TPS563200: Output Voltage Resistor Equation 0.768 constant seems wrong

Robert Butts

Part Number: TPS563200

I used the Webench to come up with a 12V to 3.3V 2A regulator circuit. It is the basic circuit in the TPS563200 datasheet on Figure 19 with the R1 = 33.2k Ohms and R2 = 10k Ohms but I'm getting a voltage output of 4.75 volts.

When I use these values the 0.768 constant in the Vout equation under section 8.2.2.1 actually turns out to be 1.131. Is there something I'm missing or should I select R1 to be 19.5k Ohms with R2 = 10k Ohms?

Thank you

over 5 years ago

0 Shawn Nie over 5 years ago

TI__Genius 10070 points

Hi,

Please refer below form and equation to configure the circuit.

Shawn

0 Robert Butts over 5 years ago in reply to Shawn Nie

Intellectual 260 points

I used the document at:

www.ti.com/.../tps563208.pdf

And I used the circuit of Figure 19 in section 8.2. I want Vout to be 3.3V so I used the recommended resistor values of R1 = 33.2k and R2 = 10k found on Table 2.

My actual Vout voltage measures 4.75V

Do I just have to calculate out what the ACTUAL equation constant (told to be 0.768) instead of using 0.768?

0 Shawn Nie over 5 years ago in reply to Robert Butts

TI__Genius 10070 points

Hi,

Could you please share your schematic? The Vout should be 3.3V, let's see if there is any issue on your schematic.

Shawn

0 Robert Butts over 5 years ago in reply to Shawn Nie

Intellectual 260 points

I tried clicking the paper-clip to attach a pdf of the schematic but it's trying to insert it in the reply and the schematic is not right.

mkr-4-pg2.pdf

0 Robert Butts over 5 years ago in reply to Robert Butts

Intellectual 260 points

It looks like it worked. Needless to say the converter is U2. Thanks

0 Shawn Nie over 5 years ago in reply to Robert Butts

TI__Genius 10070 points

Hi,

There is no issue for the schematic, please double check the soldering and the values of R1 and R2.
Also, have you tried to replace the IC with a new one?

Shawn

0 Robert Butts over 5 years ago in reply to Shawn Nie

Intellectual 260 points

I've used two ICs, the solderings all look good and the resistor values have been verified. At this point using the Vout I'm getting with the resistor values I'm using I recalculated Vout = 1.014 x (1 + R1/R2) to give me R1 = 22.56k and R2 = 10k

I'll be getting the new resistors in soon and will try that. I DO NOT like surrendering and having to fudge it like this but I've tried the suggested values with two circuit boards and two sets of components with the same result of Vout = 4.75V

0 Shawn Nie over 5 years ago in reply to Robert Butts

TI__Genius 10070 points

Hi,

Could you please share the layout? Let's check it.

Shawn

0 Robert Butts over 5 years ago in reply to Shawn Nie

Intellectual 260 points

Hi Shawn,

I don't know if these two pictures are what you're looking for. Let me know if it is something else you need.

Thanks,

Rob

0 Shawn Nie over 5 years ago in reply to Robert Butts

TI__Genius 10070 points

Hi,

The layout is not good:

1. Very thin trace is used for the power stage.

2. The input and output GND are not connect to IC GND directly with large area. And you use via for C7 GND.

3. the SW trace is under the IC, we do not recommend this, it is easy to introduce noise to FB.

Shawn

0 Shawn Nie over 5 years ago in reply to Shawn Nie

TI__Genius 10070 points

Hi

I am thinking the system is unstable, could you share the waveform (VIN, VOUT,SW,IOUT)?

Shawn

0 Robert Butts over 5 years ago in reply to Shawn Nie

Intellectual 260 points

Ok Shawn, I see the recommended layout in Figure 36 of the datasheet. I'm attaching pictures of the signal plots on a scope but I'm going back to the drawing board and redoing the layout.