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LM36274 glitches in output current

AJIT PAL
Prodigy 100 points

Hi,

Iam using LM32674 EVM. I am seeing glitches in the LED current as well as inductor current. Please see waveforms below.

The setup is : VIN = 3.7V, 3P5S congif, duty cycle of PWM = 6%, rise/fall time = 5ns, Freq = 10kHz

The problem is threre for 6% duty . At higher duty, it vanishes and at lower duty it is lesser...most pronounced at 6%. 

The current is 30mA maximum, so 6% = 1.8mA.

  • 0
    TI__Intellectual 2140 points
    Hi,
    These glitches occur due to the internal DAC settling as the current changes. The DAC settling manifests itself as glitches in the current sinks. The glitches are concentrated mainly in the major transition codes (i.e. 01111111111 to 10000000000). The duration of the glitch will be no longer than 2.5us. This is a low duty cycle event compared to the time spent at each code during ramping and will not cause any issues in the system where it's being used.
  • 0 in reply to Angelos Tsiros
    Prodigy 100 points
    Hi Angelos,

    I am not changing current here. This is statis 6% duty cycle PWM at PWM input. From where is the question of ramping comes here? The PWM is supplied through function generator and it is very clean input.

    -Ajit
  • 0 in reply to AJIT PAL
    TI__Intellectual 2140 points

    Hi Ajit,

    The spikes you are observing are due to the fact that the corresponding brightness code that results from the PWM input signal is around a major transition. It appears that 2-bits of PWM hysteresis is not enough to filter out all the jitter at the PWM input, so the brightness code bounces between codes on either side of a major transition. To eliminate them, increase the PWM hysteresis setting to 4-bits (register 0x03, b[1:0]). This assumes that you are using a PWM sample frequency of 4MHz. If 1MHz sample frequency is used, you may need to use 6-bits of hysteresis.