LTC1646
W U U
U
APPLICATIO S I FOR ATIO
(RTOL = ±1%, ±2% or ±5%) and standard sense resistor
values. Equation 7 can be used to calculate the nominal
value from the maximum value found by Equation 6:
VCB(MAX)
RSENSE(MIN)
ITRIP(MAX,NEW)
=
=
VCB(MAX)
65mV
=
= 9.8A
RSENSE(MAX)
⎡
⎤
0.0065
RTOL
100
⎛
⎜
⎝
⎞
⎟
⎠
RSENSE(N0M) • 1–
RSENSE(NOM)
=
⎢
⎥
⎦
RTOL
100
⎛
⎜
⎝
⎞
⎟
⎠
⎣
(7)
1+
Since ITRIP(MAX, NEW) > ILOAD(MAX), a larger value for
RSENSE should be selected and the process repeated again
to lower ITRIP(MAX, NEW) without substantially affecting
Often, the result of Equation 7 may not yield a standard
sense resistor value. In this case, two sense resistors with
the same RTOL can be connected in parallel to yield
RSENSE(NOM)
The last step requires calculating a new value for
ITRIP(MAX)(ITRIP(MAX, NEW)) based on a minimum value for
RSENSE (RSENSE(MIN)) and the upper limit for the circuit
breaker threshold, VCB(MAX). Should the calculated value
for ITRIP(MAX, NEW) be much greater than the design value
for ITRIP(MAX), a larger sense resistor value should be
selected and the process repeated. The new value for
ITRIP(MAX, NEW) is given by Equation 8:
ILOAD(MAX)
.
.
Output Voltage Monitor
The status of both 5V and 3.3V output voltages is moni-
toredbythepowergoodfunction.Inaddition,thePCI_RST#
signal is logically combined on-chip with the HEALTHY#
signal to create LOCAL_PCI_RST# (see Table 4).
Table 4. LOCAL_PCI_RST# Truth Table
PCI_RST#
HEALTHY#
LOCAL_PCI_RST#
LO
LO
HI
LO
HI
LO
LO
HI
VCB(MAX)
RSENSE(MIN)
LO
HI
ITRIP(MAX,NEW)
=
(8)
HI
LO
⎡
⎤
RTOL
100
⎛
⎜
⎝
⎞
⎟
⎠
where RSENSE(MIN) = RSENSE(NOM) • 1–
If either of the output voltages drop below the power good
thresholdformorethan50µs,theHEALTHY#signalwillbe
pulledhighandtheLOCAL_PCI_RST#signalwillbepulled
low.
⎢
⎥
⎦
⎣
Example: A 5V supply exhibits a nominal 5A load with a
maximum load current of 6.8A (ILOAD(MAX) = 6.8A), and
sense resistors with ±5% RTOL will be used. According to
Equation 6, VCB(MIN) = 50mV and RSENSE(MAX) is given by:
Precharge
The PRECHARGE input and DRIVE output pins are in-
tended for use in generating the 1V precharge voltage that
is used to bias the bus I/O connector pins during board
insertion. The LTC1646 is also capable of generating
precharge voltages other than 1V. Figure 8 shows a circuit
that can be used in applications requiring a precharge
voltage less than 1V. The circuit in Figure 9 can be used for
applicationsthatneedprechargevoltagesgreaterthan1V.
Table 5 lists suggested resistor values for R1 and R2 vs
precharge voltage for the application circuits shown in
Figures 8 and 9.
VCB(MIN)
ITRIP(MAX)
50mV
6.8A
RSENSE(MAX)
=
=
= 0.0074Ω
The nominal sense resistor value is (Equation 7):
RSENSE(MAX)
0.0074Ω
RSENSE(NOM)
=
=
= 0.007Ω
RTOL
100
5
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
1+
1+
100
And the new current-limit trip point is Equation 8:
1646fa
14
Linear [ Linear ]
