WEBENCH® Tools/TPS54360-Q1: High frequency for small inductor size compromising efficiency

Hi John,

Thank you very much for such valuable information. You are right, I meant physically the same components.

I've decided to use 0603 to be the smallest package throughout the overall design and there are cases where you cannot find smaller packages such as 47uF which is 1210 for 6.3V endurance voltage.

At this point, I would like to ask if I can use any approach to lower these output capacitor values.

Does putting TVS diode at output helps to lower output capacitors? I also would like to build a failsafe design so I was considering TVS option anyway, thus I would like to hear your opinion about putting TVS at output for a failsafe design.

Ogulcan

Okay, I may consider to put electrolytic capacitors here if I have the chance to put it away from the IC or the inductor like 20mm distance.

Another problem I am facing is the inductor selection. Here I check the test frequencies of the inductors (in addition to inductance and saturation current). Thus, what can be the test frequency of the inductor if I intent to use 604 KHz as the switching frequency?

What do you think about the effect of the TVS diode at the output for a more failsafe design?

Ogulcan

Hi John,

I've used WEBENCH tool to simulate the values, used it directly.

But I'm planning to put extra 3000uF output capacitance all around the PCB (150mmx100mm area) in order to make it more durable to short electric cuts. If I can find space, I may put some extra capacitance (few hundreds) at the input as well.

My only specific concern is its being failsafe and handle harsh automotive current spikes and/or voltage instabilities. Apart form that, I just need a general overview, that's all.

Regards,
Ogulcan

Hi,

Here is my latest schematic for review.

What do you think about the output capacitance?

Regards,

Ogulcan

Hi John,

Thanks for your feedback, I've been working on the compensations for a while.

I managed to put larger capacitors to the output, so I made the calculations according to the above circuit;

I had some difficulty in building the circuit in TINA-TI, however I managed to do the calculations using Octave (octave.org). Copying and pasting the above code will do the work;

Ioutmax = 3.5; # maximum output current
Vout1 = 5; # 5V output
Vout2 = 3.8; # 3.8V output
RESR = 40e-3; # ESR at output (roughly same)
Cout1 = 724e-6; # output capacitance of 5V
Cout2 = 1448e-6; # output capacitance of 3.8V
gmps = 12; # power stage analog transconductance = 12A/V
gmea = 350e-6; # error amplifier transconductance = 350uA/V
Vref = 0.8; # reference voltage
fSW = 619.76e3;# switching frequency

RESRref = 2.5e-3; 
Coutref = 58.3e-6;

################################################################################
# REF #### REF #### REF #### REF #### REF #### REF #### REF #### REF #### REF ##
################################################################################
# Reference design calculations for clarity
fPref = Ioutmax / (2*pi*Vout1*Coutref) # 1911 Hz
fZref = 1/(2*pi*RESRref*Coutref) # 1092 kHz
fcoref1= sqrt(fPref*fZref) # 45.68 kHz
fcoref2= sqrt(fPref*600000/2) # 23.93 kHz
fcoref = min(fcoref1,fcoref2)
Rref = (2*pi*fcoref*Coutref/gmps)*(Vout1/(Vref*gmea)) # 13 k --goal 13k
C1_ref = 1/(2*pi*Rref*fPref) # 6.38 nF --goal 6.4nF
C2_ref = Coutref*RESRref/Rref # 11.17 pF --goal 11.2pF
################################################################################
################################################################################

################################################################################
# 5V #### 5V #### 5V #### 5V #### 5V #### 5V #### 5V #### 5V #### 5V #### 5V ##
################################################################################
# calculating fP1, fZ1, fco1 for 5V switching regulator
fP1 = Ioutmax / (2*pi*Vout1*Cout1);
fZ1 = 1/(2*pi*RESR*Cout1);
fco1_1 = sqrt(fP1*fZ1)
fco1_2 = sqrt(fP1*fSW/2)
fco1 = min(fco1_1,fco1_2)
# 5V resulting values
R1 = (2*pi*fco1*Cout1/gmps)*(Vout1/(Vref*gmea)) # 6.2251 k (6.2k)
C1_1 = 1/(2*pi*R1*fP1) # 166.15 nF (150nF)
C2_1 = Cout1*RESR/R1 # 4.652 nF (4.7nF)
################################################################################
################################################################################

################################################################################
# 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ### 3V8 ##
################################################################################
# calculating fP2, fZ2, fco2 for 3.8V switching regulator
fP2 = Ioutmax / (2*pi*Vout2*Cout2);
fZ2 = 1/(2*pi*RESR*Cout2);
fco2_1 = sqrt(fP2*fZ2)
fco2_2 = sqrt(fP2*fSW/2)
fco2 = min(fco2_1,fco2_2)
# 3.8V resulting values
R2 = (2*pi*fco2*Cout2/gmps)*(Vout2/(Vref*gmea)) # 5.427 k (5.49k)
C1_2 = 1/(2*pi*R2*fP2) # 289.69 nF (220nF)
C2_2 = Cout2*RESR/R2 # 10.67 nF (10nF)
################################################################################
################################################################################

So, what I'm not sure about this compensation loop values is whether I put the nearest values or the smaller (or larger) values. This depends on whether the values are calculated specifically or could be under or below a limiting point.

What I mean is, for example; I set the first resistance to be 6.2k. Do this value determine the lower limit so that it can also be 5k, or it has to be specifically 6.2k?

Thank you.

Regards,
Ogulcan

Hi John,

Sorry for such a long thread.

I will continue my simulations in mostly TINA-TI and hopefully finish today.

In the meantime, here are my PCB drawings in 2D and 3D:

Note that some capacitor 3D models are wrong, so no worries about that issue.

Regards,

Ogulcan