LM1084: LM1084-adj

Richard Jenkins

Part Number: LM1084
Other Parts Discussed in Thread: LP395

Tool/software:

I'm looking to regulate a 24-28V 5A supply down to 22V, removing any ripple etc. Is this a suitable device? Is there a recommended configuration or precautions?

4 months ago

0 Ishaan Shetye 4 months ago

TI__Expert 6320 points

Hi Richard,

Yes, this is a suitable device for your application. At 5A, you are right at the edge of the max current for this device and with the max dropout voltage of 1.5V, you have 0.5V of headroom on VIN at 24V Input. So please pay close attention to input voltage droop (which can make the LDO go out of regulation) and inrush currents (which may cause current limiting). The configuration in fig 7.1 of the datasheet is recommended.

Thank you and best regards

Ishaan

0 Richard Jenkins 4 months ago in reply to Ishaan Shetye

Prodigy 20 points

Thanks. To confirm: the power dissipation in this regulator is (28-22V) * 5A = 30W?

0 Ishaan Shetye 4 months ago in reply to Richard Jenkins

TI__Expert 6320 points

That is correct.

0 Richard Jenkins 4 months ago in reply to Ishaan Shetye

Prodigy 20 points

With the above voltages, currents, I would like to switch the device on/off with TTL. Can I use 2N30904 NPN with the two resistors, configured as in sect 7.2.3 ?

0 Ishaan Shetye 4 months ago in reply to Richard Jenkins

TI__Expert 6320 points

I assume you mean 2N3904 NPN? Yes, you can use the configuration shown on fig 7-3. TI does not make 2N3904 so I cannot speak to its specific ratings however any NPN transistor with Vbe_max >TTL (High), Vce_max > Vadj_max and Ice_max > 50 mA should work. I suggest using TI's LP395 if possible.

+1 Richard Jenkins 4 months ago in reply to Ishaan Shetye

Prodigy 20 points

Yes 2N3904 NPN, I looked for a surface mount equivalent, and found FMB3904 to be closest, would this work?

+1 Ishaan Shetye 4 months ago in reply to Richard Jenkins

TI__Expert 6320 points

FMB3904 is not a TI device so I cannot advice you on it. If it satisfies the general conditions I highlighted above, then yes, it will work.

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LM1084: LM1084-adj