LTC4211CS8 datasheet(14/36 Pages) LINER

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LTC4211

4211f

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

reliable operation. A system may not get started if a timing

period is set too short in relation to the time needed for the

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

Conversely, this timing period should not be excessive as

an output short can occur at start-up allowing the external

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 operations for the LTC4211 is illustrated

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

inserted into a live backplane, the LTC4211 first performs

VCC

µA2µA

µA

PLUG-IN CYCLE

FIRST TIMING CYCLE

200

µA

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

Figure 5. Normal Power-Up Sequence

OPERATIO

Table 1. tTIMER vs CTIMER

CTIMER

tTIMER

0.0033

µF

2.0ms

0.0047

µF

2.9ms

0.0068

µF

4.2ms

0.0082

µF

5.1ms

0.01

µF

6.2ms

0.015

µF

9.3ms

0.022

µF

13.6ms

0.033

µF

20.4ms

0.047

µF

29.0ms

0.068

µF

42.0ms

0.082

µF

50.7ms

0.1

µF

61.8ms

0.15

µF

92.7ms

0.22

µF

136ms

0.33

µF

204ms

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