LM3429: Vcc drop and nDIM pin question

WU YISYUAN

Expert 2165 points

Part Number: LM3429

Hi team,

my customer use LM3429 have some question.

1. Vcc voltage: LM3429's specification，define 6.9V,but this is under no-load condition.

Reference specification Fig.33 add dim circuit, but we find the Vcc pin voltage would drop to 5.05V

Vcc pin bypass capacitor add 2.2uF, reference Specification define.

and Icc also define current limit, no normal work current suggest.

Could you provide Vcc pin Voltage drop and Icc current's correspondence?

Vcc voltage drop will affect my dimming design

2.nDIM pin I know it have UVLO function.

but I saw the specification's reference design as below, what is this function doing?

thanks.

over 6 years ago

0 Tuan Tran74 over 6 years ago

TI__Mastermind 18890 points

Hi Yisyuan,

What is Vin at when you are measuring Vcc for the 5.05V condition? VCC is a low drop out linear regulator from Vin to provide you with 6.3V -7.35V as referenced in the specification. You might want to check the circuit and resistors on figure 33 on your board for correct values and verify that there is no issue there.

We do not take data for currrent versus Vcc drop for our production testing so we cannot provide that for you.

The function that Q3 provide in figure 27 is for PWM'ing the ouput current. This will pwm the output current to the controlled level or 0 current depending on the PWM'ing input. You can read more about that in section 7.3.10

Thanks Tuan

0 Tuan Tran74 over 6 years ago in reply to Tuan Tran74

TI__Mastermind 18890 points

Hi Yisyuan,

I have not heard back from you so I believe you may have solved the issue.
The thread will close now but will reopen if you reply.

Thanks Tuan

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LM3429: Vcc drop and nDIM pin question