LTC1646IGN datasheet(14/20 Pages) LINER

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LTC1646

1646fa

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:

RTOL

SENSE NOM

SENSE MAX

()

+ ⎛

⎝⎜

⎞

⎠⎟

100

(7)

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:

where R

RTOL

TRIP MAX NEW

CB MAX

SENSE MIN

SENSE NOM

)

()

(

)

()

•–

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

100

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:

SENSE MAX

CB MIN

TRIP MAX

()

Ω

0 0074

The nominal sense resistor value is (Equation 7):

RTOL

SENSE NOM

SENSE MAX

()

+ ⎛

⎝⎜

⎞

⎠⎟

Ω

+ ⎛

⎝⎜

⎞

⎠⎟

Ω

100

0 0074

100

0 007

And the new current-limit trip point is Equation 8:

RTOL

TRIP MAX NEW

CB MAX

SENSE MIN

CB MAX

SENSE N M

)

()

() •–

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

100

0 0065

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

ILOAD(MAX).

Output Voltage Monitor

The status of both 5V and 3.3V output voltages is moni-

tored by the power good function. In addition, the PCI_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#

If either of the output voltages drop below the power good

threshold for more than 50

µs,theHEALTHY#signalwillbe

pulled high and the LOCAL_PCI_RST# signal will be pulled

low.

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

applications that need precharge voltages greater than 1V.

Table 5 lists suggested resistor values for R1 and R2 vs

precharge voltage for the application circuits shown in

Figures 8 and 9.

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