MIC2545A/2549A
Micrel
T = junction temperature
Applications Information
Supply Filtering
J
T = ambient temperature
A
θ
= is the thermal resistance of the package
JA
A 0.1µF to 1µF bypass capacitor from IN to GND, located
neartheMIC2545AandMIC2549A,isstronglyrecommended
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.
Transient Overcurrent Filter
The inrush current from the connection of a heavy capacitive
loadmaycausethefaultflagtofallfor10µsto200µswhilethe
switch is in a constant-current mode, charging the capaci-
tance.
Input transients must not exceed the absolute maximum
supply voltage (V (max) = 6V) even for a short duration.
Adding an optional series resistor-capacitor (R
) in paral-
IN
SET2
lel with R
, as shown in Figure 4, allows the transient
4.0V to 5.5V
SET
current-limit to be set to a different value than steady state. A
MIC2549A-1BM
1
2
3
4
8
7
6
5
typical USB hot-plug inrush is 2A to 3A for 10µs to 20µs. If
EN
OUT
IN
R
is 435Ω (530mA), an R
of 88Ω (2.5A) and C
of
SET
SET2
SET
FLG
GND
ILIM
1µF (RC ≈ 90µs) allows transient surge of 3A to pass for 90µs
without tripping the overcurrent flag (FLG).
0.1µF to 1µF
OUT
IN
USB Power Distribution
RSET
The MIC2545A is ideal for meeting USB power distribution
requirements. Figure7depictsaUSBHostapplication. R
SET
should be set to a value providing a current-limit >500mA.
Theaccuratecurrent-limitoftheMIC2545Awillreducepower
supply current requirements. Also, fast reaction to short
circuitfaultspreventvoltagedroopinmobilePCapplications.
Figure 3. Supply Bypassing
Power Dissipation
The device's junction temperature depends on several fac-
tors 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.
Printed Circuit Board Hot-Plug
The MIC2545A/49A is an ideal inrush current-limiter suitable
for hot-plug applications. Due to the integrated charge pump,
the MIC2545A/49A presents a high impedance when off and
slowly becomes a low impedance as it turns on. This “soft-
start” feature effectively isolates power supplies from highly
capacitive loads by reducing inrush current during hot-plug
events. Figure 5 shows how the MIC2545A may be used in
a hot-plug application.
Calculation of power dissipation can be accomplished by the
following equation:
P = R
× (I
)2
D
DS(on)
OUT
To relate this to junction temperature, the following equation
can be used:
T = P × θ + T
A
J
D
JA
where:
Ferrite
Bead
5V
(+)
10k
VBUS
4.7µF
Downstream
(–)
D+
MIC5203-3.3
USB
3.3V USB Controller
MIC2545A-2
LDO Regulator
0.01µF
D–
Port 1
4.75V min.
500mA max.
VBUS
D+
120µF
V+
OVERCURRENT
IN
OUT
ON/OFF
EN
OUT
IN
GND
1.0
µF
FLG
GND
ILIM
D–
GND
OUT
IN
GND
0.1µF
1.5k
2%
0.1µF
D+
D–
RSET
RSET(2)
CSET
GND
Bold lines indicate
0.1" wide, 1-oz. copper
high-current traces.
(optional)
Figure 4. USB Host Application
April 2004
13
M9999-041204
MICREL [ MICREL SEMICONDUCTOR ]
