MIC2550A

Micrel, Inc.

M9999-031805

8

March 2005

Applications Information

The MIC2550A is designed to provide USB connectivity in

mobile systems where system supply voltages are not avail-

able to satisfy USB requirements. The MIC2550A can oper-

ate down to supply voltages of 2.5V and still meet USB

physical layer specifications. As shown in the system dia-

gram, the MIC2550A takes advantage of USB’s supply

voltage, V

age, V

digital I/O lines (VP, VM, RCV, OE#, SPD, and SUS pins)

interfacing to the system. Internal circuitry provides transla-

tion between the USB and system voltage domains. V

typically be the main supply voltage rail for the system.

In addition, a 3.3V, 10% termination supply voltage, V

provided to support speed selection. A 0.47µF (minimum)

capacitor from V

A 1.5K resistor is required between this pin and the D+ or D–

lines to respectively specify full-speed or low-speed opera-

tion.

Power Supply Configurations

V

When the V

path between V

from V

issue can arise in systems where V

supply that can be switched off such as in the case of a

desktop PC. Adding a Schottky diode, such as the ZHCS1000

by Zetex, in series with V

during this condition. A solution is shown in Figure 8 below.

If the V

diode D1 is not necessary.

MIC2550

VIF

Note: *(Optional) See Text - Power Supply Configurations

VBUS

1µF min

VBUS

USB Device

Power Controller

*(Optional)

D1

ZHCS1000 or

equivalent

Figure 8. Solution to V

I/O Interface Using 3.3V

In systems where the I/O interface utilizes a 3.3V USB

controller, an alternate solution is shown in Figure 9. This

configuration has the advantage over Figure 8, in that no

extra components are needed. Ensure that the load on V

TRM

does not exceed 1mA total.

MIC2550

VIF

VBUS

I/O

VBUS

RCV/OE#

VTRM

USB

Controller

3.3V

Figure 9. I/O Interface Using 3.3V

Internal 3.3V Source

If the device is self-powered and has 3.3V available, the

circuit in Figure 10 is yet another power supply configuration

option. In this configuration, the internal regulator is disabled

and the 3.3V source and not V

MIC2550A

VIF

3.3V

VBUS

VTRM

Figure 10. Powering Chip

from Internal 3.3V Source

Suspend

When the suspend pin (SUS) is high, power consumption is

reduced to a minimum. V

are still functional to enable the device to detect USB activity.

For minimal current consumption in suspend mode, it is

recommended that OE# = 1, and SPD = 0.

Speed

The speed pin (SPD) sets D+/D– output edge rates by increas-

ing or decreasing biasing current sources within the output

drivers. For low speed, SPD = 0. For full speed, SPD = 1. By

setting SPD = 0 during idle periods, in conjunction with suspend

(SUS), the lowest quiescent current can be obtained. However,

designers must provide a 300ns delay between changing SPD

from 0 to 1 and transmission of data at full speed. This delay

ensures the output drivers have arrived at their proper operating

conditions. Failure to do so can result in leading edge distortion

on the first few data bits transmitted.

External ESD Protection

The use of ESD transient protection devices is not required

for operation, but is recommended. We recommend the

following devices or the equivalent:

Cooper Electronics Technologies (www.cooperet.com)

Littelfuse (www.littlefuse.com)

V0402MHS05

SP0503BAHT

Non-multiplexed Bus

To save pin count for the USB logic controller interface, the

MIC2550A was designed with V

pins. To interface the MIC2550A with a non-multiplexed data

bus, resistors can be used for low cost isolation as shown in

Figure 11.

USB Logic

Controller

(SIE)

MIC2550

10k

10k

Figure 11. MIC2550A Interface to

Non-multiplexed Data Bus