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

Revision 1.1 (03-31-05)

2

SMSC WP 1.5

WHITE PAPER

DC Voltage Drop and Current

One consideration in selecting M1 is the DC voltage drop across it, Vm1, and current

requirements,Im1. With the exception of compact Flash format interface which can support CF form

factor hard drives that can consume over 300mA, most card formats require under 100mA maximum.

The FET, M1, needs to be selected such that the minimum card voltage specs at the maximum rated

current are met. In general, a drop of less than 150mv is acceptable.

Inrush Transient

When M1 is turned on, the card supply filter capacitor, Ccard, is charged. The inrush current can drop

the voltage provided by the 3.3V regulator, VDD, until it can recover. The amount of drop is determined

by the turn on rate of M1 and the sizes of Cout, Ccard, and the response time of the regulator. If VDD

drops below 3.0V erroneous operation of the card reader can occur. The case of multiple simultaneous

card insertions must also be considered.

Short Circuits

Precautions must be taken to tolerate potential short circuits to the card power supply. This can occur

during manufacturing test or if a defective card is inserted.

External Implementations

If discrete FETs are used, they are selected to provide the required DC voltage drop at the maximum

rated card currents. Usually, this is 100mA for non-CF interfaces and 300mA+ for CF/Microdrive

interfaces. Turn-on surge currents are managed by using the intrinsic FET gate capacitance, Cg, and

a series resistor, Rg, to provide turn on delay. A value of 15 uS is usually acceptable, but may have

to be adjusted to assure that the VDD never drops below 3.0V.

Implementation using integrated FETs

SMSC’s USB2227 and USB2228 provides the power FETs for the non-CF interfaces internally and can

directly drive the other card interfaces without external components as long as the current to the card

does not exceed 100ma. This is illustrated in Figure 2, "Implementation Using Integrated FETs". The

internal FETs have fold-back current limiting and can sustain any load, including a direct short to

ground, indefinitely (See Figure 3, "Output Voltage vs. Load Current for VDDIO = 3.3V"). Since external

FETs must be sized to tolerate a short circuit, often, they must be oversized and add cost. In addition,

the integrated internal FETs have their turn on times controlled to limit supply voltage drops with

commonly available 3.3V voltage regulators and filter capacitor sizes, eliminating the need for external

components to control turn on time to eliminate supply droops.