LTC3118 datasheet(21/40 Pages) LINER

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LTC3118

3118fa

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The RUN comparators include a built-in hysteresis of

approximately 170mV, so that the turn-off threshold will

be approximately 15% lower than the turn-on threshold.

Put another way, the internal threshold levels for the RUN

comparators to disable switching from a particular input

is 1.05V.

VTURNOFF = 1.05V 1+













The RUN comparator is relatively noise insensitive, but

there may be cases due to PCB layout, very large value

resistors for RT and RB (Figure 4a), or proximity to noisy

components where noise pickup is unavoidable and may

cause the turn-on or turn-off levels to be intermittent. In

these cases, a small value filter capacitor can be added

across RB to ensure proper operation.

Selecting Priority or Ideal Diode Mode Operation

Priority Mode (SEL=0)

Prioritymodeoperationissuggestedformostapplications,

since powering from one of the sources is typically

preferred. In priority mode, the primary input is connected

to VIN1 and the auxiliary input is connected to VIN2. The

LTC3118 will maintain operation from VIN1 until either

the RUN1 or minimum VIN1 UVLO circuits transition the

LTC3118 to VIN2 operation if valid. It is important that the

RUN1 turn-off threshold programs the minimum VIN1

above 2.5V in Priority Mode unless VCC is back-fed and

held above 2.5V. This prevents an unintended soft-start

cycle from occurring if VCC hits its UVLO threshold when

the VIN1 source is removed, before transitioning to VIN2

operation.

Dependingonthemaximumloadcurrentoftheapplication,

the RUN1 and RUN2 minimum VIN turn-off thresholds

may need to set well above 2.5V to prevent VOUT from

losing regulation, especially in step-up mode. Please

refer to Maximum Load Current vs VIN curves found in

the Typical Performance Characteristics. Maximum load

current capability when VIN1 or VIN2 is less than 3.8V can

be improved if VCC is boot-strapped to 5V as shown in

Figure 7.

Ideal Diode Mode (SEL=1)

IdealDiodemodeoperationisavailableontheLTC3118for

systems with low ESR sources or where the programmed

operating range of the two inputs can be separated as will

be discussed. In Ideal Diode mode, an internal comparator

monitors the voltage on both VIN1 and VIN2 to determine

which input is higher. The comparator has approximately

800mVofhysteresistohelppreventthepartfromswitching

betweenthetwoinputsifthesourcevoltagesareequal.The

comparatorhasanapproximate250µsfilterdelaytoprevent

rapidswitchingbetweeninputsanderraticoperation.When

the LTC3118 switches between inputs, current supplied

from one source is suspended before transitioning to the

other source. Depending on the impedance of each source

and the amount of input current required to support the

load on VOUT, it is possible for the voltage ripple on one

or both inputs to exceed this comparator’s hysteresis.

As an example, if both input sources have 300mΩ of

impedance and 2A of current is being drawn from the

active source, a 600mV step will occur on the inputs

during switchover, approaching the comparator’s 800mV

of typical hysteresis. When the input voltages are equal,

theLTC3118couldtogglebetweenVIN1andVIN2operation

at high load currents. For such systems, operation in

Priority mode is recommended, unless the RUN pins can

be programmed such that the minimum operating voltage

of one input is set above the maximum source voltage of

the other input. As with priority mode, the minimum VIN

operating voltages should be set by their RUN pins above

VCC UVLO and higher if needed to support maximum load

current. Low ESR 100µF to 220µF aluminum electrolytic

capacitors close to both input pins help to reduce resonant

ringing during VIN switchover, due to cable inductances

found in some applications and bench evaluation set-ups.

operaTion

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