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Step 7VARIATIONS:USB
A USB version is possible with a few modifications. USB charging is not an option for the 3G iPod available for testing. USB supplies 5.25-4.75 volts, our target is 5 volts. Here are the changes that need to be made:
1.Swap in a USB 'A' type connector (mouser #571-7876161, $0.85)
2.Change the output voltage resistor divider (change R2 (10K) to 22K).
3.Change output protection zener (D3) to 5.6 volt 1 watt (mouser #78-1N4734A, $0.07). A 5.1 volt zener would be more exact, but zeners have error like resistors. If we try to hit a 5 volt target and our 5.1 volt zener has 10% error on the low side, all our efforts will burn up in the zener.
4.Change inductor coil (L1) to 220uH, 0.49amp (mouser # 580-22R224C , $0.59). Enter new calibration constants, as per the calibration section: Set V_out to 5 volts. Step 8&9: L_Ipeak=220*0.49=107.8=107 (round to next lowest integer, if required).
5.Modify the output set point, recalculate Table 4 and Table 5 in the spreadsheet. Table 4 – enter 5 volts as the output and replace the 10K resistor with 22K (as per step 2). We find that at 5 volts output, with a 100K/22K divider network, feedback (E1) will be 0.9 volts. Next, make any change to the voltage reference in Table 5, and find the ADC set point. With a 2.5 volt reference (MCP1525) the setpoint is 369.
6.Sample constants for USB version:
const v_out as byte=5 'output voltage to determine off-time, 5 USB, 12 Firewire
const v_ref as float=2.5 '2.5 for MCP1525, 1.72 for my stabistor, ~2.7 for a zener.
const supply_ratio as float=5.54 'supply ratio multiplier, calibrate for better accuracy
const osc_freq as float=4 'oscillator frequency
const L_Ipeak as float=107 'coil uH * coil amps continuous (220*0.49=107, round down)
const fb_value as word=369 'output voltage set point
Firmware and PCB for the USB version are included in the project archive. Only the MCP voltage reference version was converted to USB.
1.Swap in a USB 'A' type connector (mouser #571-7876161, $0.85)
2.Change the output voltage resistor divider (change R2 (10K) to 22K).
3.Change output protection zener (D3) to 5.6 volt 1 watt (mouser #78-1N4734A, $0.07). A 5.1 volt zener would be more exact, but zeners have error like resistors. If we try to hit a 5 volt target and our 5.1 volt zener has 10% error on the low side, all our efforts will burn up in the zener.
4.Change inductor coil (L1) to 220uH, 0.49amp (mouser # 580-22R224C , $0.59). Enter new calibration constants, as per the calibration section: Set V_out to 5 volts. Step 8&9: L_Ipeak=220*0.49=107.8=107 (round to next lowest integer, if required).
5.Modify the output set point, recalculate Table 4 and Table 5 in the spreadsheet. Table 4 – enter 5 volts as the output and replace the 10K resistor with 22K (as per step 2). We find that at 5 volts output, with a 100K/22K divider network, feedback (E1) will be 0.9 volts. Next, make any change to the voltage reference in Table 5, and find the ADC set point. With a 2.5 volt reference (MCP1525) the setpoint is 369.
6.Sample constants for USB version:
const v_out as byte=5 'output voltage to determine off-time, 5 USB, 12 Firewire
const v_ref as float=2.5 '2.5 for MCP1525, 1.72 for my stabistor, ~2.7 for a zener.
const supply_ratio as float=5.54 'supply ratio multiplier, calibrate for better accuracy
const osc_freq as float=4 'oscillator frequency
const L_Ipeak as float=107 'coil uH * coil amps continuous (220*0.49=107, round down)
const fb_value as word=369 'output voltage set point
Firmware and PCB for the USB version are included in the project archive. Only the MCP voltage reference version was converted to USB.
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