TPS55340: TPS55340 Layout checking

Hi David,

Before I look through the layout and give you comments, I have several things want to confirm with you: What does the input comes from? Why the vin range is 2.8V to 5.15V. And what's the maximum output voltage that you want? Considering the maximum duty cycle limit shown as below, the maximum output voltage will be limited by the maximum duty cycle 89% and minimum operating input voltage.

Hi, Zach

The input will be either

1. USB 5V

2. Single Cell Li-ion 3.7V 500mAh

My desired max output voltage is 38V. By using PMP15037, I will control VOUT from 6V to 38V.

Zack Liu said:

Considering the maximum duty cycle limit shown as below, the maximum output voltage will be limited by the maximum duty cycle 89% and minimum operating input voltage.

- May I ask you about the meaning of this duty cycle limitation, please? Curious whether there is an equation between the input voltage and the duty cycle.

- Will using a single cell Li-ion battery with a capacity lower than 500mAh become a problem?

Hope the VIN is okay with this.

- Thanks for providing the PCB comments, as well.

Hi David,

For a boost topology, the Vout to Vin equation is D=1-effi*Vin/Vout.

Assuming the minimum input voltage is 2.8V and efficiency is 0.85, duty cycle=1-0.85*2.8/38=93.7%, which is higher than the maximum duty cycle limit spec(minimum) 89% (extreme conditions). That means when li-ion battery voltage decreases down to 2.8V, due to the maximum duty cycle limit, the output voltage will not go up to 38V, approximately only 22V.

Using 89% as Dmax, the minimum operating input voltage to generate 38Vout Vin=(1-Dmax)*Vout/effi=(1-0.89)*38/0.85=4.9V. 

In summary, only the input comes from 5V USB, you could get 6V-38V Vout. If input comes from the single cell Li-ion battery, you could get 6V-22V Vout in extreme conditions.

Thanks for the clarification.

- Does the Li-ion's capacity 500mAh matters as well?

- Can you check my PCB layout, please? More vias needed?

Hi David,

Li-ion capacity decides the lifetime and maximum discharge current. You need check the maximum discharge current with battery factory.

For layout, below are my comments:

1. Input caps, inductor L, Diode locations are good. Please change output capacitors C7, C8, C9 locations close to the IC. Look the picture below, I place C7 more close to IC and the GND pins of IC are connected to C7, C8, C9 negative pads directly. The parasitic inductors of PCB could be reduced in such way. 

2. Put smaller value Cout more close to IC because ceramic caps has lower impedance.

3. Vias near the negative pad of input caps and output caps are good. 

Thanks, Zack. Like this?

I lowered the output voltage to 22V using your advice. Also, the output capacitor is closer to the IC now.

The WEBENCH is telling me to use

- two 1uF ceramic C7, C8

- one 10uF Electrolytic Capacitors  C9

Zack Liu said:

Put smaller value Cout more close to IC because ceramic caps has lower impedance.

Q1. You mean move 1uF (C7, C8) close to the boost converter as possible, not the 10uF since 1uF is a low impedance ceramic capacitor?

Q2. Considering the size, do I really need to use a 10uF Electrolytic Capacitor or can I replace this to a 10uF ceramic type?

The above layout is using a ceramic 10uF. Hope I can know the reason why I must use at least one Electrolytic Capacitor.

Zack Liu said:

Look the picture below, I place C7 more close to IC and the GND pins of IC are connected to C7, C8, C9 negative pads directly. The parasitic inductors of PCB could be reduced in such way. 

Q3. May I ask what you've meant by this; close decoupling capacitors decrease the parasitic inductors of PCB?