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LTC4211

4211fb

OPERATION

SOFT-START WITH CURRENT LIMITING

During the second timing cycle, the inrush current was

described by Equation 4. Note that there is a one-to-one

correspondence in the inrush current to CLOAD.Iftheinrush

current is large enough to cause a voltage drop greater

than 50mV across the sense resistor, an internal servo

loop controls the operation of the 10μA current source at

the GATE pin to regulate the load current to:

ILIMIT(SOFTSTART) =

50mV

RSENSE

(6)

For example, the inrush current is limited to 5A when

RSENSE = 0.01Ω.

In this fashion, the inrush current is controlled and CLOAD

is charged up slowly during the soft-start cycle.

The timing diagram in Figure 7 illustrates the operation

of the LTC4211 in a normal power-up sequence with lim-

ited inrush current as described by Equation 6. At Time

Point 5, the GATE pin voltage begins to ramp and the power

MOSFET starts to charge CLOAD.AtTimePoint5A,theinrush

current causes a 50mV voltage drop across RSENSE and

the internal servo loop engages, limiting the inrush cur-

rent to a fixed level. At Time Point 6, the GATE pin voltage

continues to ramp as CLOAD charges until VOUT reaches its

final value. The charging current reduces, and the internal

servo loop disengages. At the end of the soft-start cycle

(Time Point 7), RESET is high and SLOW COMP is armed.

Figure 7. Normal Power-Up Sequence (with Current Limiting in Second Timing Cycle)

PLUG-IN CYCLE

FIRST TIMING CYCLE

TIMER

VCC

3 4 5

RESET PULLED LOW DUE TO POWER BAD

200μA

4211 F07

2μA

VREF

10μA

GATE

VOUT

POWER BAD

VFB < VREF

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

ILOAD

RESET

SOFT-START CYCLE

SECOND TIMING CYCLE

POWER GOOD

VFB > VREF

LOAD CURRENT IS

REGULATING AT 50mV/RSENSE

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