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Turn on/off the LM5121 (start/stop switching) with UVLO, Css or FB Pin

Other Parts Discussed in Thread: LM5121, INA169

Hi,

I want to charge up a capacitor using a boost with a LM5121. To keep the charging current constant I use a INA169 current sense monitoring sensing the boost´s input current. The INA169 output is connected to the Error Amplifier. So the input current is regulated, not the output voltage. To turn off switching (with a comparator) when the output voltage is reached and to turn on again when it falls I see 3 possibilites:

1) using the UVLO Pin: if this pin is pulled to ground, the LM5121 stops swtiching, but shut-down-wake-up takes to much time (about 400us I measured on the evalboard). Pulling UVLO  to 0,5V...1,17V also stops switching immediately, stand-by-wake-up time is good enough for my application (about 20us I measured on the evalboard). But pulling UVLO to the required voltage with a diode as recommended in the datasheet is a very precariously way - over the temperature range -40...+70°C. I have designed a solution with a an external voltage reference, but it is not a "keep it simple design"...

2) using the SS-Pin: pull down the SS-Pin works fine and fast, but I want to have the soft-start only during start up and not during normal operation (output capacitor discharged by pulsing load and reloaded again). I thought I can avoid th Css, because the current is regulated. But Css is also needed to ensure the BST voltage for the High Side FET. So maybe also not a recommended solution.


3) using the FB-Pin: I pulled the FB-Pin to 2.5V to switch off and it works fast switching off, but  disconnecting the 2.5V and the regulator starts with a delay of 2ms and needs a lot of time (ms) to regulate (in my simulation).

What Pin would you recommend to use for  fast turn-on/off switching?

I appreciate any comments,

br

Andy