MIC2544/2548
Micrel
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 capaci-
tance.
Adding an optional series resistor-capacitor (R
SET2
) in paral-
lel with R
SET
, as shown in Figure 4, 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
1µF (RC = 100µs) allows transient surge of 3A to pass for
100µs without tripping the overcurrent flag (FLG).
USB Power Distribution
The MIC2544 is ideal for meeting USB power distribution
requirements. Figure 4 depicts a USB Host application. R
SET
should be set to a value providing a current-limit >500mA.
The accurate current-limit of the MIC2544 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 MIC2544/48 are ideal inrush current-limiters suitable for
hot-plug applications. Due to the integrated charge pump,
the MIC2544/48 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 MIC2544 may be used in a
hot-plug application.
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located near
the MIC2544 and MIC2548, 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.
4.0V to 5.5V
MIC2548-1BM
On/Off
1
2
3
4
EN
FLG
GND
ILIM
OUT
IN
OUT
NC
8
7
6
5
Power
Output
R
SET
0.1µF to 1µF
NOTE: MSOP PACKAGE OPTION USES PIN 5 FOR I
LIM
.
PIN 4 IS NOT CONNECTED (NC).
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.
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:
5V
(+)
4.7µF
(–)
V
BUS
D+
D–
GND
D+
D–
Bold
lines indicate
0.1" wide, 1-oz. copper
high-current traces.
GND
R
SET
R
SET(2)
C
SET
(optional)
1.5k 2%
MIC5203-3.3
LDO Regulator
IN
OUT
GND
1.0
µF
10k
3.3V USB Controller
V+
ON/OFF
OVERCURRENT
100k
MIC2544-2BM
EN
FLG
GND
ILIM
OUT
IN
OUT
NC
0.1µF
120µF
Ferrite
Bead
V
BUS
D+
0.01µF
D–
GND
Downstream
USB
Port 1
4.75V min.
500mA max.
NOTE: MSOP PACKAGE OPTION USES PIN 5 FOR I
LIM
.
PIN 4 IS NOT CONNECTED (NC).
Figure 4. USB Host Application
MIC2544/2548
10
June 2000
MICREL [ MICREL SEMICONDUCTOR ]
