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TPS5401 Output drops when Vin is low

Daniel Bandeira de Melo
Prodigy 60 points
Other Parts Discussed in Thread: TPS5401, TPS54240

My Circuit was designed to output 3.3 volts from ranges 3.5 to 42 volts. When the input falls below 5.1 Volts, to be exact, the output voltage drops to about 2.5V.

It is almost clear that the IC is not switching fast enough to charge the output capacitor. The circuit was designed using WEBENCH but all component values were

verified. The circuit is on a breadboard. The is not the one recommended on the datasheet. Anyone had a similar problem? In the simulation the circuit checks out

perfectly of course. I tried increasing the output capacitor and changing the shift frequency. Anyone had similar problems like this one?

  • 0
    Prodigy 60 points
    So I figured it out. I was testing the circuit without any loads. When I put a 600 Ohm resistor on across the output to simulate a load the IC started converting voltage inputs below 5V. My question now is: How can I create a load on my output that will always be around 600 Ohm? Most of the time the circuit is not consuming anything but when it starts working it will have its load and I am afraid that if I just leave a resistor across the circuit there will be too much load when the circuit is not dormant. Any thoughts?
  • 0 in reply to Daniel Bandeira de Melo
    TI__Guru**** 191445 points

    TPS5401 belongs to the same family as TPS54240.  For these devices, as Vin is lowered towards Vout, duty cycle increases.  It can even extend beyond 100%.  It is only limited by charge on the BOOT capacitor.  When the BOOT voltage falls below the UVLO, the high side FET trns off so the BOOT cap can recharge.  For higher load conditions, the circuit is operating in CCM and when the high side FET turns of the SW node is clamped one diode drop below GND and the BOOT cap can charge to approximately VIN.  If the circuit is at light load, and there is no inductor current, then SW node is at Vout.  Now since Vin is close to Vout, Vout must discharge to Vin - 2.1V before the BOOT cap can charge above its UVLO (2.1 V) and switching can resume.  You can pre-load your circuit to stay out of eco-mode like you are doing or try one of the techniques in this app note.  they are applicable to TPS5401 as well.

    www.ti.com/.../slva547a.pdf

  • 0 in reply to JohnTucker
    Prodigy 60 points
    Thanks John. I solved the problem by connecting a diode from the output pin to the SW pin.

    The problem I am having now is with the excess ripple in the output. I have read that something common to do is to design RC snubber from PH

    to GND. Any information on this subject? Thanks in advance for all the help
  • 0 in reply to Daniel Bandeira de Melo
    TI__Guru**** 191445 points

    RC snubber will help with ringing and overshoot on the SW node.  You can see this as "switching noise" on the output.  Enclosed is a document about it.

    7853.RC Snubber.doc