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LM27762: Output Current and Power Dissipation Estimation

ZX_W
Prodigy 50 points
Part Number: LM27762

Hi,

I would like to understand the output current allowable for this LM27762 is 250mA for each OUT+ and OUT- or both sum up to be 250mA?

Secondly, for the power dissipation  calculation:

PD = PIN – POUT = VIN × (–IOUT– + IOUT+ + IQ) – (VOUT+ × IOUT+ + VOUT– × IOUT–) 

For example, If I am trying to generate a OUT- = -5V and OUT+ = 5V with input of about Vin=5.2V, when plug in the values to the calculation its actually almost cancel out each other in the above calculation.

Please correct me if I a wrong. I just need to calculate estimation Pd with about 100mA on each OUTPUTs.

Please advise

Thank you

  • 0
    TI__Mastermind 36681 points

    Hello CH,

    Please see formula and sample calculation below to estimate the IC power dissipation.

    PIN = Ptot_VOUT- + Ptot_VOUT+ + VIN * IQ

    Ptot_VOUT- = (VOUT- * IOUT-) / EFF

                           = (5.0V * 100mA) / 82%) = 609.756mW

    Ptot_VOUT+ = (VOUT+ + VLDO+) * IOUT+

                             = (5.0V + 45mV) * 100mA = 504.5mV

    VIN * IQ = 5.2V * 390uA = 2.028mW      

    PIN = 609.756mW + 504.5mW + 2.028mW = 1.116W

    where:

                  EFF: refer to Fig. 9 (Efficiency for OUT-) of the LM7662 datasheet

                  VLDO+: refer to Section 6.5 (Electrical Characteristics) of the datasheet

                  IQ: refer to Section 6.5 (Electrical Characteristics) of the datasheet

     

    POUT = VOUT- * IOUT- + VOUT+ * IOUT+

               = 5.0V * 100mA + 5.0V * 100mA = 1.0W

     

    PD = PIN – POUT

    PD = 116mW

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    TI__Mastermind 36681 points

    Hello CH,

    Do you have further queries?

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    TI__Mastermind 36681 points

    Hi CH,

    If you don't have further queries, I will close this thread.

    Thank you.

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    Prodigy 50 points

    Hi Excel,

    Sorry for late reply...Thank you for you details explanation.

    I just want to clarify for Ploss_VOUT+ equation, the term of "VOUT-" as highlighted below is that shall be "VOUT+"?

    For typical LDO dissipation that I aware it suppose to be (Vout-Vin )*Iout, is that the VOUT+ of LM27762 is different from the typical LDO and hence ?Please correct me if I am wrong.

    PIN = PLoss_VOUT- + PLoss_VOUT+ + VIN * IQ

    PLoss_VOUT- = (VOUT- * IOUT-) / EFF

                           = (5.0V * 100mA) / 82%) = 609.756mW

    PLoss_VOUT+ = (VOUT- + VLDO+) * IOUT+

                             = (5.0V + 45mV) * 100mA = 504.5mV

    From the above calculation, I assumed that LM27762 shall be capable to source 250mA on each channels. 

    Please advise

    Thank you

  • 0 in reply to ZX_W
    TI__Mastermind 36681 points

    Hi CH,

    Yes, you are correct. It iwas a typo error. The Ploss_VOUT+ equation, the term of "VOUT-" should be "VOUT+" but the calculation still correct.

    By the way, I change the term in the equation from "Ploss" to "Ptot" to avoid confusion. "Ptot" means the total power consumption (Pdiss + Pout) of the particular rail. Hence, the equations for both Vout- and Vout+ were still correct. Please refer to the updated calculation below.

    PIN = Ptot_VOUT- + Ptot_VOUT+ + VIN * IQ

    Ptot_VOUT- = (VOUT- * IOUT-) / EFF

                           = (5.0V * 100mA) / 82%) = 609.756mW

    Ptot_VOUT+ = (VOUT+ + VLDO) * IOUT+

                             = (5.0V + 45mV) * 100mA = 504.5mV

    VIN * IQ = 5.2V * 390uA = 2.028mW      

    PIN = 609.756mW + 504.5mW + 2.028mW = 1.116W

    where:

                  EFF: refer to Fig. 9 (Efficiency for OUT-) of the LM7662 datasheet

                  VLDO+: refer to Section 6.5 (Electrical Characteristics) of the datasheet

                  IQ: refer to Section 6.5 (Electrical Characteristics) of the datasheet

     

    POUT = VOUT- * IOUT- + VOUT+ * IOUT+

               = 5.0V * 100mA + 5.0V * 100mA = 1.0W

     

    PD = PIN – POUT

    PD = 116mW

     

     

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    Prodigy 50 points

    Hi Excel,

    Thank you for your details explanation.