LTC4240IGN datasheet(21/28 Pages) LINER

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LTC4240

4240f

Transient Power Dissipation

There are certain transient events that can significantly

increase the power dissipated by the external FET. If the

LTC4240 5V supply (at 5V + 10%) powers up into a 1.5V

short (potentially manifested as a short to two diodes in

series), then the FET can potentially have 4V across it with

8.8A flowing. This implies a power dissipation of 35.1W.

The amount of time the FET will dissipate 35.1W will

depend on the relative values of the TIMER and GATE

capacitances. For the values specified on the front page

application circuit, the GATE pin will ramp high signifi-

cantly faster than the TIMER pin, hence transient power

dissipation will be set by the TIMER pin capacitance.

The dissipated 35.1W, the ramp time of the TIMER pin

(50ms will be used for this example), and the FET thermal

resistance will determine the internal junction tempera-

ture of the FET. Most FETs will specify a maximum internal

junction temperature of 150

°C.TheFETdatasheetsshould

have a transient thermal impedance graph. This graph has

a family of curves listing the FET transient thermal imped-

ance as a function of duty cycle. The duty cycle refers to

what percentage of the time the FET is in the short circuit

condition. If we choose the Si7880DP FET and assume

that the board on which the FET is placed has minimal heat

sinking capability, and further assume that the user will

turn on the board every 2.5 seconds (0.02 duty cycle:

50ms on, 2450ms off), then by looking at the junction-to-

ambient curve we note that with a 70

°C ambient tempera-

ture, the Si7880DP internal junction temperature will be

172

°C. This is above the absolute maximum rating of the

FET, and although operating at this temperature will not

damage the FET immediately, it does affect its long term

reliability. Conversely, if we assume that there is a perfect

heat sink for the Si7880DP package, then we would use the

junction-to-case curve and calculate a value of 117

°C with

a 70

°C ambient temperature. The Si7880DP comes in a

thermally enhanced package whose drain lead is a large

piece of metal that can conduct heat away from the internal

junction of the FET. To achieve best performance, the drain

of the Si7880DP should be connected to a piece of copper

(as large as possible) on the board. Note that if the output

is shorted to ground, the current foldback feature will cut

the power dissipation by at least a factor of two.

APPLICATIO S I FOR ATIO

When the LTC4240 is turned on and the large 5VOUT

output capacitor (2000

µF or more) is charged, it is pos-

sible that the 5V FET will dissipate as much as the 35.1W

described above. If there is no DC load at 5VOUT, then 8.8A

will charge the 2000

µF in less than 2ms, which should not

pose any thermal problems for the Si7880DP. If the DC

load at 5VOUT approaches the current limit, then the above

analysis should be used to calculate the internal junction

temperature of the FET.

Output Voltage Monitor

The DC level of all four supply outputs is monitored by the

power good circuitry. When any of the four supply outputs

falls below its specified level (see DC electrical specifica-

tions) for longer than 10

µs, the PWRGD (HEALTHY#)

open drain pin will be deasserted and the LOCAL_PCI_RST#

signal will be asserted low. This does not generate a fault

condition.

The LOCAL_PCI_RST# signal (RESETOUT pin) is derived

from the HEALTHY# (PWRGD pin), PCI_RST# (RESETIN

pin), and Bit 3 of the command latch (see Table 5).

Table 5. LOCAL_PCI_RST# Truth Table

Bit 3 (C3 )

PCI_RST#

HEALTHY#

Command Latch

LOCAL_PCI_RST#

XHI

Precharge

The PRECHARGE input and DRIVE output pins are used to

generate the 1V precharge voltage that biases the bus I/O

connector pins during board insertion and extraction

(Figure 10). The LTC4240 is capable of generating

precharge voltages other than 1V. Figure 11 shows a

circuit that can be used in applications requiring a precharge

voltage less than 1V. The circuit in Figure 12 can be used

for applications that need precharge voltages greater than

1V. Table 6 lists suggested resistor values for R11A and

R11B vs precharge voltage for the application circuits

shown in Figures 11 and 12.

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