TPS53015: TPS53015: UVLO

Alexis Tantot 

Question 0 - Can the device be damaged or VOUT falls below 5V when VIN gets too low?

No, the TPS53015 will not be damaged if VIN drops too low, but

Yes, the output voltage will drop below 5V when the input voltage can no longer support the 5V output due to the maximum dutycycle limitation imposed by the minimum TOFF time of 230ns (typical). When the input voltage drops to about 5.7V (Max Duty Cycle of 88%) the output voltage will begin to drop and track 88% of VIN, until VOUT drops to about 3.4V, when Under Voltage Protection will trigger and shut-down the converter.

Question 1 - Does the device disable itself without any external circuitry if VIN gets too low for the required VOUT (5V)?

No.  The Under Voltage Lock-out (UVLO) disables switching when VIN can no longer maintain VREG = 4.0V, which would compromise the integrity of the internal MOSFET switching and the analog circuitry that operates the TPS53015 control logic.

Question 2 - Is there a way to disable the device when VIN goes below a specific voltage (around 8V)

Yes.  You can add external logic to drive the EN pin low when VIN is less than 8V.  Typically, this would be accomplished with an external comparator with a built in reference voltage and a resistor divider from VIN.  If the converter generating the 12V supply offers a "Power Good" function, that power-good function could also be used to drive the EN pin.

Unfortunately, the TPS53015's EN pin was not designed with sufficient precision in it's threshold to suppose the EN pin being used as a UVLO pin with a divider from VIN to GND directly connected to the EN pin.

If you need a power controller or converter that offers a programmable UVLO function, let me know more about your application needs, such as load current, and other features, and I may be able to recommend something.

Hi Alexis Tantot,

0/ The TPS53015 will have a minimum off time (typ 230ns mentioned in datasheet) that will limit its max allowed duty cycle (D~= Vout/Vin), and as Vin get lower closer to the target output 5V and even goes below it, the controller will hit a point that it cannot increase duty cycle any further to maintain and regulate the output voltage to the intended value. The output voltage will go down as the part tries to operate in this max duty cycle condition.

1/ If Vin continues to go below such that the VREG5 (internal LDO with its input tied to Vin) goes below its UVLO, the device will shut off and stop switching, and output will drop to 0V.

2/ The TPS53015 does not have a precise EN threshold specified in the datasheet, otherwise, if it did, you could just add a resistor divider from VIN to the EN pin to set the VIN voltage at which you want the regulator to be disabled. The TPS53015 specifies a min logic high EN threshold 1.6V and max logic low EN threshold 0.5V. This wide undefined range in between would not allow for a precise disable point of ~8V while guaranteeing the part is also enabled at your nominal input voltage(12V?). Other buck regulators like TPS543620 or dual-output TPSM5D1806 specify a tight EN threshold range such that a resistor divider to EN pin can be used to set precise and repeatable VIN on/off thresholds. If you're open to using other devices, I suggest using WEBENCH and entering your input and output voltage and current requirements to see the available parts that meet your operating requirements. Most newer parts like the ones I've mentioned have precise EN thresholds compared to the logic levels in the TPS53015. If you must use TPS53015, considering looking for a voltage supervisor to precisely detect the voltage at which to disable the TPS53015, with the logic output of the supervisor driving the TPS53015 EN pin.

Regards,
Kris