Cheap USB cables can only be suitable for low power devices
On a Bus Pirate v4 (board by Sandbox Electronics) connected after a 5.25V USB boost converter (to help improve regulation of the +5V output since some systems, especially portable devices can have a problem with a low USB voltage output), there was a significant voltage drop in both directions (+5V and GND combined) on the seller-supplied USB cable between the Bus Pirate (tested with a 22 ohm (150mA) load on the +3.3V line when enabled) and the 5.25V USB boost converter, which helps the Bus Pirate maintain a constant +5V at its output pin (when enabled) with a maximum 150mA load.
This problem basically disappeared when I used a better quality USB cable (HP part number 412213-001 to be more precise) since loss on this cable (again, in both directions) was less than 100mV when both the (enabled) +5V and +3.3V outputs were fully loaded at the MIC5025 current limit of 150mA.
The original Bus Pirate v4 design did not incorporate PTC fuses and an input choke (which causes significant voltage loss under the above conditions), so I removed them and bridged them out and therefore, eliminated this loss and relied on current limiting of the voltage regulators and the USB port it is connected to.
Note that the USB +5V input (in parallel with a external DC input pin separated with a diode) is only connected to the input of three MIC5025 series regulators (165mV typical dropout @ 150mA) and is monitored by the VBUS input on the PIC microcontroller via a high value resistor.
This problem basically disappeared when I used a better quality USB cable (HP part number 412213-001 to be more precise) since loss on this cable (again, in both directions) was less than 100mV when both the (enabled) +5V and +3.3V outputs were fully loaded at the MIC5025 current limit of 150mA.
The original Bus Pirate v4 design did not incorporate PTC fuses and an input choke (which causes significant voltage loss under the above conditions), so I removed them and bridged them out and therefore, eliminated this loss and relied on current limiting of the voltage regulators and the USB port it is connected to.
Note that the USB +5V input (in parallel with a external DC input pin separated with a diode) is only connected to the input of three MIC5025 series regulators (165mV typical dropout @ 150mA) and is monitored by the VBUS input on the PIC microcontroller via a high value resistor.
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