Designing DC-DC convertor : 24V - 5V Buck convertor, 3Amps
Buck converter--> Basic DC-DC convertor topology which is used for stepping down the Voltage.
So below is the simplistic view of the buck converter, don't wanna make the content boring by explaining how it works and all.
I wanna document the calculation behind the value of the inductor and capacitor. So let's go straight into the matter.
The TI "LMR51430XDDCR" IC is selected for a 24V to 5V converter which is capable of a maximum of 3Amps.
Input Specification:
Input Voltage: 4.5-V to 36-V input voltage range.
Required output voltage: 5V.
Continuous Output Current: 3 Amps
Frequency: 500-kHz
Calculations:
1. Calculating Duty cycle, D:
`D=\frac{V_{out}}{V_{i n} }`
`D= 5/24`
`D = 0.2084`
2. Feedback resistor calculations:
`R_{FBT}=\frac{V_{out}-V_{ref}}{V_{ref} } \times R_{FBB}`
`R_{FBT}= ((5 - 0.6) / 0.6) \times 13.7 \times 10^ 3 `
`R_{FBT}=100.4kohm`
3. Inductor calculation:
`L_{min}=\frac{V_{i n-max} - V_{out}}{I_{out}\times K_{i nd}} \times \frac{D}{F_{sw}}`
`L_{min}=\frac{24 - 5}{3\times 0.2} \times \frac{0.2084}{500\times1000}`
`L_{min}=13.19uH`
K_ind is the ripple factor which is usually 0.2 or 0.3 of the output current. Multiplying K_ind and I_out gives the Output ripple current.
4. Output capacitor calculation:
`C_{out}=\frac{I_{i n-max} \times K_{i nd}}{8 \times F_{sw}\times \triangle V_{out}}`
`C_{out}=\frac{3 \times 0.2}{8 \times 500\times 1000 \times 5\times 0.001}`
`C_{out}=30uF`
So as per datasheet recommendation 44uf is used (two 22uf cap in parallel)
3-D View:
5. Input capacitor selection:
- 2*4.7uF X7R dielectric capacitor with 50V rating
- 0.1uF capacitor for high frequency
6. PCB Trace width calculation:
Saturn PCB toolkit has been used to calculate the trace width of 3 Amps carrying PCB trace.
- Temperature rise 10 deg
- PCB thickness 1.5748(Standard)
- Conductor width 2.3mm (two plane is used: one in top layer and in bottom layer and connected through vias)
- Copper weight 1oz (35um)
Schematic:
3-D View:
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