Micrel, Inc.
MIC2546/2547
T
J
= P
D
×
θ
JA
+ T
A
where:
T
J
= junction temperature
T
A
= ambient temperature
θ
JA
= is the thermal resistance of the package
Transient Overcurrent Filter
The inrush current from the connection of a heavy
capacitive load may cause the fault flag to fall for 10μs to
200μs while the switch is in a constant-current mode,
charging the capacitance.
Adding an optional series resistor-capacitor (R
SET2
) in
parallel with R
SET
, as shown in Figure 3, allows the
transient current-limit to be set to a different value than
steady state. A typical USB hot-plug inrush is 2A to 3A
for 10μs to 20μs. If R
SET
is 435Ω (510mA), an R
SET2
of
88Ω (2.5A) and C
SET
of
μF
(RC = 100μs) allows transient
surge of 3A to pass for 100μs without tripping the
overcurrent flag (FLG).
USB Power Distribution
The MIC2546 is ideal for meeting USB power distribution
requirements. Figure 3 depicts a USB Host application.
RSET should be set to a value providing a current-limit
>500mA. The accurate current-limit of the MIC2546 will
reduce power supply current requirements. Also, fast
reaction to short circuit faults prevent voltage droop in
mobile PC applications.
Printed Circuit Board Hot-Plug
The MIC2546/47 are ideal inrush current-limiters suitable
for hot-plug applications. Due to the integrated charge
pump, the MIC2546/47 presents a high impedance when
off and slowly becomes a low impedance as it turns on.
This “softstart” feature effectively isolates power supplies
from highly capacitive loads by reducing inrush current
during hot-plug events.
Application Information
Supply Filtering
A 0.1μF to 1μF bypass capacitor from INX to GND,
located near the MIC2546 and MIC2547, is strongly
recommended to control supply transients. Without a
bypass capacitor, an output short may cause sufficient
ringing on the input (from supply lead inductance) to
damage internal control circuitry.
Input transients must not exceed the absolute maximum
supply voltage (V
IN max
= 6V) even for a short duration.
Supply Bypassing
Power Dissipation
The device's junction temperature depends on several
factors such as the load, PCB layout, ambient
temperature and package type. Equations that can be
used to calculate power dissipation and junction
temperature are found below.
Calculation of power dissipation can be accomplished by
the following equation:
P
D
= R
DS(ON)
× (I
OUT
)
2
To relate this to junction temperature, the following
equation can be used:
January 2008
13
M9999-012508-A
MICREL [ MICREL SEMICONDUCTOR ]
