Automotive buck converter

Thread Starter

Jesu84

Joined Aug 29, 2021
4
Hello!
Designing my first pcb and I want to hear your oppinion about desing.
Purpose of the pcb is to step down cars battery voltage to 3.3V for future microcontroller design.
Is the design suitable for this purpose and do I need to change something?

1707046518034.png1707046476113.png
 

Ian0

Joined Aug 7, 2020
9,831
Not bad, but one thing is really important in DC-DC converters is to keep the area of the current loop small.
The current through the inductor takes one of two paths from input capacitor to output capacitor, one through the MOSFET (inside the device) and one through the diode. Make these paths as short as possible.
There is an example in the datasheet:
Screenshot from 2024-02-04 13-14-05.pngThe important ones are input bypass capacitor, catch diode, output inductor and output filter capacitor.
The others really don't matter anywhere near as much.

This isn't one of TI's better pinout designs, because the ground current track has go through the boot capacitor unless you use two vias.
The LMR38010 has a better pinout with the boot capacitor between pins 7 and 8.
 

Papabravo

Joined Feb 24, 2006
21,225

BobTPH

Joined Jun 5, 2013
8,991
How much current do you need at 3.3V? A buck converter is unlikely to be better than a linear regulator for 10s of mA.
 

crutschow

Joined Mar 14, 2008
34,455
I agree that, unless the circuit will be using a lot of current, or you want the circuit to operate for a long period with the engine off and the vehicle battery not being charged, a simple linear regulator would likely be a better choice.
 

Thread Starter

Jesu84

Joined Aug 29, 2021
4
Good point my friends.
I will reposition those components.

I did not take into account load dump at all :D Thanks for reminder. Maybe add a TVS diode?

I need at ~1A current. Maybe more later. Engine off consumption is not a problem becouse car have battery disconnect switch and device is not needed when car is not running.
 

crutschow

Joined Mar 14, 2008
34,455
I need at ~1A current.
That will dissipate some over 10W with a linear regulator, so a switcher does seem appropriate here.

Load dump is a rare occurrence, but a TVS diode will help protect against it.
A small resistor (e.g. 10Ω) in series with the 12V, with a large capacitor to ground at the regulator input, will also provide protection.
 

Ian0

Joined Aug 7, 2020
9,831
That will dissipate some over 10W with a linear regulator, so a switcher does seem appropriate here.

Load dump is a rare occurrence, but a TVS diode will help protect against it.
A small resistor (e.g. 10Ω) in series with the 12V, with a large capacitor to ground at the regulator input, will also provide protection.
Calculating for 100% efficiency, 3.3V @1A would require 9V @366mA (worst case battery voltage whilst cold cranking)
The 10Ω resistor will dissipate at least 1.3W.
 

crutschow

Joined Mar 14, 2008
34,455
Calculating for 100% efficiency, 3.3V @1A would require 9V @366mA (worst case battery voltage whilst cold cranking)
The 10Ω resistor will dissipate at least 1.3W.
I don't think how much the resistor dissipates during the short cold cranking period (where accessory power is normally off) is a problem.
But you have a good point, as a 10Ω series resistor will dissipate more than a watt, even at normal charging battery voltage.

A 5Ω resistor would thus be better, which will dissipate about a half watt with a nominal 14V charging battery voltage, so a 1W resistor would be sufficient.
 

Ian0

Joined Aug 7, 2020
9,831
I don't think how much the resistor dissipates during the short cold cranking period (where accessory power is normally off) is a problem.
But you have a good point, as a 10Ω series resistor will dissipate more than a watt, even at normal charging battery voltage.

A 5Ω resistor would thus be better, which will dissipate about a half watt with a nominal 14V charging battery voltage, so a 1W resistor would be sufficient.
The choice of TVS voltage is interesting as well. The TPS54233 will stand 28V, so a 24V TVS will protect the device against load dump transients. However, if the transient exceeds 24V the TVS will dissipate almost twice as much power than a 15V TVS, but the transient will exceed 24V far less often than it will exceed 15V.
 

Papabravo

Joined Feb 24, 2006
21,225
Good point my friends.
I will reposition those components.

I did not take into account load dump at all :D Thanks for reminder. Maybe add a TVS diode?

I need at ~1A current. Maybe more later. Engine off consumption is not a problem becouse car have battery disconnect switch and device is not needed when car is not running.
Careful. Every automaker has standards that your design must pass. As a minimum I would try to find out wha they are so you can make a determination about what you need to do.

2nd point: There is way more RF polution in and around a vehicle than there was 35 yeas ago when I was in your shoes. This problem is a two-way street: you cannot effect or disturb the radios and data links, and you must not be disturbed by them.
 

smanville

Joined Mar 24, 2010
2
For automotive usage, you defiantly and absolutely want to add a TVS on the input and add a clamping diode on the output (to protect all 3.3V sensitive downstream components or devices). The TI buck referenced in your initial design is capable of delivering 2 Amps on the output which will handle your ~1A requirement.
 
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