MIC2545A Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Programmable Current Limit High-Side Switch

MIC2545A/2549A
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located near
the MIC2545A and MIC2549A, 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
MIC2549A-1BM
1 _EN
OUT ⁸
2 _FLG
IN ⁷
3 _{GND OUT} 6
4 _ILIM
IN ⁵
0.1µF to 1µF
R_SET
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:
T_J = P_D × θ_JA + T_A
where:
Micrel
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 (530mA), an R_SET2 of 88Ω (2.5A) and C_SET of
1µF (RC ≈ 90µs) allows transient surge of 3A to pass for 90µs
without tripping the overcurrent flag (FLG).
USB Power Distribution
The MIC2545A is ideal for meeting USB power distribution
requirements. Figure 7 depicts a USB Host application. R_SET
should be set to a value providing a current-limit >500mA.
The accurate current-limit of the MIC2545A 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 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.
5V
(+)
4.7µF
(–)
V_BUS
D+
D–
GND
MIC5203-3.3
LDO Regulator
IN
OUT
1.0
GND
µF
1.5k
2%
10k
3.3V USB Controller
V+
ON/OFF
OVERCURRENT
D+
D– _GND
0.1µF
Bold lines indicate
0.1" wide, 1-oz. copper
high-current traces.
MIC2545A-2
EN
OUT
FLG
IN
GND OUT
ILIM
IN
R_SET
R_SET(2)
C_SET
(optional)
120µF
0.1µF
Figure 4. USB Host Application
Ferrite
Bead
0.01µF
V_BUS
D+
D–
GND
Downstream
USB
Port 1
4.75V min.
500mA max.
MIC2545A/2549A
12
June 2000