MIC2342 Datasheet, PDF (20/32 Pages) Micrel Semiconductor – Dual-Slot PCI Express® Hot-Plug Controller

Micrel, Inc.
3VOUT[A/B], and when the internal 3VOUT[A/B] “Power
Good” flag is asserted is a simple matter of substituting
R_3VSENSE[A/B] for R_{12VSENSE[A/B]}, C_{LOAD(3VOUT[A/B])} for
C_{LOAD(12VOUT[A/B])}, and V_UVTH(3V) for V_UVTH(12V).
Even though individual internal “Power Good” flags may
be asserted, the conditions under which the MIC2342’s
external /PWRGD[A/B] and /DLY_PWRGD[A/B] digital
outputs are asserted is described in the section labeled
“/PWRGD[A/B] and /DLY_PWRGD[A/B] Digital Outputs.”
Power-Down Cycle
When a slot is turned off, resistors internal to the
MIC2342/MIC2342R are connected to each of the
outputs to provide a discharge path for capacitors
connected to the part’s outputs. The nominal output
discharge resistance values for each rail are found in the
“Electrical Characteristics” table.
Use of an External Gate Capacitor to Control Inrush
Current Profile
In PCI Express applications where the 12VOUT[A/B] and
the 3VOUT[A/B] maximum load capacitance is 1000 µF
(2000uF on the 12V rail for x16 modules), the PCI
MIC2342/2342R
Express power control specification clearly states that
any inrush current rate-of-change shall not exceed
0.1A/µs. This situation is most likely to happen when the
controller is charging these large load capacitances in
current limit. Under these circumstances, it may be
preferable to modify the gate drive by using GATE
voltage control instead of active current regulation to
charge the load. As shown in Figure 5, an external
capacitor connected from each 3VGATE[A/B] to AGND
can be used. For the 12VGATE[A/B], an optional Miller
capacitor (Gate-Drain) can be used in conjunction with a
Gate-Source capacitor to form a Miller integrator to
control the output slew rate. The optimal capacitor value
is best determined empirically as the magnitude of the
inrush current slew rate is a function of the power
MOSFET’s input capacitance (C_ISS), the load
capacitance (C_{LOAD(3VOUT[A/B])} and C_{LOAD(12VOUT[A/B])}), and
the current-limit sense resistor (R_3VSENSE[A/B] and
R_12VSENSE[A/B). Using an external capacitor to control the
gate voltage slew rate for large load capacitance may
affect the MIC2342/MIC2342R’s specified system turn-
on and turn-off time performance.
Figure 5. (Optional) External Gate Slew Control Components
June 2008
20
M9999-062008-B