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SN6505B: Voltage drop issue

GARETH OU
Mastermind 8775 points
Part Number: SN6505B

Hi 

SN6605B question, the attachment is T4 SPEC, Vin: 5V, Vout: 6V, Iout:> 0.2A. At present, it is found that Vout is 7.4V when starting up, but Vout will slowly drop to 1.24V during the starting process (see the figure below). This power is supplied to U7033 and U7076, but the voltage will still drop when U7033 and U7076 are disconnected. Up to 1.24V, is it caused by T4? If so, is there a suggestion for Turns ratio? Does T4 sense value need attention?

Measuring point

CH1: T4 Pin 4 (frequency is 409.2KHZ)

CH2: Vout

760390014-1724450.pdf

  • 0
    TI__Genius 17100 points

    Hi Gareth,

    Thank you for posting to E2E! I appreciate your inclusion of waveforms, schematics, and datasheet in this post.

    The transformer V-t product and turns ratio seem appropriate for a 5V to 6V application, so external factors like temperature or unexpected load transients could be affecting the push-pull transformer operation.

    Have any LDOs or other components been damaged? At what temperatures is this system operating when Vout drops to 1.24V?

    If possible, please confirm the voltage levels at the LDO inputs (DAC_6V) and pin 1 or 3 of the SN6505B during the events shared above. It will be helpful to confirm the SN6505B's switching behavior during this state and that the LDOs' recommended input voltage range is not being exceeded.


    Thank you, and happy holidays!
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Mastermind 8775 points

    Hi Manuel

    The current test is to connect EN Pin to VCC, so that Power can be output normally (Figure 2).
    EN PIN is connected to 1.8V GPIO, which meets SPEC requirements EN> 0.7V (Figure 1), but it is found that the output can only be normally output at the moment of power-on, and then the voltage will drop. Does EN have special requirements?

  • 0 in reply to GARETH OU
    TI__Genius 17100 points

    Hi Gareth,

    Thank you for sharing this additional information. To confirm, have you found the output (DAC_6V) is normal when SN6505B's EN pin is connected to Vcc directly and when EN pin is connected to a 1.8V GPIO, the output is normal during power-on and then drops?

    The logic HIGH threshold for EN and CLK pins of SN6505 is a proportion of the supply voltage, Vcc. As written in the datasheet and in Figure 1 you shared above, this threshold level is equal to a maximum of 70% of the Vcc level -- or 0.7x Vcc. At Vcc = 5V, the device could turn on if the EN pin voltage is between 1.5V and 3.5V (considering the logic LOW threshold and 0.7 x 5V = 3.5V), but it is specified to reliably turn on once the EN pin voltage meets or exceeds the 3.5V upper limit.

    We strongly recommend controlling this EN pin with a 5V GPIO, connecting EN to Vcc permanently using a 4.7kΩ resistor, or reducing the supply voltage to 2.25V (if the 1.8V GPIO must be used since 0.7 x 2.25V = ~1.6V).


    Respectfully,
    Manuel Chavez