Table 9-1 lists the input parameters for this design example.
Table 9-1 Design Requirements
| DESIGN PARAMETERS |
EXAMPLE VALUE |
| Input operation voltage |
5 V |
| Rating current |
3 A or 4.5 A |
| Minimum current limit |
3 A |
| Maximum current limit |
5 A |
When choosing a power switch, there are several
general steps:
- Determine what is the power rail, 3.3 V or 5 V,
and then choose the operation range of the power switch that can cover the power
rail voltage range.
- Determine what is the normal operation current.
For example, the maximum allowable current drawn by portable equipment for a USB
2.0 port is 500 mA, so the normal operation current is 500 mA and the minimum
current limit of power switch must exceed 500 mA to avoid false trigger during
normal operation.
- Determine what is the maximum allowable current
provided by up-stream power, and then decide the maximum current limit of the
power switch that must lower it to ensure the power switch can protect the
up-stream power when an overload is encountered at the output of the power
switch.
Note: Choosing power switch with tighter current limit
tolerance can loosen the up-stream power-supply design.