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UCD90xxx for sequencing LMZ31xxx simple switcher

JulianE
Prodigy 170 points
Other Parts Discussed in Thread: LMZ31506, UCD9090, LMZ31520, LMZ31503

Hello,

we want to sequence several LMZ31xxx (LMZ31503, LMZ31506 and LMZ31520) simple switcher power modules. In the LMZ31xxx datasheets it is mentioned to use a external MOSFET to control the INH pin. Is it possible to use the UCD90xxx GPIO (configured as open-drain) directly to drive the INH pin? On a Xilinx Eval Board i have seen this configuration using an additional pull-down resistor to limit the voltage on the INH pin to ~3.6V (see attached schematic). My question is if this circuit is valid when using the above mentioned simple switcher modules together with the UCD9090? This would allow the saving of several external MOSFETs...

Best regards,

Julian

ucd90xxx and lmz31xxx

  • 0
    TI__Genius 9845 points

    Hi Julian,

    This design is risky. The two paralleled lower resistors give 4K. So when Vin=14V, the voltage on UCD9090's GPIO is 14V/14K*4K=4V, exceeding UCD9090's absolute maximum rating (V33A+0.3). Even if you adjust the resistor divider value to give GPIO pin 3.3V @ Vin=14V, you need to consider power sequence and over voltage condition: 1. if Vin applies before V33A is present, the voltage on GPIO pin will still exceed its absolute maximum rating; 2. if Vin is over voltage, the voltage on GPIO pin will again exceed its absolute maximum rating. 

    Besides, adding a parallel resistor will also change LMZ's UVLO threshold, not sure if this is what you want.

    Thanks,

    Zhiyuan

  • 0 in reply to Zhiyuan Hu
    Prodigy 170 points

    Thanks Zhiyuan for your remarks. I used external MOSFETs. Its the more saver way...