24V->12V,5V @ 2A Buck Converter Board
- Michael Botros
- Daniel Puratich
Status: Rev 1 Order Placed, functionality validated
Owner & Designer: @Michael Botros
Reviewer: @Daniel Puratich
Initial Requirements:
24V-12V @ 2A Buck Converter Board -
24V-12V Synchronous Buck Converter PCB @ 2A Max Load Current Consumption (24W)
I/O:
Input Voltage Connector
+24V
GND
Output Voltage Connectors (3x)
+12V
GND
Current Sense Connector
V_sense+ and V_sense-, or:
I_sense and GND
Features:
24V-12V Buck Converter @ 2A
Reverse Polarity Protection
Determine for given PMOS selection if conduction power loss is acceptable. If unacceptable, consult your leads to discuss compromising solutions
Status LEDs for both +24V and +12V voltage rails
Optional current sense interface
Should also be able to provide 5V @ 3A with the change of a few passives
This “requirement” is optional depending on the difficulty level we’re looking for. It would be useful for the use case of the PCBA, but it increase project difficulty.
Dimensions:
20mm x 30mm
No vertical design constraints
Documentation:
Component Selection
Buck Converter Option
https://datasheet.octopart.com/LMR16030SDDA-Texas-Instruments-datasheet-66077754.pdf
Simplified Schematic
Pin Functions
Output voltage
Output voltage controlled by voltage divider
-Recommended range 10k-100k ohms 1% tolerance
-Vref=0.6V is used.
Enable
-EN pin turns converter on and off
-A voltage of less than 0.95V shuts off device and 1.36V starts the device
-EN voltage cannot be greater than Vin+0.3V
-Do not apply EN voltage when Vin=0V
-Recommended to connect Vin to EN
Bootstrap Voltage
-Recommended to add a 0.1μF capacitor (ceramic X7R or X5R with voltage rating of 16V or higher) between CB and SW
Option 1:
Buck Converter
4.5V-36V input
2A max output
Overcurrent and Short Circuit Protection, temperature protection
2.46$ digikey (https://www.digikey.ca/en/products/detail/texas-instruments/LMR51420XFDDCR/16705137)
Option 2:
Buck, Boost, Flyback, forward and inverting switching modes.
2.7V-25V Input
3A max output
More complex, 14 pin IC (not all of them needed), larger footprint size
24.8$ digikey (https://www.digikey.ca/en/products/detail/analog-devices-inc/LT1371CSW-PBF/889199)
Option 3:
Buck Converter
10V-38V Input (12V output max)
3.5A max output
8 pins, requires many peripheral components
1.97$ mouser (Qorvo ACT4533AYH-T )
Option 4:
Buck Converter
4.3V-60V Input (1V-50V output)
3A output
8 pins, not too many peripheral components needed
6.75$ mouser (Texas Instruments LMR16030PDDA )
Option 5:
Buck Converter
4.2V-24V Input (1V-13V output)
3A output
8 pins, fewer peripheral components
2.28$ mouser (Monolithic Power Systems (MPS) MP2393GTL-Z )
Component Selection
Selected Buck Converter IC
https://datasheet.octopart.com/LMR16030SDDA-Texas-Instruments-datasheet-66077754.pdf
Output Voltage Divider
For 12V Output
RFBB = 16kΩ (0.10$)
https://www.digikey.ca/en/products/detail/yageo/RC0805FR-0716KL/727620
RFBT = 240kΩ (0.10$)
https://www.digikey.ca/en/products/detail/yageo/RC0805FR-07240KL/727762
with adjusted resistor values, VOUT = 12V
For 5V Output
RFBB = 16kΩ (0.10$)
https://www.digikey.ca/en/products/detail/yageo/RC0805FR-0716KL/727620
RFBT = 90.667kΩ
use RFBT = 91kΩ (0.10$)
https://www.digikey.ca/en/products/detail/yageo/RC0805FR-0791KL/728202
with adjusted resistor value, VOUT = 5.016V
The output voltage divider connects the output voltage rail to the Feedback pin. The buck converter has a precise reference voltage of 0.75V inside which it compares against the stepped down voltage of the voltage divider. It is noted that this 0.75V reference remains constant across operating temperature. By feeding in the desired output voltage into the feedback pin via a voltage divider, this allows the IC to do error amplification and compensate constantly for deviation to maintain a constant output voltage.
