MC34844 - design

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MC34844 - design

224 Views
jaeyonglee
Contributor I

Hi, NXP.....

ask for MC34844.

 

Q1.
We found to update your datasheet, that is add resistor from VDC1 to VDC3.
ask to this point.
1. if don't apply what happens if we don't apply this resistor.
2. Reason to add this resistor.

 

Q2.
About circuit.
1. if add resistor(about 10ohm) to pin#10~pin#19 (LED feedback line) what happens.
2. What is the difference between MC34844EP and MC34844AEP?

Tags (1)
3 Replies

116 Views
reyes
NXP TechSupport
NXP TechSupport

Hi,

[A1] The MC34844/MC34844A may be slow to startup and provide LED drive current at ambient temperatures below -20 deg C. This is remedied by the addition of an external 1K ohm resistor between VDC1 and VDC3.

The data sheet application schematics and VDC3 pin functional descriptions have been updated to reflect this information.

 

[A2] Since it is a small value resistor, there should probably not be any negative effect by adding a 10 ohm resistor in series in the Ix lines (LED feedback lines). I’m not sure of why you want to add this series resistor, probably to make an RC filter for these lines, but it is not needed, I would recommend you to connect this lines directly (or with the 220pF on each line) as shown in the datasheet.

The 220pf caps are in order to compensate the effect of the added inductance by the long traces on each LED string.

 

About the second part of this question, the differences between these two devices (MC34844EP and MC34844AEP) were:

  • Up to 50 mA on the 34844 LED current per channel
  • Up to 80 mA on the 34844A LED current per channel
  • PWM frequency programmable or synchronizable from 100 to 25,000 Hz for the 34844
  • PWM frequency programmable or synchronizable from 110 to 27,000 Hz for the 34844A

 

MC34844EP is no longer manufactured, but the MC34844AEP is the replacement for this part number as can be seen in the PCN15664: http://cache.freescale.com/files/shared/doc/pcn/PCN15644.htm


Have a great day,
Jose

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116 Views
jaeyonglee
Contributor I

Thanks for your answer.....but i have more question...


Q1

i want to know what happens if don't apply resistor from VDC1 to VDC3,

and if apply resistor to feedback line.

I know it's bad, but I want to know what happens.

 

Q2

I wonder if there is any difference in the function of the IC.

 

Have a good day~~^^

YONG.

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116 Views
reyes
NXP TechSupport
NXP TechSupport

Hi Yong,

If you do not apply resistor from VDC1 to VDC3 you may have slow to startup.

About the resistors on feedback lines, nothing wrong would happened, you would probably add more consumption on each line.


Have a great day,
Jose

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Note: If this post answers your question, please click the Correct Answer button. Thank you!
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