LM3481-Q1: LM3481 input capacitance recommendation in datasheet

Zoe Yang

Intellectual 2210 points

Part Number: LM3481-Q1
Other Parts Discussed in Thread: LM3481

Dear experts,

One of my customer using LM3481 as voltage regulator to output 12Vdc in EV/HEV application.

Input voltage is sensed by filter before Output 14V.

Some input parameter as followed:

Vin=13.6~15.3V, Vout=14V ,input capacitor recommend 100uF~200uF. But my customer is using 68uF instead.

They have tested and mass production with 68uF input capacitance for a few years and did not appear any failure on this point. They raise this concern because their end OEM customer challenge this spec which beyond our datasheet recommendation.

So here is a question need your advice:

What if input capacitance choose lower than 100uF, say 68uF？ 68uF capacitor with 27mohom ESR, 100uF capacitor with 35mohom ESR. Working frequency @ fsw= 280kHz.

Below ch3 waveform is inductor current.

I am looking forward to your reply! Thanks!

over 3 years ago

0 Harry Hofner over 3 years ago

TI__Mastermind 19595 points

Hi Zoe,

Chapter 8.2.1.2.7 of the datasheet which you have provided refers to a Boost topology.

But looking at the Input- and Output-Voltages it seems that the customer is using a SEPIC or maybe a Flyback instead.

Section 8.2.2.2.6 refers to the SEPIC and states: "the input capacitor is not as critical in a SEPIC application".

Even if the text mentions that a value in the range of 100 µF to 200 µF _should_ be used, the schematic shows a smaller capacitance.

It depends on the environment and the conditions around the converter circuitry which capacitor value will work and where the impedance interactions may come in.

Chapter 9 also mentions that the external conditions (e.g. long wires at the input) play an important role and have to be taken into account.

In the end it is the customer's responsibility to verify and prove if - in the given environment - a value of 68 µF will work stable under all conditions or not.

Best regards,

Harry