USB2227_0503 Datasheet, PDF (1/5 Pages) SMSC Corporation – Flash Card Reader Media Power Considerations

WP 1.5
USB2227 and USB2228 USB2.0 Flash Card
Reader Media Power Considerations
Introduction
Bus-powered USB2.0 Flash Media Card Readers are enjoying an explosive growth due to the
increasing popularity of digital photography and portable music players. The basic card formats now
stand at four types, but all of these require the careful implementation of their power control. Historically,
this is provided via discrete power FETs which are external to the card reader controller (e.g. SMSC’s
USB2223 and USB2224 devices), and requires careful design and component selection to avoid
several possible system issues. This paper addresses some of those issues.
In addition, fourth generation controllers, such as SMSC’s USB2227 and USB2228 devices, integrate
some of the power FETs internally to the controller itself, to further save system cost. The design of
these integrated FETs must account for these issues also in order to obtain successful operation.
Design Considerations
Figure 1 illustrates a typical card reader implementation and the card power supply circuit.
3.3V
Regulator
Cin
Cout
VBUS
USB
VDD
Rg
USB2.0
Flash Card
Reader
Card
Power
Control
Card Detect
Cg
Ccard
I_m1
M1 _Vm1
Figure 1 Card Reader Implementation/Card Power Supply Circuit
In operation, when the user inserts the card into its socket, the card insertion is signaled to the Reader
Controller which then applies power, by driving the power FET, M1, on. In many cases, the turn on time
of the FET is managed via Rg/Cg.
SMSC WP 1.5
WHITE PAPER
Revision 1.1 (03-31-05)