Hawaslsh
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Hello,
I am thinking of using a simple power sequencing chip from Ti, LM3880, to toggle the enable pins for a few voltage regulators. I posted the block diagram above. The chip's operation is quite simple. You toggle an enable pin and you get the following timing diagram for the three flags (outputs). On the left is the power up sequence, and the right power down.
I must be missing something crucial. Below is the recommended schematic. We are placing pull up resistors to the open drain outputs of the three flags (outputs). There is where my question lies. With no power to the LM3880, won't the open drain outputs be high impedance until a voltage is applied to the gate? Wouldn't this mean when Vcc is applied to the circuit, the flags would be pulled high by the pullups until the MOSFET turns on (has a votlage applied to its gate)? Clearly not, or TI wouldn't be saying so, what am I missing?
I had one other question. If one of the regulator's enable pin has a maximum rated voltage of the flag's output value. Could I simply use a voltage divider to drop the votlage to the regulators enable pin as below? The en pin draws 250uA according to the data sheet, seems ok?
Thanks in advance!
I am thinking of using a simple power sequencing chip from Ti, LM3880, to toggle the enable pins for a few voltage regulators. I posted the block diagram above. The chip's operation is quite simple. You toggle an enable pin and you get the following timing diagram for the three flags (outputs). On the left is the power up sequence, and the right power down.
I must be missing something crucial. Below is the recommended schematic. We are placing pull up resistors to the open drain outputs of the three flags (outputs). There is where my question lies. With no power to the LM3880, won't the open drain outputs be high impedance until a voltage is applied to the gate? Wouldn't this mean when Vcc is applied to the circuit, the flags would be pulled high by the pullups until the MOSFET turns on (has a votlage applied to its gate)? Clearly not, or TI wouldn't be saying so, what am I missing?
I had one other question. If one of the regulator's enable pin has a maximum rated voltage of the flag's output value. Could I simply use a voltage divider to drop the votlage to the regulators enable pin as below? The en pin draws 250uA according to the data sheet, seems ok?
Thanks in advance!