LTC1646
W U U
APPLICATIO S I FOR ATIO
U
GATE
10V/DIV
GATE
10V/DIV
5V
3V
OUT
OUT
5V
3V
OUT
OUT
5V/DIV
5V/DIV
TIMER
5V/DIV
TIMER
5V/DIV
BD_SEL#
5V/DIV
BD_SEL#
5V/DIV
HEALTHY#
5V/DIV
HEALTHY#
5V/DIV
LCL_PCI_RST#
5V/DIV
LCL_PCI_RST#
5V/DIV
PRECHARGE
5V/DIV
PRECHARGE
5V/DIV
20ms/DIV
1646 F02
10ms/DIV
1646 F03
Figure 2. Normal Power-Up Sequence
Figure 3. Normal Power-Down Sequence
Power-Down Sequence
allows the chip to power up CPCI boards with widely
varying capacitive loads on the supplies. The power-up
time for either of the two outputs is given by:
When BD_SEL# is pulled high, a power-down sequence
begins (Figure 3).
Internal switches are connected to each of the output
supply voltage pins to discharge the bypass capacitors to
ground. The TIMER pin (Pin 2) is immediately pulled low.
The GATE pin (Pin 10) is pulled down by a 200µA current
source to prevent the load currents on the 3.3V and 5V
supplies from going to zero instantaneously in order to
prevent glitching the power supply voltages. When either
oftheoutputvoltagesdipsbelowitsthreshold,HEALTHY#
pulls high and LOCAL_PCI_RST# will be asserted low.
CLOAD(XVOUT) • XVOUT
ILIMIT(XVOUT) – ILOAD(XVOUT)
tON (XVOUT) = 2 •
(2)
Where XVOUT = 5VOUT or 3VOUT. For example, for
CLOAD(5VOUT) = 2000µF, ILIMIT = 7A, and ILOAD = 5A, the
5VOUT turn-on time will be ~10ms. By substituting the
variables in Equation 2 with the appropriate values, the
turn-on time for the 3VOUT output can also be calculated.
The timer period should be set longer than the maximum
supply turn-on time but short enough to not exceed the
maximumsafeoperatingareaofthepasstransistorduring
a short-circuit. The timer period for the LTC1646 is given
by:
Once the power-down sequence is complete, the CPCI
card may be removed from the slot. During extraction, the
precharge circuit will continue to bias the bus I/O pins at
1V until the 5V and 3.3V long connector pin connections
are separated.
CTIMER •1.25V
tTIMER
=
(3)
5µA
Timer
As a design aid, the timer period as a function of the timing
capacitor using standard values from 0.01µF to 1µF is
shown in Table 2.
During a power-up sequence, a 5µA current source is
connected to the TIMER pin and current limit faults are
ignored until the voltage exceeds 1.25V. This feature
1646fa
11
Linear [ Linear ]
