LTC4211_12 datasheet(28/40 Pages) LINER

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LTC4211

4211fb

APPLICATIONS INFORMATION

VCC

TIMER

GATE

RESET

VCC – VSENSE

FILTER

VOUT

SLOW COMPARATOR ARMED

FAST COMPARATOR ARMED

tOFF

tFILTER

tOFF

VREF

>50mV

GATE

VFB < VREF

VOUT

FPD

tRESET

VSENSE = 50mV

REGULATED

LOAD CURRENT

10μA

4211 F19

2μA

ON/FAULT

DUTY CYCLE =

(tFILTER << t1, t2 AND tOFF)

tOFF + t1 + t2

To increase the RC delay, the user may either increase

CAUTO or RAUTO. However, increasing CAUTO > 2μF will

actually limit the RC delay due to the reset sink-current

capability of the FAULT pin. Therefore, in order to increase

the RC delay, it is more effective to either increase RAUTO

or to put a bleed resistor in parallel with CAUTO to GND.

For example, increasing RAUTO in Figure 18 from 1M to

3.2M decreases the duty cycle to 0.8%.

HOT SWAPPING TWO SUPPLIES

Using two external pass transistors, the LTC4211 can

switch two supply voltages. In some cases, it is necessary

to bring up the dominant supply first during power-up but

ramp them down together during the power-down phase.

The circuit in Figure 20 shows how to program two dif-

ferent delays for the pass transistors. The 5V supply is

powered up first. R1 and C3 are used to set the rise and

fall times on the 5V supply. Next, the 3.3V supply ramps

up with 20ms delay set by R6 and C2. On the falling edge,

both supplies ramp down together because D1 and D2

bypass R1 and R6.

OVERVOLTAGE TRANSIENT PROTECTION

Good engineering practice calls for bypassing the supply

rail of any analog circuit. Bypass capacitors are often placed

at the supply connection of every active device, in addition

to one or more large value bulk bypass capacitors per sup-

ply rail. If power is connected abruptly, the large bypass

capacitors slow the rate of rise of the supply voltage and

heavily damp any parasitic resonance of lead or PC track

inductance working against the supply bypass capacitors.

The opposite is true for LTC4211 Hot Swap circuits mounted

on plug-in cards. In most cases, there is no supply bypass

capacitor present on the powered supply voltage side of

the MOSFET switch. An abrupt connection, produced by

Figure 19. Autoretry Timing

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