MIC2580A Datasheet, PDF (19/21 Pages) Micrel Semiconductor – Hot-Swap PCI Power Controller

MIC2580A
Application Information
Whenever voltage is applied to a highly capacitive load, high
inrush currents may result in voltage droop that may bring the
supply voltage out of regulation for the duration of the
transient. The MIC2580A solves this problem by specifically
controlling the current and voltage supply ramps so that the
system supply voltages are not disturbed. Very large capaci-
tive loads are easily supported with this device.
Figure 1 shows the timing during turn-on. When /ON is forced
low, all supplies are turned on at a slew rate determined by the
external capacitor, C_SLEW.
Figure 2 shows the foldback characteristics for the supply
voltages. This foldback affect bounds the magnitude of the
current step when the supplies are turned on or shorted. This
specifies the compact PCI specification of 1.5A/ms, thereby
ensuring reliable operation. In discrete FET implementa-
tions, this magnitude can exceed several amps and may
cause the main supply to go out of regulation during this
transient event. This, in turn, could cause the system to
behave unpredictably. In addition, should a fault occur, the
MIC2580A will prevent system malfunctions by limiting the
current to within specifications.
MOSFET Selection
The external MOSFET should be selected to provide low
enough dc loss to satisfy the application’s voltage regulation
requirements. Note that the voltage across the sense resistor
Micrel, Inc.
must also be added to the dc voltage drop across the
MOSFET to compute total loss. In addition to meeting the
voltage regulation specifications, thermal specifications must
also be considered. During normal operation very little power
should be dissipated in the MOSFET. DC power dissipation
of the MOSFET is easily computed as I²R_DS where I is the
drain current and R_DS is the specified on-resistance of the
MOSFET at the expected operating drain current. However,
during excessive drain current or short-circuit faults, the
power dissipation in the external MOSFET will increase
dramatically. To help compute the effective power dissipated
during such transients, MOSFET manufacturers provide
transient thermal impedance curves for each MOSFET.
These curves provide the effective thermal impedance of the
MOSFET under pulsed or repetitive conditions; for example,
as will be the case when enabling into a short circuit fault.
From these curves the effective rise in junction temperature
of the MOSFET for a given condition can be computed. The
equation is given as:
peak T_J = PDM × Z_θJA + T_A
where PDM is the power dissipated in the MOSFET usually
computed as V_IN x I_DRAIN and Z_θJA is the thermal response
factor provided from the curves. Since the MIC2580A re-
duces the current to 30% of full scale even under severe faults
such as short-circuits the MOSFET power dissipation is held
to safe levels. This feature allows MOSFETs with smaller
packages to be used for a given application thereby reducing
cost and PCB real-estate requirements.
Power
Supply
+12V
+5V
+3.3V
GND
–12V
+5V
PCI Hot-Plug
Controller
/CIRST
BUS EN
February 2005
10mΩ _IRF7413
10mΩ IRF7413
MIC2580A
12VIN
12VOUT
5VSENSE
5VGATE
5VIN
5VOUT
3VSENSE
3VGATE
3VIN
3VOUT
VPCHG
/POR
/PCIRST
CRST
/ON
/LPCIRST
CSLEW
/FAULT
/EPWDGD CSTART
/PWDGD
GND
M12VIN M12VOUT
12V
5V
3.3V
–12V
/CIRST
D0
D0
D1
D1
Bus
D2
D2
Switch
Dn
Dn
Figure 6. Hot-Plug PCI Application
12V/500mA
5V/5A
3.3V/7.6A
GND
–12V /100mA
/CIRST
19
MIC2580A