LT1640AL datasheet(15/40 Pages) LINER

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LTC4211

4211fb

OPERATION

The CTIMER value is vital to ensure a proper start-up and

reliable operation. A system may not start up if a timing

period is set too short relative to the time needed for the

output voltage to ramp up from zero to its rated value.

Conversely, this timing period should not be too long as

an output short can occur at start-up causing the exter-

nal MOSFET to overheat. A good starting point is to set

CTIMER = 10nF and adjust its value accordingly to suit the

specific applications.

OPERATING SEQUENCE

Power-Up, Start-Up Check and Plug-In Timing Cycle

The sequence of operation for the LTC4211 is illustrated

in the timing diagram of Figure 5. When a PC board is

inserted into a live backplane, the LTC4211 first performs

Figure 5. Normal Power-Up Sequence

VCC

2μA

10μA

PLUG-IN CYCLE

FIRST TIMING CYCLE

200μA

POWER

GOOD

(VFB > VREF)

VTMR = VREF

TIMER

3 4 5

RESET PULLED LOW DUE TO POWER BAD

SLOW COMPARATOR ARMED

CHECK FOR GATE < 0.2V

FAST COMPARATOR ARMED

CHECK FOR FILTER LOW (<VREF – 80mV)

CHECK FOR

FAULT HIGH (>VREF + 50mV)

ON GOES LOW

GATE

VOUT

RESET

POWER BAD

(VFB < VREF)

SOFT-START CYCLE

SECOND TIMING CYCLE

4211 F05

Table 1. tTIMER vs CTIMER

CTIMER

tTIMER

0.0033μF

2.0ms

0.0047μF

2.9ms

0.0068μF

4.2ms

0.0082μF

5.1ms

0.01μF

6.2ms

0.015μF

9.3ms

0.022μF

13.6ms

0.033μF

20.4ms

0.047μF

29.0ms

0.068μF

42.0ms

0.082μF

50.7ms

0.1μF

61.8ms

0.15μF

92.7ms

0.22μF

136ms

0.33μF

204ms

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