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Micrel, Inc.
Application 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.
Figure 3. Supply Bypassing
Power Dissipation
The device's junction temperature depends on 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:
P D = R DS(ON) × (I OUT ) 2
MIC2545A/2549A
To relate this to junction temperature, the following
equation can be used:
T J = P D × θ JA + T A
where:
T J = Junction temperature
T A = Ambient temperature
θ JA = 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 capacitance. Adding an optional series
resistor-capacitor (R SET2 ) in parallel with RSET, 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
435 ? (530mA), an R SET2 of 88 ? (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 Dissipations
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.
June 2011
14
M9999-062711-A
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