) and COMP is also high (also about 3.6V) and the converter is not switching, another issue has been created that we need to look into, but please check these conditions first.

     

  • 0 in reply to Peter James Miller
    Prodigy 155 points

    Without a load on the converter, the output returns after the OC switch on the SS/SD pin is de-asserted. The SS/SD pin voltage returns to about 3.6 V after being near zero during shutdown. With the expected 5 V output, the COMP pin is at 1.68 V.

    However, with a load of 100 mA (or more), the converter does not turn back on after being disabled (there is no output, and the converter is not switching). The COMP pin measures 0.0 V. The SS/SD pin voltage is not rising and falling in a ramp. When the converter does not return to on after SD, the SS/SD pin sits at a constant 0.2 V

  • 0 in reply to Tim Finks
    TI__Guru 56925 points

    Tim,

     

    I will be out of touch for the next 3 weeks.  I will be traveling and unable to access the TI forums, however:

     

    Check your Open-collector switch to see if it is sinking any current.  The SS/SD pin of the TPS40057 controller only sources 2.5uA of current, if the OC switch is sinking more than this current, the SS/SD voltage will never rise.  You could try adding a 1k resistor in series with the OC switch to measure the current (if any) into the OC switch.