MIC2582_14 Datasheet, PDF (18/27 Pages) Micrel Semiconductor – Single-Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC5283
12V in this example, and the ON pin voltage exceeds its
threshold (V_ON) of 1.24V and the MIC2582/83 initiates a
start-up cycle. In Figure 5, the connection sense consisting
of a discrete logic-level MOSFET and a few resistors
allows for interrupt control from the processor or other
signal controller to shut off the output of the MIC2582/83.
R4 pulls the GATE of Q2 to V_IN and the ON pin is held low
until the connectors are fully mated.
Once the connectors fully mate, a logic LOW at the
/ON_OFF signal turns Q2 off and allows the ON pin to pull
up above its threshold and initiate a start-up cycle.
Applying a logic HIGH at the /ON_OFF signal will turn Q2
on and short the ON pin of the MIC2582/83 to ground
which turns off the GATE output charge pump.
Higher UVLO Setting
Once a PCB is inserted into a backplane (power supply),
the internal UVLO circuit of the MIC2582/83 holds the
GATE output charge pump off until VCC exceeds 2.2V. If
VCC falls below 2.1V, the UVLO circuit pulls the GATE
output to ground and clears the overvoltage and/or current
limit faults. A typical 12V application, for example, should
implement a higher UVLO than the internal 2.1V threshold
of MIC2582 to avoid delivering power to downstream
modules/loads while the input is below tolerance. For a
higher UVLO threshold, the circuit in Figure 6 can be used
to delay the output MOSFET from switching on until the
desired input voltage is achieved. The circuit allows the
charge pump to remain off until VIN exceeds
1 ^R1   1.24V. The GATE drive output will be shut
 ^R2 
down when VIN falls below 1 ^R1   1.19V. In the
 ^R2 
example circuit (Figure 6), the rising UVLO threshold is set
at approximately 9.5V and the falling UVLO threshold is
established as 9.1V. The circuit consists of an external
resistor divider at the ON pin that keeps the GATE output
charge pump off until the voltage at the ON pin exceeds its
threshold (V_ON) and after the start-up timer elapses.
5V Switch with 3.3V Supply Generation
The MIC2582/83 can be configured to switch a primary
supply while generating a secondary regulated voltage rail.
The circuit in Figure 8 enables the MIC2582 to switch a 5V
supply while also providing a 3.3V low dropout regulated
supply with only a few added external components. Upon
enabling the MIC2582, the GATE output voltage increases
and thus the 3.3V supply also begins to ramp. As the 3.3V
output supply crosses 3.3V, the FB pin threshold is also
exceeded which triggers the power-on reset comparator.
The /POR pin goes HIGH, turning on transistor Q3 which
lowers the voltage on the gate of MOSFET Q2. The result
is a regulated 3.3V supply with the gate feedback loop of
Q2 compensated by capacitor C3 and resistors R4 and
R5. For MOSFET Q2, special consideration must be given
to the power dissipation capability of the selected
MOSFET as 1.5V to 2V will drop across the device during
normal operation in this application. Therefore, the device
is susceptible to overheating dependent upon the current
requirements for the regulated output. In this example, the
power dissipated by Q2 is approximately 1W. However, a
substantial amount of power will be generated with higher
current requirements and/or conditions. As a general
guideline, expect the ambient temperature within the
power supply box to exceed the maximum operating
ambient temperature of the system environment by
approximately 20ºC. Given the MOSFET’s R_θ(JA) and the
expected power dissipated by the MOSFET, an
approximation for the junction temperature at which the
device will operate is obtained as follows:
T_J = (P_D x R_(JA)) + T_A
Eq. 11
where T_A = T_{A(MAX OPERATING)} + 20ºC. As a precaution, the
implementation of additional copper heat sinking is highly
recommended for the area under/around the MOSFET
May 23, 2014
Figure 6. Higher UVLO Setting
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Revision 5.0