SMSC WP 1.5

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Revision 1.1 (03-31-05)

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.

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.

Figure 1 Card Reader Implementation/Card Power Supply Circuit

USB2.0

Flash Card

Reader

3.3V

Regulator

Card Detect

Card

Power

Control

USB

VBUS

Cin

Cout

Cg

Ccard

Rg

M1

VDD