Stabilizing the UVLO feature on a DC/DC converter

Question

Very many DC/DC controller IC's utilize an under voltage lockout feature by way of an external pin monitoring the applied voltage and comparing it with some internal reference, putting the IC into standby if this threshold is breached. Usually this pin is named "EN" or "UVLO".

I use a high resistance voltage divider to feed the EN pin on my IC. The divider is fed directly from Vin, the battery voltage, and set to output the correct threshold voltage when my batteries are discharged to my pre-defined limit.

When powered from Li-Ion batteries ( and probably any other battery I imagine ), when the threshold voltage is broken, the IC goes into standby mode. This removes the load on the battery, which raises the battery's voltage, which pushes EN over the threshold, and the IC resumes operation again for a few ms.

This cycle continues on and on until the battery, after 100's of 1000's of cycles, finally quits and doesn't bounce back to higher than the EN threshold.

My question is: what are the techniques used to feed the EN/UVLO pins of DC/DC converters to prevent this type of bounce? Is a separate comparator with wide hysteresis used instead of directly feeding EN?

Answer 1

Given the additional information that it is an LT8610, there looks like something that can be done.

The EN/UV input has a 1.0V +/-6% threshold and it has almost no (+/-20nA) bias current. That's with the input rising, it's 40mV less typically with the input falling.

You already have a voltage divider on the input. All you need is a resistor from Vout to EN/UV.

^{simulate this circuit – Schematic created using CircuitLab}

When the UV input is RISING, \$V_{OUT} = 0\$ and thus it will trigger at \$V_{IN}\$ =\$ 1.0V \cdot \$\$(R1 + R2 || R3) \over (R2 ||R3)\$

It is easy\$^{TM}\$ to show that the hysteresis: is \$0.04V\cdot\$ \$R1\over R1||R2||R3\$ + \$V_X \cdot( \$ \$ R1 \over R3\$), where \$V_X \$is the output voltage.

Answer 2

Most DC/DC controllers have hysteresis built into the UVLO/EN pin. For example, the TI TPS63061 boost/buck datasheet states:

Under voltage lockout threshold: 1.8 1.9 2.2 V (min/typ/max)

UVLO hysteresis: 300 mV.

Therefore, assuming the 'typical' conditions are in play, when VIN drops below 1.9 V, the device will shut down. It will not resume until the input voltage rises past the threshold+hysteresis voltage - e.g. 1.9+.3 = 2.2 V, which prevents the bounce problem due to removing the load that you have described.