SCSI2SD Schematic Notes: Difference between revisions
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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. | 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 ==== | ==== Preferred Option: Switching Regulator ==== | ||
* [http://www.onsemi.com/PowerSolutions/product.do?id=NCP3170 NCP3170] | |||
* Over 90% efficiency with 5V input. | |||
* [http://au.element14.com/on-semiconductor/ncp3170adr2g/buck-3a-8soic/dp/1924872?Ntt=NCP3170 CHEAP] $1.73 | |||
* * Max load current without a heatsink is [http://www.codesrc.com/utilities/thermal-resistance.php?Tamb=70&Tj=150&r_jc=1&r_b=0&r_ha=87&efficiency=90&voltage=3.3&switching 2.75A] | |||
==== Backup Option: Linear Regulator ==== | |||
A [http://www.national.com/mpf/LM/LMS1585A.html LMS1585A] linear LDO regulator can be used to convert the 5v supply to the required 3.3v. | A [http://www.national.com/mpf/LM/LMS1585A.html LMS1585A] linear LDO regulator can be used to convert the 5v supply to the required 3.3v. | ||
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* At an expected peak current of 800mA, the regulator will dissipate: (5-3.3)*0.8 = 1.36W | * 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 | * Thermal Resistance Junction-to-Case: 2.3C/W | ||
* Max load current without a heatsink is [http://www.codesrc.com/utilities/thermal-resistance.php?Tamb=70&Tj=150&r_jc=2.3&r_b=0&r_ha=65&efficiency=66&voltage=3.3 | * Max load current without a heatsink is insufficient at [http://www.codesrc.com/utilities/thermal-resistance.php?Tamb=70&Tj=150&r_jc=2.3&r_b=0&r_ha=65&efficiency=66&voltage=3.3 0.7A] | ||
== In-circuit programming == | == In-circuit programming == | ||
Revision as of 11:45, 1 November 2011
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.
Preferred Option: Switching Regulator
- NCP3170
- Over 90% efficiency with 5V input.
- CHEAP $1.73
- * Max load current without a heatsink is 2.75A
Backup Option: 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 insufficient at 0.7A
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
