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LMR16030: Enable threshold defeats purpose UVLO control

Genatco
Guru 56083 points
Part Number: LMR16030


Hello,

The formula used to determine UVLO for Enable input ends up being 1 amp current in Rent / Renb divider or 1R111111 / 32R32. The 1.2v enable threshold is so close to ground for external UVLO input range 28v-30v to control enable pin. Oddly the Vin states +3.715v threshold internal UVLO after POR, seems to defeat +1.2v enable UVLO control purpose. Did I mess up the calculation for LMR external divider as Rhom UVLO divider runs enable pins +4v from +24v input rail.  The reason to keep Rohm divider +4v above ground, transient avoidance 2v enable threshold. With 48v on the LMR a 1.2v enable threshold, seems to suffer to battery efficiency.

  

  • 0
    TI__Mastermind 31315 points

    Hi Genatco,

    If I understood you correctly, normally you should not expect to see 1A of current running through the UVLO divider. Usually resistors in the 10-100kOHm are used. Can you provide your design schematic for the UVLO so that I can take a closer look? 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Guru 56083 points

    Hi Richard,

    It's just a simple resistor divider, above formula renders 1A of current, I=ER  28v-30v input to divider for 1.2v enable threshold. We monitor the input battery rail for UVLO not the output voltage. Perhaps IHys was wrong in the formula decimal point 3.6µA needs to be 0.000036A?

    For Rohm 2v enable UVLO we use 75K to +24v input and 18K with 100nF to ground. Recall sets enable pin peak near 4.1vdc. So the enable becomes soft start as it crosses the 2v enable threshold but remains well above ground snub transients cause false reset. 

  • 0 in reply to Genatco
    TI__Mastermind 31315 points

    Hi Genatco,

    Sorry for the delay. Let me double check this formula and I'll get back to you later this week.

    Regards,

    Richard

  • +1
    TI__Mastermind 31315 points

    Hi Gentaco,

    The formula and hysteresis current seems correct to me. 

    In your situation, assuming you want a Vstart (when you want to enable the part) of 4V, and a Vstop (when to disable the part) of 1.2V, you would want a top resistor of 777kOhm and a bottom resistor of 166kOhm. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Guru 56083 points

    Hi Richard,

    Richard Fu22 said:
    Vstart (when you want to enable the part) of 4V,

    No, we want the EN threshold to be well above +4v input and more like 28v-32v where the LDO shuts off the EN pin below that stop point. So it's after the device is powered by fresh battery supply the discharge rate determines safe bucking via LDO to downstream regulators in the chain. That also stops the LMR from powering back up when the battery is depleted until it has been recharged, 

    Tina transient analysis was more affective to determine RENT (470K) and RENB (20K) with 4v dropout minimum 1.14v minimum enable threshold 28v input.  This sown table. That top part of the formula still produces RENT 1.111111Ω. That among other issues VStart RENT formula is not working 3.6µA divided into 28v - 32v (Vstart). 

    Richard Fu22 said:
    Vstop (when to disable the part) of 1.2V,

    That is the enable threshold where it will startup and stop when enable drops below 1.05v (yellow). Yet over a range of devices with a safety margin external RENT/RENB circuit +1.6v EN seems ok and +28v input LMR still produces +23v out.

    Only two graphs in the data sheet indicate LDO was tested 5v by drop out curves Fig.6.7, Fig.6.8 are not from 24v input. The recommended operating conditions Table 6.3 Vin (60V)  Vout(50V). Oddly the high end LDO 10v and Low end LDO Vin (4.3v), Vout (0.8v). So a 3.5v in to out is below the UVLO 3.715v discussed 7.3.6. There is no way to know other than Tina transient analysis what the input dropout voltage might look like.