Power Section Notes
Scope
Build only the power section from the user-provided requirements:
- 12V DC input through a 2-pin screw terminal.
- P6SMB33CA TVS from +12V to GND.
- 220uF input bulk capacitor.
- LMR33630ADDAR buck converter generating +4V for a 4G module.
- Existing 100uF tantalum plus added 470uF low-ESR electrolytic bulk capacitance on +4V for A7672SA transmit bursts.
- AMS1117-5.0 linear regulator generating +5V from +12V.
- AMS1117-3.3 linear regulator generating +3V3 from +5V.
- Single GND return net.
Datasheet-backed implementation notes
- LMR33630ADDAR DDA pinout: PGND, VIN, EN, PG, FB, VCC, BOOT, SW, exposed pad AGND.
- LMR33630 EN must not float; for always-on operation it is tied to VIN.
- LMR33630 PG is open-drain and may be left unconnected if unused.
- LMR33630 VCC requires a 1uF capacitor to GND and must not power external loads.
- LMR33630 BOOT uses 100nF from BOOT to SW.
- LMR33630 adjustable FB reference is nominally 1.000V. For +4.0V: Rtop/Rbottom = 3. Implemented as 100k from +4V to FB and 33.2k from FB to GND.
- AMS1117 fixed regulators use SOT-223 pinout: pin 1 GND/ADJ, pin 2 VOUT, pin 3 VIN, tab VOUT. Output tantalum capacitance >=22uF is required for stability; user requested 220uF.
Load currents and thermal validation
User-provided load currents:
- +3V3 rail: 60mA typical, 100mA maximum.
- +5V rail direct loads: 30mA typical, 50mA maximum.
- +4V rail: 200mA average, 2A peak A7672SA transmit bursts.
Thermal calculations:
- U_3V3 dissipates (5V - 3.3V) * I_3V3 = 1.7V * 0.10A = 0.17W maximum.
- Because +3V3 is derived from +5V, U_5V supplies both direct +5V load current and U_3V3 input current. Worst-case U_5V current is about 50mA + 100mA = 150mA, so U_5V dissipates (12V - 5V) * 0.15A = 1.05W maximum. Typical U_5V dissipation is about 7V * (30mA + 60mA) = 0.63W.
- U_5V therefore still needs careful copper heat spreading and temperature verification under maximum load; it is not validated by considering only the direct +5V load.
- The requested 10x10mm copper pads and thermal vias should be implemented during PCB layout, but note that AMS1117 SOT-223 tab is VOUT, not GND. Any thermal vias directly tied to the tab pad must connect to the corresponding output rail, not GND.
Open validation assumptions
- +4V buck load is 200mA average and 2A peak burst, within the LMR33630 3A rating if layout, inductor current rating, and local module decoupling are adequate.
- P6SMB33CA is a bidirectional TVS with 28.2V standoff and about 45.7V clamp at pulse current; it protects against large surges but does not protect AMS1117 devices from sustained moderate overvoltage above their 15V abs max.