Frequency Setting Resistor
Switching Frequency of 500kHz selected. Select RT = 49.9kΩ (0.10$)
https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0749K9L/727265
The switching frequency resistor programs a set operating frequency that the IC switches at. A higher RT results in a lower operating frequency in a non-linear fashion. The switching frequency determines how often one switching cycle occurs. A higher frequency means a higher resolution which essentially means less output ripple. The cost of operating at higher frequencies is that switching losses are greater. There is a balancing act to be made with these factors as well as many others not mentioned here.
Output Inductor Selection
For 5V Output
Only need to consider 5V@3A condition since that inductor will also support 12V@2A.
L = 9.1μH
Initial option (very much subject to change)
New Calculations
L = 22μH
Inductor option
https://www.digikey.ca/en/products/detail/würth-elektronik/7447709220/1638648
Low resistance, good price and size.
The output inductor is a critical part of the buck converter and is used in tandem with a capacitor to deliver an output voltage by switching a voltage across the inductor really quickly such that there is an average current that the buck converter operates at. This voltage is also kept approximately the same by a charging and discharging output capacitor. This allows current and voltage to be output without passing through much resistance which results in lots of power losses. Larger inductance can handle higher voltages but takes up more room, costs more and has a longer transient time.
Output Capacitor Selection
From both voltage scenarios, it was determined that ESR<0.1Ω and that Cout>77μF.
Option 1:
In order to compensate for DC Bias at 5V and 12V, two parallel 47μF, 25V capacitors were selected to have large safety margins.
Also the effective resistance of the capacitor is within calculated parameters at the operating frequency.
https://www.digikey.ca/en/products/detail/murata-electronics/KRM55WR71E476MH01K/2782167
Option 2:
Using 4 of these 22µF ±10% 25V Ceramic Capacitor X7R 1210 in order to reach the desired capacitance for 5V (0.78$ each). Many capacitors are cheaper but have a capacitance that falloff to less than 10µF due to DC-Bias at 5V.
https://www.digikey.ca/en/products/detail/murata-electronics/GRM32ER71E226KE15K/13904980
The output capacitor in the buck converter works in tandem with the output inductor to apply a voltage across the inductor and allow current through to pass through the load. The capacitor takes time to charge and this means that turning the supply voltage on and off at a certain duty cycle maintains the output capacitor at an average voltage, similarly to the output inductor. This capacitor needs to have a rated voltage that is much larger than the desired output voltage to account for any voltage spikes. In addition to this, the capacitance needs to be large enough to supply a worst case duty cycle where the capacitor is discharged for a long time. When there is a large load transient, a large amount of power is needed from the capacitor and therefore enough capacitance is needed to provide the power for a sudden change in load.
Schottky Diode Selection
Breakdown voltage should be 25%>Vin and current rating should be same as maximum current output
https://www.digikey.ca/en/products/detail/diodes-incorporated/B330B-13-F/724947
The Schottky diode is critical in a buck converter as when the input switching transistor is off, the voltage across the inductor suddenly becomes negative and the current through the inductor begins to decrease. The Schottky diode supplies this current from ground since the voltage of the inductor terminal falls below ground as seen in the diagram below.
A Schottky diode is more efficient than a regular diode since it has a lower forward voltage and can work with much higher switching speeds as a result compared to a regular diode.
Input Capacitor Selection
Bulk capacitor and decoupling capacitor required.
