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Dual current-limiting switch for USB applications

Posted: 15 Sep 2003     Print Version  Bookmark and Share

Keywords:POWER 

/ARTICLES/2003SEP/A/2003SEP15_POW_ID_AN.PDF

February 2000 1 Application Note 32

Application Note 32 Micrel

Application Note 32

Dual Current-Limiting Switch for USB Applications

by William Mai

Overview

The need for hot insertion capability has dramatically in-

creased due to the demand of reducing system down time

and portability. The effect of hot insertion is an inrush current

or surge current which is produced when any uncharged

capacitors are connected to a power supply. The larger the

capacitors,themoreenergyisneededtochargethem.Inrush

current can cause the voltage of the power supply to droop to

a level where system logic can be confused. This application

note describes one of the techniques to minimize the inrush

current to allow the user to utilize bulk capacitors of any size

for hot insertion applications. In addition, with this solution the

user does not have to wait for a long period of time between

multiple insertions.

Circuit Description

Instead of charging the capacitor in a short period of time, the

idea is to charge the capacitor in a long period of time by

limiting the charge current. The circuit in Figure 1 uses the

MIC2545A-2 and the MIC2778. The MIC2545A-2 is a pro-

grammable current limit high-side switch that is capable of

having current-limit up to 2.5A. The current-limit is set by the

external resistor. In this circuit, there are two current-limits,

100mA and 500mA. USB specifications limit the device

current to 100mA prior to enumeration. After enumeration, a

high-powered device can request no more than 500mA. This

circuit will guarantee these limits are not exceeded. The

MIC2778 is a voltage monitor with adjustable hysteresis and

that operate down to 1.5V. The high and low thresholds are

setat4Vand2Vrespectively.Theequationbelowshowshow

to calculate the resistors' value for the two thresholds and the

two current-limits.

VDD

C1

4.75F

R4

100k

R2

310k

R1

380k

R3

310k

LTH

HTH GND

/RST

RSET1

600

Q1

VN1206

LoadMIC2778

IN

EN

GND FLG

OUT

IN OUT

ILIM

MIC2545A-2

RSET2

2.2k

RL

10

CL

5005F

Figure 1. Dual Current-Limiting Switch

Micrel, Inc. 7 1849 Fortune Drive 7 San Jose, CA 95131 7 USA 7 tel + 1 (408) 944-0800 7 fax + 1 (408) 944-0970 7 http://www.micrel.com

Thresholds

V V

R1 R2 R3

R3high threshold REF=

+ +

V V

R1 R2 R3

R2 R3low threshold REF=

+ +

+

V 1.24VREF =

Set R1+R2+R3 = 1M

V 4V 1.24

1M

R3high threshold = =

R3 310k=

V 2V 1.24

1M

R2 310klow threshold = =

+

R2 310k=

R1 1M 310k 310k 380k= - - =

Current-limit

Current limit =

230

RSET

where:

Current factor = 230

100mA

230

RSET

=

R

230

100mA

2.2kSET1 =

500mA

230

R ||RSET1 SET2

=

(continued)

Application Note 32 Micrel

Application Note 32 2 February 2000

R R

230

500mA

460SET1 SET2|| = =

R 0.6kSET2

When VIN is hot-plugged to a 5V power supply, the voltage

will ramp up from 0V to 5V (Figure 2). When VIN crosses 4V,

the delay generator of the MIC2778 turns on allowing the

V/RST to remains low for a minimum of 140ms. The delay

feature is important in this application because it guarantees

VIN will be at 5V when the delay is off. It also allows the

MIC2545A-2 to charge the capacitor with less than 100mA

(Figure 4). Once the delay is off, the V/RST is pulled up to 5V.

Q1 turns on to parallel the 2.2k and 600 to set the current

limit for the MIC2545A-2 at 500mA. When VIN is unplugged,

its voltage will ramp down. Once the VIN crosses 2V, the

MIC2545A-2's current limit is reset back to 100mA.

In Figure 5, a logic control can be used to switch the current

limit from 100mA to 500mA. The difference between the two

circuits is that the MIC2778 allows the circuit to switch

automatically.

C1

4.75F

+5V

RSET2

600

Q1

VN1206

Load

IN

EN

GND FLG

OUT

IN OUT

ILIM

MIC2545A-2

Logic

Control

RSET1

2.2k

RL

10

CL

5005F

Figure 5. Logic-Controlled Current-Limit Setpoint

VIN

(10V/div)

RL = 10

CL = 05F

V/RST

(10V/div)

VOUT

(5V/div)

IIN

(200mA/div)

Figure 3. Dual Current Limiting

VIN

(10V/div)

RL = 10

CL = 5005F

V/RST

(10V/div)

VOUT

(5V/div)

IIN

(200mA/div)

Figure 4. Inrush-Current Limiting

5V

100mA

ILIMIT

Time

500mA

4V

VIN

V/RST

3V

2V

1V

0V

5V

4V

3V

2V

1V

0V

tRST

Figure 2. Timing Diagram

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or

other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.

) 2000 Micrel Incorporated





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