MIC2342 Datasheet, PDF (19/32 Pages) Micrel Semiconductor – Dual-Slot PCI Express® Hot-Plug Controller

Micrel, Inc.
MIC2342/2342R
I_{INRUSH(12VOUT[A/B])}
=
dV_{12VGATE[A/ B]}
dt
× C_{LOAD(12VOUT[A/B])}
=
I_GATE(12VSNK)
×
C_{LOAD(12VOUT[A/B])}
C_ISSP
For the same p-channel power MOSFET in the previous
example, if C_ISSP = 1700pF and C_{LOAD(12VOUT[A/B])} =
100µF, the 12VOUT[A/B] inrush current charging this
load capacitance is:
I_{INRUSH(12VOUT [A / B])}
= 25µA × ^100µF
1700pF
= 1.47A
Calculating the 12VOUT[A/B] voltage rate-of-change for
a given capacitive load can be determined by the
following expression:
dV_{12VOUT [A / B] =} I_{INRUSH(12VOUT [A / B])}
dt
C_{LOAD(12VOUT [A / B])}
and, using the same quantities in the current example, is
given by:
dV_{12VOUT[A/B ] = 1.47 A = 14.7 V}
dt
100 µF
ms
To determine (to first-order) the time point at which the
12VOUT[A/B] voltage crosses its corresponding output
“Power Good” threshold, the following equation can be
used:
( ) _{t PWRGD(12VOUT [A / B])}
=
V_{UVTH(12V )} + V_HYSPG
dV_{12VOUT [A / B]}
= ^10.53V
14.7 ^V
≈ 0.72ms
dt
ms
To determine 3VGATE[A/B] pin voltage slew rates,
inrush currents, 3VOUT[A/B] output voltage slew rates,
and time to assert its corresponding internal “Power
Good” flag into capacitive loads connected to
3VOUT[A/B], simple computations can be made using
the same equations by substituting I_{GATE(3VCHARGE)} for
I_{GATE(12VSINK)}, C_ISSN (the input gate capacitance of an N-
channel power MOSFET) for C_ISSP, C_{LOAD(3VOUT[A/B])} for
C_{LOAD(12VOUT[A/B])}, I_{INRUSH(3VOUT[A/B])} for I_{INRUSH(12VOUT[A/B])},
and V_UVTH(3V) for V_UVTH(12V).
For example, if a Si4420BDY n-channel power MOSFET
is used with the MIC2342 to control inrush currents at
3VOUT[A/B], its C_ISSN is approximately 4100pF at V_DS =
3V. The 3VGATE[A/B] pin voltage rate of change is
given by:
dV_{3VGATE[A/B ] =} I_{GATE(3VCHARGE) = 25µA = 6.1 V}
dt
C_ISSN
4100pF
ms
Assuming a 300-µF capacitive load, the 3VOUT[A/B]
inrush current charging this load capacitance is given by:
I_{INRUSH(3VOUT[A/B])}
=
25µA ×
300µF
4100pF
= 1.82 A
The 3VOUT[A/B] output voltage slew rate is given by:
dV_{3VOUT[A/B] = 1.82 A ≈ 6.1 V}
dt
300 µF
ms
and the time to assert the internal 3VOUT[A/B] “Power
Good” flag is given by:
( ) _{tPWRGD(3VOU T[A/B]) =}
V_UVTH(3V) + V_HYSPG
dV_3VOUT[A/B]
= ^2.77V
6.1 ^V
≈ 0.45 ms
dt
ms
Mode 2: Charging 12VOUT and 3VOUT Capacitive
Loads in Current Limit
In x4 and x8 PCI Express applications, capacitive loads
at 12VOUT[A/B] and 3VOUT[A/B] can be as large as
1000µF. As a result, the inrush load charging currents at
start-up can be large enough to cause a voltage drop
across the external sense resistor larger than 50mV. In
these applications, internal servo circuits at
12VGATE[A/B] and 3VGATE[A/B] modulate the drive to
the gates of their corresponding power MOSFETs to
regulate the load current to:
I_{LIMIT(12VO UT[A/B])}
= ^VTHLIMIT
R_{12VSENSE[A /B]}
=
50 mV
R_{12VSENSE[A /B]}
In the typical application circuit, the external sense
resistor connected between 12VIN[A/B] and
12VSENSE[A/B] pins was selected to be 20mΩ. The
regulated current charging the load capacitance at
12VOUT[A/B] is given by:
I_{LIMIT(12VOUT[A/B])}
=
50 mV
20 mΩ
=
2.5
A
Once current-regulation control is activated, the circuit
breaker’s t_FLT timer is also activated to protect the
external power MOSFET against potentially excessive
power dissipation. For additional information on this
timer and the MIC2342’s circuit breaker operation,
please consult the section labeled “Circuit Breaker
Function.” The output voltage rate of change at
12VOUT[A/B] during current limit charging into a 1000µF
capacitive load is given by:
dV_{12VOUT[A/B] =} I_{LIMIT(12VOUT[A/B]) = 2.5 A = 2.5 V}
dt
C_{LOAD(12VOUT[A/B])} 1000 µF
ms
In this fashion, the inrush current is controlled and the
load capacitance is charged up slowly during the start-up
cycle. The gate drive circuits will maintain control of the
inrush current until the 12VOUT[A/B] or 3VOUT[A/B]
voltages have reached their corresponding “Power
Good” thresholds (V_{UVTH(12V)[A/B]} or V_{UVTH(3V)[A/B]},
respectively) at which time the inrush current
approaches its nominal steady-state level, the voltage
across the external sense resistor drops below the circuit
breaker’s V_THLIMIT threshold, and the corresponding
internal “Power-is-Good” flag is asserted. For the
12VOUT[A/B] example, its internal “Power-is-Good” flag
is asserted at:
( ) _{tPWRGD(12VOUT[A/B]) =}
V_UVTH(12V) + V_HYSPG
dV_12VOUT[A/B]
=
10.53V
2.5 ^V
≈ 4.2 ms
dt
ms
Calculating the current limit for charging the 3VOUT[A/B]
load capacitance, the output voltage slew rate at
June 2008
19
M9999-062008-B