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LM5118: Facing line regulation problem in power board.

Sam mishra
Prodigy 120 points
Part Number: LM5118

hello,

I have designed a buck-boost  DC to DC converter with LM5118 for 9V - 36V to 24V, 2A using WEBENCH Design tool.

I'm facing problem in line regulation of this circuit.The output of circuit is 24V for 9 - 36 Vin with no Load, But with Load it gives variation in output voltage. i have simulated this circuit on TINA TI simulation software and circuit is working here but not on my Power board.

Readings and schematic are below.

Please any one can tell me where is the problem in this circuit .

Vout with no load = 24.32V
In Put Current = 30mA
line regulation test for 9V-36Vin to 24v, 2A supply
Load resistor = 41.3 ohm    
Reading from 8V to 30.9V     Reading for 30V to 8V    
Vin (Volt) Iin(A) Vout(Volt) Vin (Volt) Iin(A) Vout(Volt)
8 0.61 13.12 8 0.6 13.06
9 0.68 14.83 9 0.68 14.76
10 0.76 16.43
10 0.76 16.44
11 0.83 18.16 11 0.83 18.05
12 0.83 18.67 12 0.8 18.6
13 0.53 15.56 13 0.47 14.69
14 0.5 15.78 14 0.5 15.82
15 0.54 17 15 0.54 16.99
16 0.57 18.03 16 0.57 18.03
17 0.6 19.14 17 0.6 19.17
18 0.63 20.19 18 0.63 20.22
19 0.66 21.29 19 0.67 21.34
20 0.7 22.39 20 0.7 22.4
21 0.73 23.51 21 0.73 23.44
22 0.75 24.32 22 0.28 14.79
23 0.71 24.32 23 0.29 15.5
24 0.68 24.32 24 0.32 16.14
25 0.66 24.33 25 0.32 16.84
26 0.63 24.33 26 0.33 17.48
27 0.6 24.33 27 0.34 18.16
28 0.58 24.33 28 0.35 18.8
29 0.55 24.33 29 0.36 19.48
30 0.53 24.33 30 0.39 20.11
30.9 0.39 20.7

  • 0
    TI__Guru* 76005 points
    You barely has not output voltage regulation. If both the Webench and Tina sim verified the design but your board does not, you have something wrong on your board, either some wrong connection, or wrong components placed, or the layout is poor. Recommend to order an LM5118 EVM as a start point.
  • 0 in reply to Youhao Xi
    Prodigy 120 points

    Screen shots.pdf9_36Vin_TO_24VDC.xlsxThanks for reply.

    please take a look of BOM and screen shots of schematic and layout attached below.

    All components and layout is as per TI recommendation.

    without load  this circuit is working but with load it doesn't work. please give us solution for this problem if any.

    please mention If there is problem in layout.

  • 0 in reply to Sam mishra
    TI__Guru* 76005 points

    First, your layout does not seem following the datasheet's layout guideline. Please read that section carefully.

    Since you get the Vout regulation at no load but lose it with load, it sounds like related to the current sense noise. Your schematic doe not have an RC filter reserved. Please refer to the datasheet Figure 19, and add R11 and C22 to your circuit, and C22 should be placed directly across the IC pins, and see if this help.

  • 0 in reply to Youhao Xi
    Prodigy 120 points

    i have read layout section from data sheet but don't get what changes need to do, please tell me if you notice any particular problem in layout.

    And still circuit is not working after adding R11 and C22 in the circuit.

  • 0 in reply to Sam mishra
    Prodigy 120 points
    Please could you update on this. We are waiting the solution and hold our design for development of this power supply.

    Let us know how we could proceed further on this. We know there are 12V EVM board available for this. IS this 12V EMV tested for line and load regulation for full load?

    www.ti.com/.../LM5118EVAL

    We could purchase this 12V EVM board but want to make sure this board is tested for specifications. If any test data is available for this 12V EVM board , let us share.

    If any 24V EVM is available, please suggest or we are planning to start with 12V EVM and would modify it for 24V voltage? Would this approach correct? we want to isolate the issue of layout if any in our design.

    We are worried since 24V design is working fine in simulation , but it is not working on actual board( that we design with taking care of all layout recommendation).

    Please suggest, Your early reply is much appreciated.
  • 0 in reply to Sam mishra
    TI__Guru* 76005 points
    Yes the EVM has been tested for output regulation against line and load variations. You can modify the EVM for 24V output. Just pay attention to the MOSFETs and output capacitor voltage ratings.