SCSI2SD Schematic Notes

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Details for the circuit design of SCSI2SD.

SMT Type

  1. 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 200mA

42mA excl. peripherals.
See IDD(REG)(3V3), Table 6, LPC1751 datasheet.

(160mA @ 80% efficiency)
SD Card 200mA

Peak value from [1]

(160mA @ 80% efficiency)
UCC5617 N/A 440mA

4V - 5.25V

74HCT05 N/A 150mA (50mA * 3)

4.5V - 5.5V

Total 400mA 910mA

5V supply from a hard drive molex connector should provide more than sufficient current. The 3.3v supply will be regulated from the 5v supply to share input capacitance and therefore reduce complexity (since each set of input filter caps will need a resistor+disconnect circuit to decrease inrush current).

Switching Regulator Requirements

  • Require at least 83% duty cycle, to allow operation down to Vin = 4v.
  • Require >= 90% efficiency to reduce heat.
  • > 500mA output.
  • Fsw >= 1MHz for small output filtering capacitance.
  • Easy hand-soldering.

MAX1951 Design

  • MAX1951
  • Supports Vout == Vin.
    • Regulator won't dropout if the 5V rail temporarily drops down to 3.3V.
  • Over 90% efficiency with 5V input.
  • 2A output
    • Max load current without a heatsink is 1.36A
  • 1MHz fixed switching
  • Digital soft-start
  • Reasonably priced for single units $4.41
  • Designed for use with small ceramic capacitors.
    • Only 10uF is required for both the input & output filters.
    • Additional bulk capacitance will be provided on input to deal with the 5V IC's.

We will take 400mA as the expected load; in truth, it is likely to be much less. Derating to provide a safety margin will be done as a last step in chosing components - this derating should allow a higher load without any problems.

Output Inductor

The MAX1951 recommends a 2μH inductor, making selection very simple.

  • 2A minimum current rating, 3A minimum saturation.
  • 20mΩ maximum DC resistance.

Given LIR = 30% (Inductor ripple %, recommended range 20%-40%):

Peak inductor current = <math>\left ( 1 + \frac{LIR}{2} \right ) \times I_{OUT} = \left ( 1 + \frac{0.3}{2} \right ) \times 0.4 = 460mA</math>

Given the above values, the chosen inductor is TDK SPM6530T-2R2M ($1.41 for single units).

Inductance Tolerance DC Resistance Current
2.2μH 20% 19mΩ 8.4A

Filter Capacitors

The MAX1951 recommends a 10μF input and output ceramic capacitor. There is no point calculating ripple current/voltages etc, because manufacturers don't bother supplying a ripple voltage spec on their datasheets; the low ESR and ESL of ceramic capacitors makes it somewhat irrelevant.

The value of ceramic capacitors decreases significantly (80% or greater) as they approach their rated voltage. For this reason, the filter caps will be over-rated to 25V, and the bypass caps to 10V.

The chosen capacitors are:

Use Value Voltage Type Package Device
Input and Output filtering 10μF 25V X5R or X7R 0805 Murata GRM21BR61E106KA73L
Input Bulk 47μF 25V X5R or X7R 1206 TDK C3216X5R1E476M
Bypass 100nF 25V X5R or X7R 0805 TDK C2012X7R1E104K
Compensation 220pF 50V

See Table 2 in datasheet.

C0G/NP0 0805 TDK C2012C0G1H221J

Ferrite Bead

A ferrite bead will be used on the incoming +5V line to reduce EMI being conducted back to the host.

  • Ideally, the bead should reject all frequencies from the switching frequency (1MHz) to the CPU Frequency (100MHz). Realistically, very few beads attenuate from 1MHz.
  • 2A current rating.

To be safe, we'll chose a ferrite bead with a very minimal DC resistance.

DC Resistance Peak current Case style Impedence Part
5mΩ 6A 1206 48Ω @ 100MHz Taiyo Yuden FBMJ3216HS480NT

Inrush Current Limiting

The low-ESR ceramic input capacitors will act as a short to ground when the device is turned on. A 5Ω power resistor will be used to limit the initial inrush current to a maximum of 1A, dissipating 5W. 5W SMD resistors are very large, so we'll make do with a 2W resistor. Most power resistors are rated for 5x there rated power for a short duration (~ 5seconds). The resistor must be bypassed soon after the circuit turns on allow it to cool down, and to improve efficiency. The disconnection will be done with a P-Channel enhancement mode MOSFET, with the gate connected to the output of the 3.3W switching regulator via a transistor. ie. We'll make use of the built-in soft-start circuit of the regulator as a time delay for the MOSFET.

Resistor

DC Resistance Power Rating Peak Power Current Limit Part
5.0Ω 2W 10W for 5 seconds 1A (5V) PWR4318W5R00JE Bourns PWR4318 series

Bypass MOSFET

On Resistance (Vgs = -4.5V) On Resistance (Vgs = -2.5V) Continuous Drain Current Power Rating Power disapation @ 1A drain Part
40mΩ 70mΩ 4.6A 1.25W 40mW Diodes Inc. DMP2066LDM-7

MOSFET Gate pullup resistor (to +5v). The current through this resistor, when grounded, will be collector->emitter current of NPN transistor.

Voltage DC Resistance Current
5V 2kΩ 2.5mA

Switching NPN Transistor (used to bring the MOSFET gate to ground to turn the MOSFET on).

Max Collector Current (Ic) Collector Emitter breakdown voltage collector-emitter saturation base-emitter saturation Part
200mA 40V 300mV 0.95V Diodes Inc. MMBT3904-7-F

The base-emitter voltage will be 3.3V, which will certainly cause saturation (only need < 1V). The collector-emitter saturation voltage will bring the gate-to-source voltage of the MOSFET down to -4.7V (5V input - 300mV in the transistor). This is sufficient to saturate the MOSFET (need -4.5V).

Transistor Base current limiting resistor. We'll drive at 2.5mA so saturation isn't a problem (same as the intented collector current) .

Voltage DC Resistance Current
3.3V 1320Ω 2.5mA

In-circuit programming

The LPC17xx micro will be programmed via JTAG using Open OCD.

The standard ARM 0.1" 20-pin JTAG header will be used (see http://www.keil.com/support/man/docs/ulink2/ulink2_hw_connectors.htm for connector and necessary pull-up/pull-down details).

Serial programming of the LPC1751 is performed via the UART0 TX and RX pins. To enter programming mode, P2.10 must be low on RESET. The active-low P2.10 and RESET lines will be pulled up to +3.3V via a 10kΩ resistor to ensure the micro isn't reset.

Termination

  • The ucc5617 will be powered by +5v, not TERMPWR. This enables testing the device without connecting to a live SCSI bus. The PHY essentially connects the outputs back to the inputs, but we still need the terminator powered to provide pullups.
  • A DIP Switch will be used to connect the DISCNCT pin of the ucc5617 to ground if the user wants to disable termination. The pin will be pulled-up to +5V via a 10k resistor.

Switches

  • Parity and SCSI ID will be set via a set of DIP switches to ground.
  • The micro GPIO port pull-ups will be enabled (this is the default anyway).
  • Parity requires 1 bit, SCSI ID requires 3 bits, SCSI Terminator DISCNT requires 1 bit. (5-way DIP switch required)