> 7 | USB Design

> This reference design is aimed at USB equipment, but can be used for all cases with similar requirements. According to USB specification (revision 2.0 Chapter 7.3.2) power supply from a high power port is:

> Table 3: | USB Host / powered Hub voltage specification

> Parameter | Min | Max
> Supply Voltage at USB port [V] | 4.75 | 5.25
> Supply Voltage at end of USB cable [V] | 4.625 | 5.125
> Current limitation [mA] | 500 |

> A power supply of 500mA is enough for GSM modules, but not enough for GPRS class 10 or HSDPA/UMTS modules. This reference design is therefore only suitable for GSM and GPRS class 8 modules. For designs with GPRS class 10 or UMTS a battery supply with battery charging current from USB or another additional power supply is recommended.
> NOTE: According to the USB specification there is also a low power port. A low power port does not deliver enough power for a GSM/GPRS/HSDPA/UMTS application. Also, there is a voltage loss on the USB connection cable under maximum load. It is defined as 250mV (125mV each in Vusb and Vgnd). The user should include this fact in a calculation.
> When a USB unit is plugged into the network, it has a certain amount of capacitance between VBUS and ground (C1). In addition, the regulator supplies current to it’s output capacitor (C3)as soon as power is applied. Consequently there could be a surge of current into the device, which might pull the VBUS below its minimum operating level. This problem must be solved by limiting the inrush current during the power up phase.
> According to the USB specification the maximum load that can be placed at the downstream end of a USB cable is 10µF in parallel with 44Ohm resistance.
> Figure 10: USB power supply
> In the design above the red circuit limits the inrush current of the 6mF capacitor C1 at 40mA. Make sure that the module is being turned on after C1 is full charged. The delay has to be approximately 3s.