LTC3774 데이터 시트보기 (PDF) - Linear Technology

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LTC3774

Dual, Multiphase Current Mode Synchronous Controller

Linear Technology 

LTC3774

OPERATION

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 VOSNS+ pin voltage is not

within ±7.5% of the 0.6V reference voltage. The PGOOD

pin is also pulled low when the RUN pin is below 1.14V

or when the LTC3774 is in the soft-start or tracking up

phase. When the VOSNS+ pin voltage is within the ±7.5%

regulation window, 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. The PGOOD pin will flag power good

immediately when the VOSNS+ pin is within the regulation

window. However, there is an internal 45µs power-bad

mask when the VOSNS+ goes out of the window.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>7.5%) as well as other more serious condi-

tions that may overvoltage the output. In such cases, the

top MOSFET is turned off and the bottom MOSFET is turned

on until the overvoltage condition is cleared.

Undervoltage Lockout

The LTC3774 has two functions that help protect the

controller in case of undervoltage conditions. A precision

UVLO comparator constantly monitors the INTVCC voltage

to ensure that an adequate gate-drive voltage is present.

It locks out the switching action when INTVCC is below

3.75V. To prevent oscillation when there is a disturbance

on the INTVCC, the UVLO comparator has 500mV of preci-

sion hysteresis.

Another way to detect an undervoltage condition is to

monitor the VIN supply. Because the RUN pin has a preci-

sion turn-on reference of 1.22V, one can use a resistor

divider to VIN to turn on the IC when VIN is high enough.

An extra 4µA of current flows out of the RUN pin once the

RUN pin voltage passes 1.22V. The RUN comparator itself

has about 80mV of hysteresis. One can program additional

hysteresis for the RUN comparator by adjusting the values

of the resistive divider. For accurate VIN undervoltage

detection, VIN needs to be higher than 4.75V. Always

set the VIN undervoltage detection threshold higher than

the power stage UVLO threshold so that the LTC3774 is

enabled after the power stage is.

3774f

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For more information www.linear.com/LTC3774

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