MIC2591B-2YTQ Micrel Inc, MIC2591B-2YTQ Datasheet - Page 18

• Manufacturer Part Number: MIC2591B-2YTQ
• Description: IC CTRLR HOTPLUG PCI DUAL 48TQFP
• Manufacturer: Micrel Inc
• Type: Hot-Swap Controllerr

Specifications of MIC2591B-2YTQ

• Applications: General Purpose, PCI Express
• Internal Switch(s): No
• Voltage - Supply: 3.3V, 12V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 48-TQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 576-1098

are enabled by asserting ON[A/B], the 3VGATE[A/B] and
12VGATE[A/B] pins are each connected to a constant cur-
rent supply. These supplies are each nominally 25µA. For a
slot’s 3VGATE pin, this is a current source; for the 12VGATE
pin, this is a current sink.
Inrush Current and Load Dominated Start-up
The expected maximum inrush current can be calculated by
using the following equation:
where I
or I
C
MOSFET and any external capacitance connected from
the GATE output pin to the GATE reference – GND or
source).
For the 3.3V outputs and 12V outputs (if no external 12VGATE
output capacitors are implemented), the following equation
is used to determine the output slew rate.
Consequently, the overcurrent timer delay must be pro-
grammed to exceed the time it will take to charge the output
load to the input rail voltage level.
MAIN Outputs (Start-up Delay and Slew-Rate Control)
The 3.3V outputs act as source followers. In this mode of
operation,V
output reaches 3.3V. The voltage on the gate of the MOSFET
output reaches 3.3V. The voltage on the gate of the MOSFET
will then continue to rise until it reaches 12V, which ensures
minimum R
command to a slot and the appearance of voltage at the slot’s
command to a slot and the appearance of voltage at the slot’s
3.3V output. This delay is the time required to charge the
3VGATE output up to the threshold voltage of the external
MOSFET (typically about 3V).
The source (output) side of the external MOSFET will reach
the drain voltage in a time given by:
For the 12V outputs, each MOSFET is confi gured as a Miller
integrator (by virtue of C
the MOSFET’s gate and drain). In this confi guration, the
feedback action from drain to gate of the MOSFET causes
the voltage at the drain of the MOSFET to slew in a linear
fashion at a rate which satisfi es the following equation:
A delay exists between the ON command to a slot and the
appearance of voltage at the slot’s 12V output. For a slot’s
12V output, that delay is given by the time required for the
capacitor from the gate of the MOSFET to its source (typically
fi ve times the value of C
March 2005
GATE
GATE(12VSINK)
GATE(12VSINK)
GATE(12VSINK)
INRUSH � I
is the total GATE capacitance (C
is the total GATE capacitance (C
GATE
t
dV
t
3V(SOURCE_DRAIN)
3VDLY
DS(ON)
DS(ON)
DS(ON)
SOURCE
OUT
 is the GATE pin current, I
�dt �
�
, C

. Note that a delay exists between the ON
dv/dt(12V) � 
GATE
C
LOAD
I
= [V
GATE(3VCHARGE)
GATE
C
I
LIM(3V�12V)
LOAD(3V�12V)
MILLER

GATE
� V
= t
MILLER
is the load capacitance, and
is the load capacitance, and
C
C
LOAD
GATE
3VDLY
GS(TH)
– V
, which is connected between



) to charge to the threshold
 25A �
+
TH(ON)
TH(ON)
TH(ON)
C

I
GATE
MILLER

C
LOAD
] until the associated



ISS
I
GATE(3VCHARGE)
GATE(3VCHARGE)
GATE(3VCHARGE)
LIM(3V)
C
C
 V
LOAD
GATE
of the external
DRAIN

18
voltage of the MOSFET (typically about 3V). In this instance,
the delay before the output voltage starts ramping can be
approximated by:
where C
MOSFET, any external capacitance from the GATE output of
MOSFET, any external capacitance from the GATE output of
the MIC2591B to the source of the MOSFET, and C
(external, if used).
Table 1 approximates the output slew-rate for various values
of C
(external C
ternal MOSFET for the 3.3V rail; C
*
Power-Down Cycle
When one or more PCI slots are disabled via the MIC2591B
output control pins, ON[A/B] or AUXEN[A/B], the output volt-
age for each supply will discharge as a function of the RC
time constant produced by the controller’s internal resistance
(R
(C
in the Electrical Characteristics Table. The charts below in
Figure 7 display curves of the fall time (90% - 10%) as a
function of the output load capacitance for both the 3V and
12V MAIN outputs.
Standby Mode
Standby mode is entered when one or more of the MAIN
supply inputs (12VIN and/or 3VIN) is below its respective
UVLO threshold or OFF. The MIC2591B also supplies
3.3V auxiliary outputs (VAUX[A/B]), satisfying PCI Express
Table 1. 3.3V and 12V Output Slew-Rate Selection for
DIS
LOAD
C
1200
1000
Figure 7. 3V and 12V Output Discharge vs. Load
800
600
400
200
GATE
Values in this range will be affected by the internal parasitic capaci-
tances of the MOSFETs used, and should be verifi ed experimentally.
GATE
Function of Load Capacitance
) connected to the output and the load capacitance
0
0
0
0
0
0
0
3V Output Discharge as a
). The typical value of R
0.022µF*
0.022µF*
0.022µF*
0.047µF
0.047µF
0.047µF
0.01µF*
0.01µF*
GATE(TOTAL)
GATE(TOTAL)
GATE(TOTAL)
Gate Capacitance Dominated Start-up
0.1µF
0.1µF
0.1µF
when start-up is dominated by GATE capacitance
5
5
50
5
or C
0
t
GATE
12VDLY
FALL TIME (ms)
100
MILLER
from GATE pin to ground plus C

150

C
| I
is the sum of the C
200
GATE(TOTAL)
GATE
Capacitance
250
I
| = 25µA
GATE
× V
DIS
1200
1000
800
600
400
200
GS(TH)
Function of Load Capacitance
MILLER
0
0
0
0
0
for each supply is listed
12V Output Discharge as a
dv/dt (load)
dv/dt (load)
0.532 V/ms
0.532 V/ms
0.532 V/ms
1.136V/ms
1.136V/ms
1.136V/ms
0.250V/ms
0.250V/ms
0.250V/ms
2.5V/ms
2.5V/ms
500

GS
FALL TIME (ms)
for the 12V rail).
1000 1500 2000 2500
of the external
M9999-033105
GS
of the ex-
MILLER