LTC4211CS8 datasheet(27/36 Pages) LINER

27 / 36 page

LTC4211

4211f

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. As

an example, increasing RAUTO from 1M to 3.2M decreases

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

different 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

APPLICATIO S I FOR ATIO

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

µA

4211 F19

µA

ON/FAULT

DUTY CYCLE =

(tFILTER << t1, t2 AND tOFF)

tOFF + t1 + t2

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 supply 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

Figure 19. Autoretry Timing

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