LM3404HV: Fluctuation of output current

Dice-K

Part Number: LM3404HV
Other Parts Discussed in Thread: LM3404

Hi support team,
Please let us know about the specification and operation of the LM3404HVMR.

I am prototyping based on "Design Example 1" of the data sheet.

If the input voltage (24V) is fluctuated in 1V increments in the range of 18V to 36V, the output current will fluctuate by several mA.

18V: 132.2mA (actual measurement value)
36V: 140.2mA (actual measurement value)

Is this current fluctuation within the standard range of the product?
Or is there anything that can be inferred as a factor that causes the output current to fluctuate?
A prototype circuit is attached, so please let us know if you notice anything.

Regards,
Dice-K

over 3 years ago

0 Sean Zhou over 3 years ago

TI__Mastermind 22745 points

Hi Dice-K,

From your schematic, you have used the 1.65 as the "CS" resistor to configure the output current, the typical value should be about 200mV/1.65ohm=121mA. I guess your resistor will have some variation which caused some variation for the output current.

For the variation of the current that you observed, it seems a little large than the spec shown in "Figure3".

One caution is that you need to measure the output current when it is stable, there is some transition during you change the supply voltage. So please wait some time to measure again.

Here I would recommend not measuring the output current while measuring the voltage on the "CS" pin since for our LED driver, we will regulate the voltage on the "CS" pin to achieve constant output current.

Sean

0 Dice-K over 3 years ago in reply to Sean Zhou

Mastermind 6500 points

Hi Sean,
Thank you for your reply.

Page-25, SNVS465G

From Equation (34), does the IF increase as the VIN increases?

Best regards,
Dice-K

0 Sean Zhou over 3 years ago in reply to Dice-K

TI__Mastermind 22745 points

Hi Dice-k,

No, ideally IF doesn't have a direct relationship with the Vin. IF should keep stable without the change of the VIN.

Sean

0 Dice-K over 3 years ago in reply to Sean Zhou

Mastermind 6500 points

Hi Sean,
Thank you for your support.

Realistically, how is the relationship between IF and VIN?

From equations (1) and (34),

it becomes the following equation.

IF = (0.2 / RSNS) - {(VO * tSNS) / L)} + [{(6.7 * 10^-11 * RON) / L} * {1 - (VO / VIN)}]

RSNS, VO, tSNS, L and RON are constants.
The larger the VIN, the larger the third term, which is a positive component.

Does this equation represent a realistic relationship between IF and VIN?

Best regards,
Dice-K

0 Sean Zhou over 3 years ago in reply to Dice-K

TI__Mastermind 22745 points

Hi Dice-k,

There is no relationship between "Vin" and "IF", 'IF" is regulated by IF=VREF-REG / RSNS. And you don't need to concern about the VIN.

Sean

0 Hirotsugu Kobayashi over 3 years ago in reply to Sean Zhou

Mastermind 8675 points

Sean-san,

Since the LM3404 is COT controlled, the valley current of the inductor can be determined by Rsns and Vref, which is Vin independent.
However, the average output current is the valley current plus half the ripple current.
As the input voltage increases, the on-time decreases, but the peak current value of the inductor increases.
Does the average output current fluctuate with Vin?

Best Regards,

0 Sean Zhou over 3 years ago in reply to Hirotsugu Kobayashi

TI__Mastermind 22745 points

Hi Hirotsugu,

As long as the M3404HV can regulate the supply voltage, the average current is only dertamined by the internal reference voltage and the resistor connected to the "Rsns" pin.

Sean

0 Hirotsugu Kobayashi over 3 years ago in reply to Sean Zhou

Mastermind 8675 points

Sean-san,

From Rsns and Vref can be set the valley current value of the inductor.
The average current value is the valley current value plus the half ripple current value.
Is my understanding wrong?