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LP5012: Connect a P-MOSFET at the output drive a high current LED

Danilo A.
Mastermind 24715 points
Part Number: LP5012

Hi Team, 

We've received an inquiry from a customer if it is possible to connect a P-MOSFET to the output of LP5012 to drive an LED at higher current?

Does he need to add a pull up resistor to keep the MOSFET off when the OUT pins of LP5012 is not sinking current? He prefers to use 1Mohm.

The customer wants to know if it is important to set the resistor for iREF if the pull-up resistor is 1M ohm? 

Regards,

Danilo

  • 0
    TI__Expert 6695 points

    Hi Danilo,

    sorry to not reply in timely.

    if you choose this scheme to increase the max current, the pull up resister should keep the MOSFET off. When the output pin is off, the voltage is pulled up to VDD and the P-MOS is off. When the output pin turn on, the Vgs of PMOS is smaller than Vth and the P-MOS is on.

    in addition, the absolute Maximum ratings of voltage on OUTx is  6V and the VDD should not be too high.

    Best Regard

    Monet Xu

  • 0 in reply to Monet Xu
    TI__Mastermind 24715 points

    Hi Monet,

    The supply voltage in the customer's application is 3V. I understand that the value of pull-up resistor will depend on the characteristics of the MOSFET that he will use. If he choose a 1M ohm pull up resistor, does he need to put a ~1M ohm resistor to the iREF. What is the highest resistance that can be connected to the iREF pin? 

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 6695 points

    Hi Danilo,

    LP50xx scales up the reference current (IREF)set by the external resistor(RIREF) to sink the output current(IOUT) at each output port. the following formula can be used to calculate the external resistor(RIREF):

    RIREF = KIREF * VIREF / ISET (KIREF = 105, VIREF = 0.7V)

    When connect a P-MOSFET to the output of LP5012 to drive an LED at higher current, the current sink(LP5012_OUTx)  provide the drive current to the P-MOS.

    Too small drive current will lead to insufficient drive capacity. Please choose the proper resistor to the IREF refer to your P-mos.

    Best Regard

    Monet Xu