SC2041 Semtech Corporation, SC2041 Datasheet - Page 10

• Manufacturer Part Number: SC2041
• Description: HotSwitch USB Power Distribution Switch With Shutdown
• Manufacturer: Semtech Corporation

Shutdown Signal From USB Controller
July 18, 2000
APPLICATIONS INFORMATION (Cont.)
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
© 2000 SEMTECH CORP.
Flag Output To USB Controller
VBUS IN
R1
C1
0.1uF
1
2
3
4
U1
GND
IN
IN
SHDN
SC2041
OUT
OUT
OUT
FLG
8
7
6
5
HotSwitch™ USB Power Distribution
Switch With Shutdown
C2
0.1uF
C3
22uF
VBUS OUT
P
P
T
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
D M A X
D M A X
J
652 MITCHELL ROAD NEWBURY PARK CA 91320
(
(
T
)
)
A
OUT
(
( . )
I
0 5
O U T
JA
A
= 500mA and r
)
= 85°C,
2
2
P
D M AX
0 1 5 0
(
r
.
O N M AX
(
)
)
JA
3 7 5
= 130°C/W (package thermal
.
m W
ON(MAX)
= 150m
SC2041
(at V
IN
10
=