LM3409-Q1: LM3409 loop parameter question

Meng Liu

Intellectual 1765 points

Part Number: LM3409-Q1

Hi Expert

in the EVM:

1. my customer change the parameter is:

Roff: 1K

Coff: 470pf

RSNS: 75mohm

EN PWM: 140HZ

PWM min pulse: 5uS

min stable current: 385mA

customer used internal PWM method, control EN pin.

situation: When external PWM(control EN) reduce to 130HZ, circuit output will close.

Question 1:the relationship with EN PWM frequency with output current? do you have equation to explain it?

Question2: when used EN control mode, device will use internal PWM, could you please tell us the internal control frequency?

Queation3: we want to optimization output current rise and fall time, do you have any guide to optimize it?

over 1 year ago

0 Meng Liu over 1 year ago

TI__Intellectual 1765 points

Hi expert

i have future question:

can we use sent PWM to IADJ and the EN at the same time for control output current?

if yes, how to calculate output current?

0 Tuan Tran74 over 1 year ago in reply to Meng Liu

TI__Genius 17640 points

Hello Meng,

Please give our expert a couple of days to answer to you on this request.

Thanks Tuan

0 Xiaoguang1 over 1 year ago in reply to Meng Liu

TI__Genius 14030 points

Meng,

When PWM is added to EN PIN, the device runs between on/off and the inductor current charges from 0A to regulated current. See below. Some part of the current charges output capacitor. It is possible that the LED has no chance to turn on if PWM pulse is low.

You can use shunt FET pwm control mode which described in the datasheet(8.3.8 External Parallel FET PWM Dimming). The inductor current always keeps regulated at designed value.

Regards,

+1 Xiaoguang1 over 1 year ago in reply to Xiaoguang1

TI__Genius 14030 points

More comment: It's not recommend to add PWM to IADJ pin. For stability and good current regulation consideration, DC voltage is expected on IADJ pin.

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LM3409-Q1: LM3409 loop parameter question