SCSI2SD Schematic Notes
Details for the circuit design of SCSI2SD.
SMT Type
- 0805 sized components will be used where applicable. These represent a good tradeoff between hand-solderability and PCB board space.
Crystal Oscillator
- LCP1751 requires a 25MHz crystal, which results in a 100MHz clock with x4 PLL
- The crystal requires 2 caps for stability. The required value is:
2 * (CL - CS)
Where CL is the crystal's load capacitance, as specified by the crystal manufacturer, and CS is the PCB's stray capacitance (around 5pF for a reasonable PCB).
TXC - 9C-25.000MEEJ-T Load capacitance 18pF. Therefore, use 2x 22pF standard ceramic capacitors.
Power Supply
Power Requirements
| 3.3V | 5V | |
|---|---|---|
| LPC1751 | 600mA
Rated at 100mA per supply pin. |
0 |
| SD Card | 200mA | 0 |
| UCC5617
Optionally powered by SCSI TERMPWR |
0 | 440mA |
| 74HCT05 | 0 | ?
Likely to be insignificant |
| Total | 800mA | > 440mA |
5V supply from a hard drive molex connector should provide more than sufficient current. A regulator will be required to convert the 5v supply to 3.3V.
Option 1: Linear Regulator
A LMS1585A linear LDO regulator can be used to convert the 5v supply to the required 3.3v.
- 5A max current is more than enough
- Easy TO-220 mounting
- 1.3V dropout @ 3A allows for 5V supply to drop to 4.6V
- Significantly cheaper than a switching regulator
- Simpler than a switching regulator.
- At an expected peak current of 800mA, the regulator will dissipate: (5-3.3)*0.8 = 1.36W
- Thermal Resistance Junction-to-Case: 2.3C/W
- Max load current without a heatsink is 1A
Option 2: Switching Regulator
- Only devices with an internal switch are considered for now for simplicity.
- Use 12V rail input, 5V doesn't appear high enough for stable operation.
LM2576: 52kHz, 75% efficient with 12V input, 3A load. @ 1A load, will dissipate ((1/0.75) - 1) * 3.3 = 1.1W. A bit too high to try without a heatsink. Minimum 6V input.
LM2676: 260kHz, 86% efficient with 12V input, 3A load. @ 1A load, will dissipate ((1/0.86) - 1) * 3.3 = 0.54W. No need for a heatsink! [2] Will run under 80 degrees. The surface-mount TO-263 has a lower thermal resistance than the TO-220, so may as well use this (just need 1Oz copper the size of the device under the pad.). $5.39
NCP3170: 90% efficient with 5V input @3A load. CHEAP $1.73
In-circuit programming
UART0 pins, + active low program enable power supply while programming ? Custom header ? Custom bootloader as well ? Reuse for serial debug output ?
Switches
TERMPWR, terminator, parity, SCSI ID
