Micrel, Inc.
MIC2544A/2548A
Application Information
Supply Filtering
A minimum 1μF bypass capacitor from IN-to-GND,
located near the MIC2544A and MIC2548A, 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. An additional 22µF
input capacitor placed close to the IC is required if a bulk
input capacitor is further than 3 inches away from the IC.
Input transients must not exceed the absolute
maximum supply voltage (VIN max = 6V) even for a short
duration.
Figure 4. Flag Glitch with COUT = 120µF
Adding an optional series resistor-capacitor (RSET2), in
parallel with RSET, and as shown in Figure 8, 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 RSET is 309Ω (525mA), an RSET2 of
100Ω (2.3A) and CSET of 1μF (RC = 100μs) allows
transient surge of 3A to pass for 100μs without tripping
the over current flag (FLG), as shown in Figure 5.
Figure 3. Supply Bypassing
Power Dissipation
The device's junction temperature depends upon 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:
Figure 5. ILIMIT Filter with COUT = 120µF
2
PD = RDS(on) × (IOUT
)
To relate this to junction temperature, the following
equation can be used:
Figure 6 circuit can also be used to filter out transient
FLG assertion. The value of the RC time constant should
be selected to match the length of transient. Figure 7
shows the FLAG pin waveform due to the inrush of
current surge.
Tj = PD × θJA + TA
where:
Tj = junction temperature
TA = ambient temperature
θJA = is the thermal resistance of the package
Transient Over Current 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, as shown in Figure 4.
Figure 6. Flag Filter Circuit
M9999-043010
April 2010
16
MICREL [ MICREL SEMICONDUCTOR ]
