USB2227 and USB2228 USB2.0 Flash Card Reader Media Power Considerations

Revision 1.1 (03-31-05)

4

SMSC WP 1.5

WHITE PAPER

Special Cases

In some cases, it may be desired to have specific non-CF interfaces also use external power FETs.

These would be interfaces that are expected to require greater than the 100mA maximum supply

current that the USB2227 and USB2228’s internally integrated power FETs can supply. In this case, it

would be desired that this interface, as well as the CF interfaces use external FETs, while the

remaining card interfaces use the internal FETs. The USB2227 and USB2228 can be configured via

its attribute bit settings contained in its optional external serial EEPROM to select which interfaces use

the internal versus external FETs.

Integrated FET Parallel Operation

The design of the USB2227 and USB2228’s internal power FETs are such that they may be paralleled

to supply increased current. Two of the internal power FETS are capable of 100ma output operating

current, while one output (pin 76, GPIO11) is capable of supplying 200ma by itself. Therefore, up to

400ma may be obtained by paralleling the internal power FETs in a single card interface application.

The firmware of the USB2227 and USB2228 allows any combination of the three internal FETs to be

used for any card interface. The FETs are designed not only to sustain individual short circuits, but

simultaneous shorts as long as the operating ambient temperature and power supply voltage limits are

not exceeded.

Summary

Proper card power supply design and component selection is necessary for error free operation of USB

card readers. Whether using external FETs or the internally integrated FETs of advanced controllers,

such as SMSC’s USB2227 and USB2228, problem free implementations are possible by following

several simple guidelines.