LM2735: 3.7 V to 5V only regulates from Vin = 4.2

Hi Ignacio,

Thanks for reaching out.

1. Can you show your schematic and layout here? 

2. What is the effective value and saturation current of the inductor?

3. Can you also take the waveforms of Vin, Vout, SW and iL?

Regards,

Bryce

Hi Brice,

Thank you for your reply.

1) Schematic

2) Layout

Note: the compensation capacitor is soldered over the resistor R2 (Sandwich).

3) Inductor: Wurth 74477610. Imax 2.98 A, Isat = 3.24 A

4) Waveforms

4.1) Situation #1: Vin = 4.2

Vin

Vout

VSW

4.2) Situation #2: Vin = 3.7V

Vin

Vout

VSW

VSW --> Zoom

I cannot measure the IL, sorry for that. My first guess is that the inductor value (10 uH) is too big for this application

Please, let me know your thoughts

Hi Ignacio,

Thanks for your clear scopes. Below are my comments:

1. What is your input supply when you experiment? The battery or DC source? Why the input voltage is unstable at 3.7Vin?

2. I think the inductor current has decreased to zero from the scope of SW in 4.2Vin-5Vout, please double check the inductor value. It will be very helpful if you can measure the inductor current by putting a wire between pad and inductor and probe it.

3. From the scopes of 3.7Vin, the minimum input voltage has been smaller than UVLO falling voltage, which could shutdown the device.

4. I have check the layout, the gnd of Cout is far away from GND pin, which increases the parasitic parameter of the switching loop. The loop parasitic inductance in high di/dt current path will generate high voltage spikes at output and across power MOSFETs. Suggest it put close to GND pin like recommendation. Also, suggest remove the copper under the inductor.

Regards.

Bryce

Hello Bryce,

1. It is a DC source. It is stable but when I go down to 3.7 V when supplying the converter there is this variation. It disappears when I increase the power supply voltage to 4.2 V and the converter regulates.

2. I have ordered new inductors, once I perform the tests I will let you know.  I am going to beg for a current probe or purchase one in the meanwhile  

3. I think so too.

4. You are right, let's see what can I do with this PCB.

I will keep you posted!

Thank you so much

Ignacio

Bryce Xun said:

uctor current has decreas

Hi Ignacio,

Thanks for your update.

1. What is your supply current limit when you experiment? The minimum input current limit should not be less than 2A, assuming the efficiency is 0.8, Iin=5V*1A/(0.8*3.7)=1.7A. Please ensure that DC source current limit is enough.

2. Can you repeat the unstable phenomenon on other boards? It's good if you can capture the inductor current waveform.

Regards,

Bryce

Hi Ignacio,

Congratulation! Very glad to know that.

Regards,

Bryce