Decoupling capacitor is 0.1μF and rated at double the voltage of the input (50V) and also has a rating of XR7.
https://www.digikey.ca/en/products/detail/murata-electronics/GCE21BR71H104KA01L/7363229
Bulk capacitor is recommended to be between 4.7μF and 10μF. Selected capacitor is rated for 50V and is 10μF with a rating of X5R.
https://www.digikey.ca/en/products/detail/murata-electronics/GRM21BR61H106KE43L/10326316
Two types of capacitors are needed at the input to ensure the input voltage into the buck converter is steady. A decoupling capacitor is usually 0.1μF and is put on the input to act as a low pass filter. Hooking it up to ground allows high frequencies to pass through and short to ground while the more DC like voltages go through the buck converter. A bulk capacitor is also added at a voltage much higher than the input in order to steady the input DC voltage. An average input voltage charges a bulk capacitor to the nominal input voltage but any sudden changes to the input voltage can be levelled by the bulk capacitor which will charge or discharge in case the voltage goes too high or too low. This requires more capacitance to be able to discharge or charge for longer so a larger capacitance is needed for a more unsteady input voltage.
Bootstrap Capacitor Selection
0.1μF recommended capacitance. Should be rated for 16V or higher. Should be high quality and X7R or X5R grade dielectric for temperature stability
https://www.digikey.ca/en/products/detail/murata-electronics/GCM188R71C104KA37D/1641648
The bootstrap capacitor in this application is used to provide the drive voltage for the high side MOSFET. Every switching cycle the capacitor is charged by the output while the low side MOSFET is active and the capacitor is discharged into the high side MOSFET to activate it.
It can be noted that the bootstrap capacitor isn’t directly going into the gate voltage of the high side MOSFET and as a result there is a current flow which means that the bootstrap capacitor can only supply the required voltage for a limited number of switching cycles before it recharges.
Soft-Start Capacitor Selection
A 16nF capacitor at a voltage like 16V will give a 4ms start-up time.
A 18nF capacitor was found at 16V which will give a slightly longer start-up time.
https://www.digikey.ca/en/products/detail/murata-electronics/GRM155R71C183KA01D/2175203
Jumper Selection
A jumper is used to switch between 5V @ 3A output and 12V @ 2A output. Only the output resistors are changed in this buck board so switching modes should be easy.
The following jumper connector was sourced as it is cheap and has a relatively small pitch.
https://www.digikey.ca/en/products/detail/sullins-connector-solutions/STC02SYAN/76372
Using a simple 2 pin header with the same pitch can be used with the connector to make a jumper.
https://www.digikey.ca/en/products/detail/würth-elektronik/61300211121/4846823
Reverse Polarity Protection
PMOS can be found below. Max VDS is 30V but the board only runs on 24V and its unlikely that there is 24V between both sides of reverse polarity protection since the input goes to Vin.
VGS is maximum 12V but activates at 2.5V. By using a 6.2V zener diode to drop one side of PMOS down from 24V to 6.2V, this gives sufficient activation voltage while not exceeding VCC.
The MOSFET is also rated for 3.3A which is more than the 3A amount we will draw.
https://www.digikey.ca/en/products/detail/diodes-incorporated/DMP3068L-7/5223214
For the 6.2V Zener diode, the BZX884B6V2L-G3-08 was used. This is used in conjunction with a 10KΩ resistor from the WARG library to ensure enough current is going through the zener diode to activate it while also not drawing too much current.
LTSpice simulation of reverse plugged in source showing 0V output
When plugged in properly, 24V is shown on the output and only 1.8mA passes through the 10k ohm resistor to ground.
Also, zener diode working properly since Vs (green) and Vg (purple) are about 7 volts apart which means that Vgs is within the maximum.
Maximum and Minimum Input Voltage
So for Vout = 5V, Vin is 7V-26V
for Vout = 12V, Vin is 14V-26V
Maximum input current is limited by high side MOSFET current limit in datasheet.
At Vin = 12V, current limit can be as low as 3.8A. 3.4A current limit was selected with a 0.4A buffer.