LT3757 datasheet(19/48 Pages) AD

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LTC7841

Rev. 0

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OPERATION

In pulse-skipping mode, if VIN is between 100% and 110%

of the regulated VOUT voltage, TG turns on if the inductor

current rises above a certain threshold and turns off if the

inductor current falls below this threshold. This thresh-

old current is set to approximately 6%, 4% or 3% of the

maximum ILIM current when the ILIM pin is grounded,

floating or tied to INTVCC, respectively. If the controller is

programmed to Burst Mode operation under this same

VIN window, then TG remains off regardless of the induc-

tor current.

If VIN rises above 110% of the regulated VOUT voltage in

any mode, the controller turns on TG regardless of the

inductor current. In Burst Mode operation, however, the

internal charge pump turns off if the chip is asleep. With

the charge pump off, there would be nothing to prevent

the boost capacitor from discharging, resulting in an

insufficient TG voltage needed to keep the top MOSFET

completely on. To prevent excessive power dissipation

across the body diode of the top MOSFET in this situation,

the chip can be switched over to forced continuous mode

to enable the charge pump; a Schottky diode can also be

placed in parallel with the top MOSFET.

Power Good

The PGOOD pin is connected to an open drain of an

internal N-channel MOSFET. The MOSFET turns on and

pulls the PGOOD pin low when the VFB pin voltage is

not within ±10% of the reference voltage. The PGOOD

pin is also pulled low when the corresponding RUN pin

is low (shut down). When the VFB pin voltage is within

the ±10% requirement, the MOSFET is turned off and the

pin is allowed to be pulled up by an external resistor to a

source of up to 6V (abs max).

Overvoltage Protection

An overvoltage event is defined as when the output feed-

back voltage (VFB) is greater than 110% of its normal

regulated point set by the DAC. During an overvoltage

event, TG1/TG2 are turned on continuously until the over-

voltage condition is cleared, regardless of whether Burst

Mode operation, pulse-skipping mode, or forced continu-

ous 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 LTC7841

is in sleep during an overvoltage condition, which dis-

ables the internal oscillator and BOOST-SW charge pump.

So the BOOST-SW voltage may discharge (due to leak-

age) if the overvoltage conditions persists indefinitely.

If BOOST-SW discharges, then by definition TG would

turn off.

BOOST Supply Refresh and Internal Charge Pump

Each top MOSFET driver is biased from the floating boot-

strap capacitor, CB, which normally recharges during

each cycle through an external diode when the bottom

MOSFET turns on. There are two considerations for keep-

ing 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 gener-

ates 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. There is also an internal charge

pump that keeps the required bias on BOOST. The charge

pump always operates in both forced continuous mode

and pulse-skipping mode. In Burst Mode operation, the

charge pump is turned off during sleep and enabled when

the chip wakes up. The internal charge pump can normally

supply a charging current of 55μA.

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