LTC4244IGN datasheet(17/28 Pages) LINER

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LTC4244/LTC4244-1

42441f

Calculating RSENSE

Determining the most appropriate value for the sense

resistor first requires knowing the maximum current needed

by the load under worst-case conditions. Two other pa-

rameters affect the value of the sense resistor. First is the

tolerance of the LTC4244’s circuit breaker threshold volt-

age. The LTC4244’s nominal circuit breaker threshold

voltage is VCB(NOM) = 52mV; however it exhibits ±5mV

tolerance over process and temperature. Second is the

tolerance (RTOL) of the sense resistor. Sense resistors are

available in RTOL’s of

±1%, ±2% and ±5% and exhibit

temperature coefficients of resistance (TCR’s) between

±75ppm/°C and ±100ppm/°C. How the sense resistor

changes as a function of temperature depends on the

I2 • R power being dissipated by it. The power rating of the

sense resistor should accommodate steady-state fault

current levels so that the component is not damaged

before the circuit breaker trips.

Table 2 lists ITRIP(MIN) and ITRIP(MAX) versus some sug-

gested values of RSENSE. Table 7 lists manufacturers and

part numbers for these resistor values.

Table 2. ITRIP vs RSENSE

RSENSE (1% RTOL)

ITRIP(MIN)

ITRIP(MAX)

0.005

Ω

9.31A

11.5A

0.007

Ω

6.6A

8.2A

0.011

Ω

4.2A

5.2A

Output Voltage Monitor

The status of all four output voltages is monitored 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 3). As a result,

LOCAL_PCI_RST# will be pulled low whenever HEALTHY#

is pulled high independent of the state of the PCI_RST#

signal.

If any of the output voltages drop below the power good

threshold for more than 14

µs, the PWRGD pin will be

pulled high and the LOCAL_PCI_RST# signal will be

asserted low.

Table 3. LOCAL_PCI_RST# Truth Table

PCI_RST#

HEALTHY#

LOCAL_PCI_RST#

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 and extraction. The LTC4244 is also capable of

generating precharge voltages other than 1V. Figure 7

shows a circuit that can be used in applications requiring

a precharge voltage of less than 1V. The circuit in Figure 8

can be used for applications that need precharge voltages

greater than 1V.

Precharge resistors are used to connect the 1V bias volt-

age to the I/O lines with minimal disturbance. Figure 1

shows the precharge application circuit for 5V signaling.

The precharge resistor requirements are more stringent

for 3.3V and Universal Hot Swap boards. If the total leak-

age current on the I/O line is less 2

µA, then a 50k resistor

can be connected directly from the 1V bias voltage to the

I/O line. However, many ICs connected to the I/O lines can

have leakage currents up to 10

µA. For these applications,

a 10k resistor is used but must be disconnected when the

board is seated as determined by the state of the BD_SEL#

signal. Figure 9 shows a precharge circuit that uses a bus

switch to connect the individual 10k precharge resistors to

the LTC4244’s 1V PRECHARGE pin. The electrical connec-

tion is made (bus switches closed) when the voltage on the

BD_SEL# pin of the plug-in card is pulled-up to 5VIN,

which occurs just after the long pins have made contact.

The bus switches are electrically disconnected when the

short, BD_SEL# connector pin makes contact and the

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