TPS62404-Q1: TPS62404-Q1 Vout1 is 300mV lower

Hi,

The marking looks okay for the TPS62404-q1 device. Could you provide more detail about the schematics and operating conditions? 

Best regards,

Tanvee

Thanks for replying Tanvee! So this picture shows my TPS62404-Q1 circuit. Sorry I cannot show you the rest of my circuit because it's mostly proprietary. But I've cut the traces on PCB so that Vout1 (V1P575) is now isolated from the rest of the circuit to eliminate the possibility of a heavy load causing this issue. My input voltage is 5V.

Is there anything I should look for in the switching waveforms? I've checked that the switching frequency was around 1.8MHz for the Vout1 buck.

Do I need to keep this buck converter operating in CCM?

Here's what I've done so far to investigate this issue (in this order):

1. Cut the trace to isolate Vout1 from the rest of the circuit, Vout1 measures 1.2V.

2. Added 30ohm resistor at Vout1 to ensure the output current is more than 51mA and in CCM operation, Vout1 measures 1.2V.

3. Disconnect the MODE pin (pin 2) from GND and connect it to Vin (5V) to put device in constant PWM, Vout1 still measures 1.2V.

Hi,

The Schematic looks okay.  Yes, it would be good if you provide the switching waveform with input voltage and inductor current? Because it is a fixed frequency device.  

Best regards,

Tanvee

Hi Tanvee,

I did another test today. I cut the DEF_1 trace and connected it to 5V and then measured precisely 1.9V at Vout1. In fact, it is 1.9V even when I removed the 30ohm load at the output, which tells me that Vout1 is not impacted by the current load (i.e. CCM vs DCM). This device that I have is behaving exactly like the TPS62404, not TPS62404-Q1.

So these are the changes that I've made so far (in this order):

1. Cut the trace to isolate Vout1 from the rest of the circuit, Vout1 measures 1.2V.

2. Added 30ohm resistor at Vout1 to ensure the output current is more than 51mA and in CCM operation, Vout1 measures 1.2V.

3. Disconnect the MODE pin (pin 2) from GND and connect it to Vin (5V) to put device in constant PWM, Vout1 still measures 1.2V.

4. Cut the DEF_1 pin (pin 5) from GND and connected it to Vin (5V). Vout1 measures precisely 1.9V, with and without the 30 ohm resistor at the output.

I'll get you the switching waveforms later, because it'll take me a while to rework my board to probe the inductor current waveform. But the observations so far seems to tell me that I didn't get the correct part.

If you have some known good TPS62404-Q1 on hand, can you please send a few to me so I can replace the ones that I currently have? This should ultimately confirm whether the TPS62404-Q1 I got from the vendor is different from TI's sample.

Hi,

Thanks for the update.

The measurement gives more understanding of the device is behavior. The marking seems correct for the device and it is decreasing chances to get the wrong device. I would suggest to buy it from the TI store. Another possibility is the device might be damaged during the measurement.

Best regards,

Tanvee 

Hi Tanvee,

I ordered this EVM from digikey a couple of days ago (https://www.digikey.com/products/en?mpart=TPS62400EVM-167&v=296). I replaced the on-board TPS62400 device with my TPS62404-Q1 that I ordered from TI store, and the result is the same, Vout1 is 1.2V when DEF1=low and 1.9V when DEF1=high regardless of MODE/DATA setting. Based on the schematic in http://www.ti.com/lit/ug/slvu178a/slvu178a.pdf, I had to remove R5, R6, R7, C3,  and C8. R4 was replaced with a 0 ohm resistor. I've captured the SW1 waveforms below.