MIC2546/2547
Micrel
P = R
× (I
)2
Applications Information
D
DS(on)
OUT
To relate this to junction temperature, the following equation
can be used:
Supply Filtering
A 0.1µF to 1µF bypass capacitor from INX to GND, located
neartheMIC2546andMIC2547,isstronglyrecommendedto
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.
T = P × θ + T
A
j
D
JA
where:
T = junction temperature
j
T = ambient temperature
A
Input transients must not exceed the absolute maximum
supply voltage (V
θ
= is the thermal resistance of the package
JA
= 6V) even for a short duration.
IN max
Transient Overcurrent Filter
4.0V to 5.5V
The inrush current from the connection of a heavy capacitive
loadmaycausethefaultflagtofallfor10µsto200µswhilethe
switch is in a constant-current mode, charging the capaci-
tance.
MIC2546-1BM
1
2
3
16
INA
OUTA
0.1 to 1µF
15
14
OUTA ILIMA
Adding an optional series resistor-capacitor (R
) in paral-
SET2
lel with R
, as shown in Figure 3, allows the transient
ENA
GNDA
FLGB
ENB
OUTB
NC
SET
13
12
11
4
5
6
7
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
NC
FLGA
GNDB
ILIMB
OUTB
R
is 435Ω (510mA), an R
of 88Ω (2.5A) and C
of
SET
SET2
SET
1µF (RC = 100µs) allows transient surge of 3A to pass for
100µs without tripping the overcurrent flag (FLG).
RSETA
10
9
.
8
INB
USB Power Distribution
0.1 to 1µF
RSETB
The MIC2546 is ideal for meeting USB power distribution
requirements. Figure3depictsaUSBHostapplication. R
SET
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
circuitfaultspreventvoltagedroopinmobilePCapplications.
Supply Bypassing
Power Dissipation
Printed Circuit Board Hot-Plug
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.
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 “soft-
start” feature effectively isolates power supplies from highly
capacitive loads by reducing inrush current during hot-plug
events.
Calculation of power dissipation can be accomplished by the
following equation:
5V
(+)
10k
100k
4.7µF
1.5k 2%
10k
(–)
100k
Ferrite Bead
MIC5203-3.3
LDO Regulator
MIC2546-2BM
3.3V USB Controller
VBUS
VBUS
D+
INA
OUTA
Downstream
USB
Port 1
4.75V min.
500mA max.
V+
IN
OUT
D+
D–
RSET
120µF
ON/OFF
OUTA ILIMA
ENA GNDA
0.01µF
D–
GND
OVERCURRENT
GND
GND
NC
FLGB
ENB
FLGA
VBUS
D+
D+
GNDB OUTB
Downstream
USB
Port 2
4.75V min.
500mA max.
RSET
D–
GND
ILIMB
OUTB
NC
0.01µF
D–
INB
120µF
GND
Bold lines indicate
0.1" wide, 1-oz. copper
high-current traces.
Ferrite Bead
RSET(2)
CSET
(optional)
Figure 3. USB Host Application
June 2000
11
MIC2546/2547
MICREL [ MICREL SEMICONDUCTOR ]
