MC34844AEP- Not generating output

[Ashish93]

Hi 

I am using MC34844AEP this IC for my LCD panel backlight control. From this IC I am trying to give 24V output to my LCD panel. In this I am giving 12 input to this IC but I am not getting the output form this.

So I need help about the what will the causes for this problem and how can I resolve this.

Below I am attaching my circuit design for your refernce.

Vin = 12V

Vout = 24V

Iout = 0.8A

[JozefKozon]

Hi Ashish,

please refer to the typical application figures in the MC34844 datasheet. The Anodes of the LEDs must be connected to the Ix pins. 

Please do not use the VCx pins for any other, than internal use. 

Please use a resistor and a capacitor for the COMP pin. 

Please make sure, that the EN pin is high.

With Best Regards,

Jozef

[Ashish93]

Hi 

Actually we have operated this circuit in our previous board. we have developed 3 new boards from out of 3 led driver is working on 1 board but it is not working on other 2 board. I have check all the possible ways to check the issue but everything is fine. comp circuit is as per calculation, Rslop is also as per calculation, I am not using VDCx apart from internal.

So please let me know apart of these thing what will the possible issues will have?

[JozefKozon]

Hi Ashish,

you shouldn't use the VDC2 for the D3 diode. The VDCx pins are for internal use only. 

Please use the I0 through I9 separately. I0 shouldn't be short circuited to I1, I2 to I3 etc. Have you changed anything in the registers? Or have you left them in default values. 

With Best Regards,

Jozef

[Ashish93]

Hi 

But in my previous board it was working in the same circuit design and also so as per my understanding the issue is not in the circuit.

Can you suggest me some other ways.

[JozefKozon]

Hi Ashish,

if you are sure, that the hardware/circuit is correct, then it must be software. Please read the register values on the working board, and please set the same values on the non-working boards. Please refer to the I2C bus specification in the MC34844 datasheet. 

Please follow the sequence when configuring the registers.

With Best Regards,

Jozef

Jozef

[Ashish93]

Hi ,

We are using it in manual mode so I think the resistor configuration or setting is not required and also we are following the sequencing as per the manual mode.

[JozefKozon]

Hi Ashish,

please confirm, that you follow the recommended start sequence. 

Please confirm, that the SDA and SCK pins are grounded if you are using the manual mode.

Please connect the M/~S pin high for Master mode and connect an oscilloscope probe to the CK pin and GND to check if the MC34844 works. You should measure a PWM signal on the CK pin. 

With Best Regards,

Jozef

[Ashish93]

Please find circuit attachment 

[JozefKozon]

Hi Ashish, 

it seems the circuit design wasn't attached. Please resend the file.

With Best Regards,

Jozef