TPS923650：TPS923650D2: Analog dimming circuit configuration

Dear Sir,

Thanks for reaching out! Pls kindly see my comments as shown below:

The ceramic capacitors are an excellent choice for the input decoupling of the step-up converter since they have extremely low ESR and are available in small footprints. But do take care when using only ceramic input capacitors. For example, when there is load step at the output which can induce ringing at the VIN pin. Additional "bulk" capacitor (electrolytic or tantalum) in this circumstance, recommend to be placed between CIN and the power source lead to reduce ringing that can occur between the inductance of the power source leads and CIN.

kenny Lin said:

If MLCC is the only option for IC input, are there recommended specifications?

For this design, I am concerned about the current capability here. Let's say, we take into consideration the worst case: Vin=12V, Vout=55V, Iout=0.5A. It means that the maximum inductor DC current IL(dc)_max = Iout*Vout / (Vin*Efficiency). Even if the efficiency is ideal as 100%, IL(dc)max would be around 0.5*55/12=2.29A. And we also need to take the inductor ripple current into consideration which means that the IL(peak)=IL(dc) + 0.5*IL(ripple). 

If you look at below EC table about current limit spec, you can find that for TPS923651, the minimum inductor current limit is 2.8A. 

Thus, I would recommend that you can check the current limit according to your application. Thank you!

BRs

Lucia

Dear Sir,

Yes, you can use TPS923651 but pls make sure the max inductor peak current is not greater than 2.8A. For the inductor, you need to pay attention to the inductor rated current and saturation current. the saturation current rating of the inductor is at least as large as the high-side switch current limit. And for rated current, you can leave some margin like 1.2~1.5 * inductor rms current.

Thank you!

BRs

Lucia