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LM3481: ULVO Threshold for Enable and Disable

Rohit Murkar
Prodigy 60 points
Part Number: LM3481

I have used LM2481 IC in my flyback design

my flyback should power up from 5.1V

Its max input voltage is 30V

The circuit diagram file is inserted below.

For power cycle of 50msec ON and 100 msec OFF i found that LM3481 IC is not turning ON properly.

I reduced value of C36 capacitor to 330nF which helped me to have consistent pattern. 

Can you please help me to understand 

1. Under voltage Lockout Reference Voltage" when "Ramping down" 1.43V any other tolerance for Ramping up?

2. UVLO Shutdown Voltage 0.7V. then what is  UVLO enable (Ramping Up) threshold and its tolerance

3. once under voltage is detected but but with slow ramp down of ULVO pin and fast switching of power input; how fast LM3481 will respond?

Thanks

Rohit M  

  • 0
    TI__Guru 73310 points

    Please refer to Section 7.3.5 and Figure 29.  The datasheet gives the tolerance of the  UVLO comparator reference voltage VUVLOSEN in the EC Table (page 5).  It is the same for both ramping up and ramping down.  There is also an UVLO current source IUVLO, which helps create the UVLO hysteresis and separate the actual input line voltage ramp up and down thresholds. 

    The shutdown ramping up and down have the same threshold.

    In your circuit, when C36=1uF, the time constant of your UVLO network is about 100ms, for it delays too much to respond well to your 50ms ON and 100 ms OFF power cycling.   Unless you have a special need, do not use a such a large capacitor there.  You can refer to our EVM or reference designs for the UVLO in capacitor selection.  Also use our online tool Webench to help you design.

    Thanks,

    Youhao Xi,  System and Applications Engineering, APP-BMC-BCS, TI 

  • 0
    TI__Mastermind 21720 points
    Hi Rohit,

    Sorry to hear that you are having an issue with the LM3481. It sounds like the enable and shutdown voltages might be not be set correctly. Please refer to section 7.3.5 of the datasheet. Also C36 can be removed as the UVLO pin has built in hysteresis and should not have a chattering problem.

    Thanks,

    Garrett