LTC1702AIGN datasheet(12/36 Pages) LINER

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LTC1702A

1702afa

APPLICATIONS INFORMATION

SHUTDOWN/SOFT-START

Each half of the LTC1702A has a RUN/SS pin. The RUN/SS

pins perform two functions: when pulled to ground, each

shuts down its half of the LTC1702A, and each acts as a

conventional soft-start pin, enforcing a maximum duty

cycle limit proportional to the voltage at RUN/SS. An

internal 3.5

µA current source pull-up is connected to each

RUN/SS pin, allowing a soft-start ramp to be generated

with a single external capacitor to ground. The 3.5

µA

current sources are active even when the LTC1702A is

shut down, ensuring the device will start when any exter-

nal pull-down at RUN/SS is released. Either side can be

shut down without affecting the operation of the other

side. If both sides are shut down at the same time, the

LTC1702A goes into a micropower sleep mode, and

quiescent current drops below 100

µA. Entering sleep

mode also resets the FAULT latch, if it was set.

Each RUN/SS pin shuts down its half of the LTC1702A

when it falls below about 0.55V. The maximum duty cycle

limit increases linearly between 1V and 2V, reaching its

final value of 90% when RUN/SS is above 2V. Somewhere

before this point, the feedback amplifier will assume

control of the loop and the output will come into regula-

tion. (See Figure 4).

CURRENT LIMIT

The LTC1702A includes an onboard current limit that

engages when the maximum output current exceeds a

user-programmed level. It works by sensing the voltage

drop across QB during the time that QB is on and compar-

ing that voltage to a user-programmed voltage at IMAX.

Since QB looks like a low value resistor during its on-time,

the voltage drop across it is proportional to the current

flowing in it. In a buck converter, the average current in the

inductor is equal to the output current. This current also

flows through QB during its on-time. Thus, by watching

the voltage across QB, the LTC1702A can monitor the

output current.

Figure 4. Soft-Start Operation in Start-Up and Current Limit

1.0V

VOUT

VRUN/SS

RUN/SS CONTROLS

DUTY CYCLE

RUN/SS CONTROLS

DUTY CYCLE

START-UP

NORMAL OPERATION

CURRENT LIMIT

1702A F04

COMP CONTROLS DUTY CYCLE

LTC1702A ENABLED

0.55V

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