LT7101 datasheet(17/38 Pages) AD

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LT7101

Rev. 0

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Boost Supply and Dropout Operation

By making use of an internal charge pump, the LT7101

is capable of operating at 100% duty cycle, providing the

lowest possible dropout voltage and zero switching noise

while in dropout. This charge pump delivers the small

current required to maintain the static gate voltage on

the top MOSFET switch when operating in dropout. When

not operating in dropout, the gate drive voltage required

for switching the top MOSFET switch is supplied by the

charge pump formed by the BOOST capacitor (CBST),

the bottom MOSFET switch, and an internal switch from

INTVCC to BOOST. As dropout is approached, the on-time

of the bottom MOSFET switch is lengthened as needed

to maintain an adequate supply to the floating gate driver

between BOOST and SW.

Power Good (PGOOD Pin)

The PGOOD pin is connected to the open drain of an inter-

nal N-channel MOSFET. The MOSFET turns on and pulls

the PGOOD pin low when the internal feedback voltage

(VFBI) is not within ±10% of the 1V reference voltage. The

PGOOD pin is also pulled low when the RUN pin is low

(shutdown). When VFBI 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 no greater

than 16V. There is a 24μs delay (TPG) before the PGOOD

pin goes low in response to the VFBI voltage going outside

of the ±10% window.

Overtemperature and Overvoltage Protection

In addition to the OVLO pin, which provides a user-

adjustable protection against VIN overvoltage, the LT7101

contains an internal VIN overvoltage shutdown feature. If

the VIN pin voltage exceeds 118.5V rising (112V falling),

then the top and bottom MOSFETs are held off and all

switching stops. Likewise, if the internal die temperature

exceeds 171°C rising (155°C falling), then the LT7101

disables switching until the temperature drops. Note that

the internal overvoltage and overtemperature protection

features are activated outside of the absolute maximum

range of operation, and therefore should not be relied

upon operationally. These features are only intended as

a secondary failsafe to improve overall system reliability

and safety.

OPERATION

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