SC2041

© 2000 SEMTECH CORP.

HotSwitch™ USB Power Distribution

Switch With Shutdown

July 18, 2000

652 MITCHELL ROAD NEWBURY PARK CA 91320

10

overcurrent circuitry reacts. Once the current limit cir-

cuitry trips, the current is limited to 0.85A and FLG is

asserted low. Refer to Figure 7 on page 8.

3) the load gradually increases beyond 500mA. In this

case, the current will be allowed to rise until it reaches

the Current Limit Trip Threshold, at which point the

current will drop back to 0.85A and FLG will assert low.

Refer to figures 4 and 5 on page 7 and 8.

Thermal Sensing

An internal thermal shutdown circuit turns off the power

switch when the die temperature exceeds 150°C

(typical).

The FLG pin asserts low signaling a fault

condition. Built-in hysteresis prevents the switch from

turning back on until the die temperature has cooled

approximately 20°C. The switch will continue to cycle

on and off until the fault condition is removed.

The

thermal sense circuit functions only when the switch is

enabled.

Undervoltage Lockout

An undervoltage lockout circuit monitors the input volt-

age and prevents the power switch from turning on until

the input voltage (IN) exceeds 2.7V (typical). If the input

voltage falls and drops below 2.4V (typical), the under-

voltage circuitry turns off the power switch and the FLG

pin asserts low. The undervoltage lockout functions

only when the switch is enabled.

Thermal Considerations

Since the on-resistance of the power switch is so low,

the SC2041 can pass large currents without requiring a

large package to dissipate the heat. The worst-case

power dissipation (under normal operating conditions)

is given by:

So for I

3.3V), the maximum power dissipation is:

The junction temperature can be calculated using the

following equation:

Inserting T

impedance for minimum line widths and no internal

Component Selection - General

A 0.1µF or greater ceramic bypass capacitor is recom-

mended at the device input. This should be placed

close to the input pin (IN) and routed directly to ground.

A low ESR electrolytic capacitor is recommended at the

output, higher values should be used for heavy loads to

reduce ringing at the output (and hence input). Bypass

the output with a 0.1µF to 1µF ceramic capacitor to

improve immunity to short circuit transients.

Shutdown

The shutdown pin (SHDN) input is a 3V or 5V

compatible enable/disable. Pulling this pin high shuts

down the power switch and the SC2041 will draw < 1µA

from the supply. Pulling this pin low will enable the

device.

Error Flag

The error flag (FLG) output is an open drain N-channel

MOSFET. Its output is pulled low during overcurrent,

input undervoltage and thermal shutdown conditions.

Connection of high capacitance loads to the output can

cause momentary overcurrent conditions due to inrush

current and trigger false error flag assertion. This can

be reduced by using low ESR output capacitors to

provide a low impedance source for hot-plug events.

The addition of an RC filter between FLG and the USB

controller can resolve this easily. This pin also requires

a pull-up resistor for a high signal when not asserted.

Overcurrent

There are three overcurrent situations to be consid-

ered:

1) the output is already short before the device is

enabled or power is applied. In this case, the SC2041

immediately detects the short, and the output current

will slowly ramp up to the current limit value, and FLG

will assert low. Refer to Figure 6 on page 8.

2) a short occurs while the device is enabled. In this

case, very high current may flow initially while the

VBUS IN

Flag Output To USB Controller

VBUS OUT

Shutdown Signal From USB Controller

U1

SC2041

1

2

3

4

5

6

7

8

GND

IN

IN

SHDN

FLG

OUT

OUT

OUT

C2

0.1uF

C3

22uF

R1

C1

0.1uF

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0 150

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APPLICATIONS INFORMATION (Cont.)