LM5122 VCC regulator+Driver

Giuseppe Ciavolino

Other Parts Discussed in Thread: LM5122

Hello to everyone,
the specifications of my application are:

Vin_min=5V and Vin_max=8.4V
Vout=9 V , Iout_min=10mA and Iout_max=500mA

I would not use an external supply and I was wondering if I can use my input to feed the Vcc regulator+Driver circuits.

I don't understand some of the technical terms used in the datasheet because this is the first time that I approah to DC/DC converters design. Infact, this isn't my first question about LM5122 and i'm sorry if i bother you with my stupid questions..
I have read that the VCC dropout is at least 0.5V so, if i connect the VIN pin with my input, VCC will varies from 4.5V to 7.9V?
This is very important to know, because from that depends the choice of the MOSfets.
Another question..for the LowSide mosfet the Driver switchs the gate from Vcc to Gnd while, for the High side mosfet, the driver provides a bootstrap circuit that puts in parallel a precharged Cbst to the Vgs terminal?Is it correct?
Thank you very much for the attention

over 8 years ago

0 EricLee over 8 years ago

TI__Guru 59001 points

Hello
You can supply the external 5~8.4V to VCC directly.
Max VCC dropout is 0.5V. So, if you connect the VIN pin with my input, VCC will varies from 4.5V to 8.4V
LM5122 Cbst will be charged from VCC through Dbst when SW node is pulled down to ground .
If no switching, Cbst will be charged by internal charge pump.
By the way, Your system is only 4.5W. Is there any special reason you have to use synchronous boost controller?
Thanks

0 Giuseppe Ciavolino over 8 years ago in reply to EricLee

Prodigy 230 points

Thanks for the answer, so you can confirm that with my input voltage I can feed directly the VCC regulator withoout any external supply, is it right?
By the way, i need a synchronous topology beacuase the application is for audio purpose and if the controller went to DCM, the frequency switching will decrease and maybe it will excite audio frequencies..Do you think that is unuseless?

0 Giuseppe Ciavolino over 8 years ago in reply to Giuseppe Ciavolino

Prodigy 230 points

I didn't notice when I answered but, why did you say that " VCC will varies from 4.5V to 8.4V"?It should not be "from 4.5 to 7.9?"

0 EricLee over 8 years ago in reply to Giuseppe Ciavolino

TI__Guru 59001 points

Hello
You can feed your external supply (5.0V to 8.4V) directly to VCC pin.
When VCC current is small VIN-to-VCC voltage drop is less than 0.5V.
Thanks

0 Giuseppe Ciavolino over 8 years ago in reply to EricLee

Prodigy 230 points

thank you very much..one last thing: how can I calculate the power consuption of the LM5122?

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LM5122 VCC regulator+Driver