LTC1702A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702A' PDF : 36 Pages View PDF
LTC1702A
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 3.5µA 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 3.5µA
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 100µA. 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.
12
VOUT
0V
5V
VRUN/SS
0V
2V
1.0V
0.55V
LTC1702A ENABLED
RUN/SS CONTROLS
DUTY CYCLE
COMP CONTROLS DUTY CYCLE
2V
RUN/SS CONTROLS
DUTY CYCLE
START-UP
NORMAL OPERATION
CURRENT LIMIT
1702A F04
Figure 4. Soft-Start Operation in Start-Up and Current Limit
1702afa
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