MIC2583 Micrel Semiconductor, MIC2583 Datasheet - Page 16

• Manufacturer Part Number: MIC2583
• Description: Single Channel Hot Swap Controllers
• Manufacturer: Micrel Semiconductor

MIC2582/MIC2583
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
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 V
below 2.1V, the UVLO circuit pulls the GATE output to ground
and clears the overvoltage and/or current limit faults. For a
higher UVLO threshold, the circuit in Figure 7 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 V
drive output will be shut down when V
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
elapses.
MIC2582/MIC2583
1
R2
R1
1.19V . In the example circuit (Figure 7), the rising
12V
V
IN
Undervoltage Lockout Threshold (rising) = 9.5V
Undervoltage Lockout Threshold (falling) = 9.1V
Undervoltage (Output) = 11.4V
Additional pins omitted for clarity.
IN
exceeds 1
49.9k
332k
1%
1%
R1
R2
ON
) and after the start-up timer
CC
(18V)
D1
exceeds 2.2V. If VCC falls
R2
R1
1.24V . The GATE
C1
1 F
2
IN
ON
Figure 7. Higher UVLO Setting
VCC
3
falls below
1
8
0.010
R
SENSE
5%
SENSE
MIC2582
GND
2
4
7
4
IN
16
GATE
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 over-
heating 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 20 C. Given the MOSFET’s
R
an approximation for the junction temperature at which the
device will operate is obtained as follows:
where T
implementation of additional copper heat sinking is highly
recommended for the area under/around the MOSFET.
For additional information on MOSFET thermal consider-
ations, please see MOSFET Selection text and subsequent
sections.
FB
(J-A)
6
5
T
IRF7822
(SO-8)
and the expected power dissipated by the MOSFET,
J
Q1
A
= (P
'=T
R3
10
C2
0.01 F
A(MAX OPERATING)
D
x R
(J-A)
R4
133k
1%
R5
16.2k
1%
) + T
A
'
C
220 F
LOAD
+ 20 C. As a precaution, the
12V@4A
V
OUT
(10)
April 2003
Micrel