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TPS78230 at Vdropout

Michael Litwin
Prodigy 30 points

Hello,

 

I would like to use the TPS78230 in a battery powered application.  I have the following questions.  Once Vin is below the dropout voltage:

1)   Will Vout be stable?

2)   Will Vout = Vin under the following conditions (2.75 >= Vin <= Vdropout)?

3)   What will Iq be at Vin < Vdropout?

 

Thanks.

  • 0
    TI__Expert 3300 points

    Hello Michael,

    Thank you for your post. In order to help you, I would like to understand a few more things:

    • By Vdropout, are you referring to the dropout voltage of the LDO? This is spec'd at a maximum of 250mV at 150mA load current. 
    • The minimum Vin required for stable operation is 2.2V. Thus Vin can never be less than or equal to the drop out voltage.
    • What is the output current you are requiring from this part? 
    Regards,
    Kartik

  • 0 in reply to Kartik Sinha
    Prodigy 30 points

    Hi Kartik,

    Vin to the LDO will be between 4.2V and 2.75V.  The current draw will be 20uA most of the time but the device does have different run modes when it will draw 6mA for 2 hours, 16mA for 110sec, and 20mA for 2sec.  There is also a motor that will draw 80mA at start and 42mA run, this device will run for 5sec. 

    I would like to know the characteristics of Vout when Vin is less than 3.0V.

    Thanks,Mike

  • 0 in reply to Michael Litwin
    TI__Expert 3300 points

    Hi Mike,

    Since your worst case current draw is 80mA, let us consider that:

    This part is specified at max 250mV Dropout Voltage for 150mA of current draw. Thus, your LDOmin_resistance or RDSon = (250mV / 150mA) = 1.67 Ohms

    If I multiply this by your max current draw of 80mA, I get a Dropout Voltage of = (80mA * 1.67Ohms) = 134mV

    I am assuming your Vout requirement is 2.5V. Thus, based on the above calculation you should be ok with Vin as low as (2.5V + .134V) = ~2.64V

    Providing an input below 2.64V will cut off the control loop circuitry of the LDO and may cause your Vout to be pulled down to GND by the load when your motor is drawing 80mA.

    Even though a Vin of 2.64V is ok to use in order to get a regulated output, I would not recommend it since with this input level we are operating at the boundary of the dropout region. As we get closer to this boundary region,  PSRR and transient responses are degraded.

    It is recommended to provide a minimum Vin  = Vout + 0.5V for optimal performance. 

    I hope this helps, please let me know if you have any more questions.

    Regards,

    Kartik