LT8705 datasheet(14/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

operaTion

Overvoltage Mode Selection

The OVMODE pin is used to select how the LTC3784

operates during an overvoltage event, defined as when

the output feedback voltage (VFB) is greater than 110%

of its normal regulated point of 1.2V. It is also used to

determine the light-load mode of operation when the

LTC3784 is synchronized to an external clock through the

PLLIN/MODE pin.

The OVMODE pin is a logic input that should normally be

tied to INTVCC or grounded. Alternatively, the pin can be

left floating, which allow a weak internal resistor to pull

it down to ground.

OVMODE = INTVCC: An overvoltage event causes the error

amplifier to pull the ITH pin low. In Burst Mode operation,

this causes the LTC3784 to go to sleep and TG1/TG2 and

BG1/BG2areheldoff.Inpulse-skippingmode,BG1/BG2are

held off and TG1/TG2 will turn on if the inductor current is

positive. In forced continuous mode, TG1/TG2 (and BG1/

BG2) will switch on and off as the LTC3784 will regulate the

inductor current to a negative peak value (corresponding

to ITH = 0V) to discharge the output.

When OVMODE is tied to INTVCC, the LTC3784 operates

in pulse-skipping mode when synchronized.

In summary, with OVMODE = INTVCC, the inductor cur-

rent is not allowed to go negative (reverse from output to

input) except in forced continuous mode, where it does

reverse current but in a controlled manner with a regulated

negative peak current. OVMODE should be tied to INTVCC

in applications where the output voltage may sometimes

be above its regulation point (for example, if the output

is a battery or if there are other power supplies driving

the output) and no reverse current flow from output to

input is desired.

OVMODE Grounded or Left Floating: When OVMODE is

groundedorleftfloating,overvoltageprotectionisenabled

and the TG1/TG2 are turned on continuously until the

overvoltage condition is cleared, regardless of whether

Burst Mode operation, pulse-skipping mode, or forced

continuous mode is selected by the PLLIN/MODE pin.

This can cause large negative inductor currents to flow

from the output to the input if the output voltage is higher

than the input voltage.

Note however that in Burst Mode operation, the LTC3784

is in sleep during an overvoltage condition, which disables

theinternaloscillatorandBOOST-SWchargepump.Sothe

BOOST-SW voltage may discharge (due to leakage) if the

overvoltage conditions persists indefinitely. If BOOST-SW

discharges, then by definition TG would turn off.

When OVMODE is grounded or left floating, the LTC3784

operates in forced continuous mode when synchronized.

OVMODEshouldbetiedtogroundorleftfloatingincircuits,

such as automotive applications, where the input voltage

can often be above the regulated output voltage and it is

desirable to turn on TG1/TG2 to “pass through” the input

voltage to the output.

Operation at Low SENSE Pin Common Mode Voltage

ThecurrentcomparatorintheLTC3784ispowereddirectly

from the SENSE+ pin. This enables the common mode

voltage of the SENSE+ and SENSE– pins to operate at as

low as 2.3V, which is below the UVLO threshold. The figure

on the first page shows a typical application in which the

controller’s VBIAS is powered from VOUT while the VIN

supply can go as low as 2.3V. If the voltage on SENSE+

drops below 2.3V, the SS pin will be held low. When the

SENSE voltage returns to the normal operating range, the

SS pin will be released, initiating a new soft-start cycle.

BOOST Supply Refresh and Internal Charge Pump

EachtopMOSFETdriverisbiasedfromthefloatingbootstrap

capacitor, CB, which normally recharges during each cycle

through an external diode when the bottom MOSFET turns

on. There are two considerations for keeping the BOOST

supply at the required bias level. During start-up, if the

bottom MOSFET is not turned on within 200μs after UVLO

goes low, the bottom MOSFET will be forced to turn on for

~400ns. This forced refresh generates enough BOOST-SW

voltage to allow the top MOSFET ready to be fully enhanced

instead of waiting for the initial few cycles to charge up.

Thereisalsoaninternalchargepumpthatkeepstherequired

bias on BOOST. The charge pump always operates in both

forcedcontinuousmodeandpulse-skippingmode.InBurst

Modeoperation,thechargepumpisturnedoffduringsleep

and enabled when the chip wakes up. The internal charge

pump can normally supply a charging current of 55μA.

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