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 | 200mA
42mA excl. peripherals. |
(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
Threshold voltage (min) | On Resistance (Vgs = -4.5V) | On Resistance (Vgs = -2.5V) | Continuous Drain Current | Power Rating | Power disapation @ 1A drain | Part |
---|---|---|---|---|---|---|
-0.6V | 40mΩ | 70mΩ | 4.6A | 1.25W | 40mW | Diodes Inc. DMP2066LDM-7 |
The MOSFET will turn on when the capacitor voltage has risen to the gate threshold value. Assume we have 2 x 47uF ceramic caps as the bulk capacitors with zero charge (<math>V_0</math>). The ESR will be negligible (~ 0.1 Ohm) and will be ignored.
<math> V_c = V_s \left ( 1 - e{\frac{-t}{RC}} \right )</math> <math> \frac{V_c}{V_s} = 1 - e{\frac{-t}{RC}} </math> <math> e{\frac{-t}{RC}} = 1 - \frac{V_c}{V_s}</math> <math> \frac{-t}{RC} = ln \left ( 1 - \frac{V_c}{V_s} \right )</math> <math> t = -RC * ln \left ( 1 - \frac{V_c}{V_s} \right )</math> <math> t = -5 * 0.000094 * ln \left ( 1 - \frac{0.6}{5} \right )</math> <math> t = 60us</math>
At this stage the current being used to charge the caps is low enough for the resistor to be removed.
<math>I = \frac{V_s}{R} e^{\frac{-t}{RC}} </math> <math>I = \frac{5}{5} e^{\frac{-0.000060}{5 * 0.000094}} </math> <math>I = 0.88A</math>
hmm, but if we disconnect the resistor, current will shoot up again !!!
How about just wait until the output hits 4V ? ie. get a different MOSFET ? Or why don't we just put the resistor in series with the cap, and tap off before the resistor to the regulator. The regulator has it's own soft start anyway.
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)