S39421 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
MFG CO.
'S39421' PDF : 28 Pages View PDF
S39421
Slew Rate Control
The nominal slew rate for the VGATE3 and VGATE5
outputs is set at a default value of 250V per second, which
conforms to a number of standards including that for
Compact PCI. This slew rate helps limit current spike
transients as the bypass capacitors of the add-in card are
charged. The conditions for the default slew rate are:
ISLEW input is grounded; and the CVGATE capacitance is
less than or equal to 0.08µF.
3
ISLEW
ISLEW IN
23
VGTE5
The slew rate can be extended (made slower) by adding
capacitance to the VGATE outputs. In this case it should
be assumed the ISLEW input is grounded. The VGATE
outputs can drive up to 20µA typically, so the slew rate may
be calculated as 20µA ÷ CVGATE (not exceeding 250V/s).
Refer to Graph 1 shown below.
22
VOUT5
10Ω
CVGATE
Host 5V
To Card 5V
2024 ILL14.1
300
250
200
150
100
50
0
0
FIGURE 18
0.1
0.2
0.3
0.4
Capacitance (µF)
2024 ILL13.0
FIGURE 17
The slew rate can be increased (made faster) by injecting
current into the ISLEW input. One quarter of the current
injected into ISLEW will be mirrored out of the VGATE
drivers. The resulting slew rate may be calculated as
ISLEW ÷ 4xCVGATE (not less than 250V/s). Example slew
rates are plotted to illustrate the effects of capacitance on
the VGATE output in Graph 2. The reason for the flat
portion of the graph is that the internal slew rate control
operates in parallel to add as much as 20µA (typically) to
help keep the SR at 250V/s.
1200
1000
800
600
400
200
0
0
C = 0.08µF
C = 0.2µF
250V/s
100
200
300
ISLEW Current in µA
400
2024 ILL15.0
FIGURE 19
Note that the ISLEW input is simply a diode-connected
MOSFET. As a consequence, its I-V characteristic is
temperature dependent.
2024 9.0 8/8/00
16
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