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The operation with low dropout voltage and VIN<VOUT for LM22672, LM34919C and LM2842

Atsushi Yamauchi
Genius 4810 points
Other Parts Discussed in Thread: LM22672-Q1, LM34919C-Q1, LM2842-Q1, TPS54140, LM22672

I would like to confirm the operation with low dropout voltage and VIN<VOUT for LM22672-Q1, LM34919C-Q1 and
LM2842-Q1.

For TPS54140(TPS57140), the problem occurs the condition of low dropout voltage and VIN<VOUT (like
oscillation by VIN and VOUT voltage for boot capacitor (charge and discharge)).

Are there same operation by LM22672-Q1, LM34919C-Q1 and LM2842-Q1?

Best regards,
Atsushi Yamauchi 

  • 0
    TI__Mastermind 18940 points

    Hello Yamauchi-san, 

    The LM22672 has a forced off time (200ns typ, 300ns max) each cycle. During this time the boot capacitor should be able to charge up. When the part reaches dropout condition (Vin getting close to Vout), it will operate at maximum duty cycle and Vout will follow Vin - the drop out voltage. There should not be oscillation. I have tried 5Vin with 5V output setting and 0.5A load and did not observe oscillations. 

    It looks like you can estimate the corner of the input voltage where dropout occurs by using the equation on page 9 of the LM22672 datasheet. It depends on the load current, FET Rdson, inductor DCR, and switching frequency. 

    Regards, 

    Denislav

  • 0
    TI__Intellectual 2320 points

    Hello Atushi-san,

    The output voltage in LM34919C will enter the voltage dropout region once the input voltage gets close to the minimum input voltage governed by the minimum forced off-time (Toff,min), switching frequency (Fsw), Load current (Io), High side (R_ds,on), low side internal (R_sense), Inductor DCR resistor, (R_dcr) and the low side external diode (Vd). You can use the following equation estimate the minimum input voltage.

    Vin_min=[ Vo + Io*R_dcr + Io*R_ds,on + Toff,min*Fsw*(Vd + Io*R_sense - Io*R_ds,on) ] / ( 1 - Toff,min*Fsw )

    You cannot use the part for Vin less than 4.5V. At high frequency and large load the minimum input voltage will be higher. There would be regulation loss once the part enter in this dropout mode.

    Regards,

    Sourav