implemention of internal regulator in high voltage dc-dc con

[edwintsu]

Dear all

In the high voltage dc-dc regulator, typically using internal low voltage regulator(Typ. 5V) to supply the high side NMOS driver with the help of external bootstrap capacitor.

My question: How to implement the internal 5V power typically? ( Using ldo? Too wide input voltage, too high voltage, and how to settle the high speed load transient? )

Thank you!

------
Edwin

 

[rajanarender_suram]

LDO will do, with high cap at the output(where 5V is Drawn)

 

[edwintsu]

Thanks! But i have no pin for high cap.

 

[rajanarender_suram]

How is ur load transient varying? (frequency and Peak transistion current)

 

[haff99]

Re: implemention of internal regulator in high voltage dc-dc

This is simple. You have to charge your bootstrap cap off of the supply voltage. Basically it goes like this input voltage-switch-diode. You have to have a circuit that monitors the bootstrap cap voltage during the charging phase(when the SW/LX pin is low). Once you reach full charge you shut off the swtich. There are other considerations like maybe you want to control the slew rate when charging the boot cap to avoid disturbing your input rail. It isn't a hard thing to do, but is a little more difficult then charging via a dedicated LDO.

 

[edwintsu]

Re: implemention of internal regulator in high voltage dc-dc

How is ur load transient varying? (frequency and Peak transistion current)

Freq. ~400K
Load Transient: 0~100mA

Added after 10 minutes:

This is simple. You have to charge your bootstrap cap off of the supply voltage. Basically it goes like this input voltage-switch-diode. You have to have a circuit that monitors the bootstrap cap voltage during the charging phase(when the SW/LX pin is low). Once you reach full charge you shut off the swtich. There are other considerations like maybe you want to control the slew rate when charging the boot cap to avoid disturbing your input rail. It isn't a hard thing to do, but is a little more difficult then charging via a dedicated LDO.

Thanks. But sorry, i didn't understand your meaning. I want to know how to get the internal power for the bootstrap circuit.

 

[haff99]

Re: implemention of internal regulator in high voltage dc-dc

If you want to charge the bootstrap cap to 5V, you do not necessarily need a 5V internal supply. You are going to have issues charging the bootstrap cap from an internal LDO if you do not have a large decoupling cap(meaning an external cap). What is going to happen is you are going to get huge current spikes on your 5V LDO when the bootstrap cap is in the charging phase,that will corrupt your whole rail, especially if everything else is running off your 5V rail(especially if your switch is large).

It may be easier for you to charge the bootsrap cap up to 5V right off your input voltage. It doesn't matter if your input voltage is 12V 24V 48V etc. All you need a switch, a diode and circuit that monitors the bootstrap cap voltage. So basically what happens is you turn the swtich on and start charging the bootstrap cap. Once you reach 5V on the bootstrap cap you shut off the switch. The advantage is your input supply already has a large decoupling cap. Therefore you do not need a totally seperate 5V LDO.

 

[edwintsu]

Thanks. Good idea, i got it. I will try the method.