LM5085: Buck Controller duty cycle and output current queries

JASBEER SINGH

Part Number: LM5085
Other Parts Discussed in Thread: LM5145

Hi Team

We are planning to use LM5085 Buck regulator in our design and we are having queries which are as below:

1) If input will be Vin<= 12V then what will be the output of LM5085 and whether that voltage will be stable or not as MP1498 has wide i/p range (4.4V- 60V)?

2) We need 60W output (system requirement) from LM5085 which means (12V/5A) or (10V/6A).Can LM5085 provide 6A to load at 10V? If yes then how can we choose PFET used in that circuit?

3) In LM5085 , external Mosfet is there which is taking more space. Is TI having any parts which work in 100% duty cycle same as LM5085 but inbuilt MOSFET?

For the better understanding image is attached below.

Looking forward to your valuable response.

over 8 years ago

0 Brani Dubocanin over 8 years ago

TI__Expert 4100 points

Hello,
We are looking in your requirements and will get back to you soon.
regards
Brani

0 JASBEER SINGH over 8 years ago in reply to Brani Dubocanin

Intellectual 925 points

Hi Team

Gentle Reminder!

Please look into this issue as our project is on critical stage.

0 Timothy Hegarty over 8 years ago in reply to JASBEER SINGH

TI__Guru* 87597 points

Hello Jasbeer,

You can select a 60V PMOS FET for this application (see section 7.3.8 of the datasheet for further information). While the PMOS FET facilitates low dropout by operating at 100% duty cycle, note that the diode power dissipation is quite high when operating at high input voltage and 5A of output current. In that case, please consider a synchronous buck controller such as the LM5145 -- this has a low Toff(min) of 140ns and is thus suitable for low dropout operation.

PS: note that the MP1498 is a 16V/2A converter.

Regards,
Tim

0 Guruprasad Patil over 8 years ago in reply to Timothy Hegarty

Prodigy 20 points

Hi,

Our requirement is as follows

Vin Range: 12V to 48V

Vout: 12V

Iout: 5A

Questions regarding LM5085:

1) If input is 12V and set to 100% Duty Cycle, can we get 12V output exactly? What will be the tolerance?

2) To achieve 100% Duty Cycle, what will be the maximum switching frequency?

0 JASBEER SINGH over 8 years ago in reply to Timothy Hegarty

Intellectual 925 points

Hi Timothy

Thanks for your Guidance.

Could you please let me know what would be output of LM5145 if:
1) Vin=Vout
2) Vin< Vout

0 Timothy Hegarty over 8 years ago in reply to JASBEER SINGH

TI__Guru* 87597 points

Hi Jasbeer,

The LM5145 with Toff(min) of 140ns would have a relatively small dropout when VIN drops to the output voltage setpoint. The drop is related to the output current, inductor DCR and high-side MOSFET Rdson, as well as the switching frequency (as this impacts the maxiumum obtainable duty cycle). The output voltage is approximately given by: VOUT = Dmax*VIN - Iout* DCR.

The dropout would remain essentially constant as VIN decreases. You can review the LM5145 quickstart tool downloadable from the product folder for further insight.

Regards,
Tim

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LM5085: Buck Controller duty cycle and output current queries