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TPS40210/11 LED application

Red Yen
Expert 4360 points

the datasheet figure 36, 12V input, 700mA LED driver up to 35V application, why the comp pin need to  add "D3, mos, C14, R15?

the page3 error amplifier Why Icomp source (250uA) and sink current (2.5mA) have differed 10 times?

  • 0
    TI__Guru 56925 points

    the datasheet figure 36, 12V input, 700mA LED driver up to 35V application, why the comp pin need to  add "D3, mos, C14, R15?

    these components provide an inverted PWM dimming capability to the circuit.  Hence the label "PWM Dimming" at the gate of the MOS device.

    When PWM Dimming goes high, the NMOS device turns on, pulling COMP low through 2 diodes.  This forces the error amplifier ouptut voltage (COMP) below the valley voltage of the PWM ramp, for forced 0% duty cycle operation.  When PWM goes low the NMOS is turned off and COMP rises again and switching resumes.

    the page3 error amplifier Why Icomp source (250uA) and sink current (2.5mA) have differed 10 times?

    The error amplifier has an asymetric ouptut stage that has significantly less source capability than sink.  This allows the error amplifier to externally driven more easily than if it had symetric 2.5mA drive capability.  This is important for the above case, when the COMP pin is pulled low and this also moderates the error amplifier ramp rate on return to regulation to minimize the possibility of overshoot.