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LTC1702

1702fa

APPLICATIONS INFORMATION

SHUTDOWN/SOFT-START

Each half of the LTC1702 has a RUN/SS pin. The RUN/SS

pins perform two functions: when pulled to ground, each

shuts down its half of the LTC1702, 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 LTC1702 is shut

down, ensuring the device will start when any external

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 LTC1702

goes into a micropower sleep mode, and quiescent cur-

rent 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 LTC1702 when

it falls below about 0.5V. Between 0.5V and about 1V, that

half is active, but the maximum duty cycle is limited to

10%. The maximum duty cycle limit increases linearly

between 1V and 2.5V, reaching its final value of 90% when

RUN/SS is above 2.5V. Somewhere before this point, the

feedback amplifier will assume control of the loop and the

output will come into regulation. When RUN/SS rises to

0.5V below VCC, the MIN feedback comparator is enabled,

and the LTC1702 is in full operation (see Figure 4).

CURRENT LIMIT

The LTC1702 includes an onboard current limit circuit that

limits the maximum output current to a user-programmed

level. It works by sensing the voltage drop across QB

during the time that QB is on and comparing 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 LTC1702 can monitor the output current.

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

2.5V

1.0V

VOUT

VRUN/SS

4.5V

RUN/SS CONTROLS

DUTY CYCLE

MIN COMPARATOR ENABLED

RUN/SS CONTROLS

DUTY CYCLE

START-UP

NORMAL OPERATION

CURRENT LIMIT

1702 F04

COMP CONTROLS DUTY CYCLE

LTC1702 ENABLED

0.55V

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