LTC1702 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702' PDF : 36 Pages View PDF
LTC1702
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 3.5µA 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 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 100µA. 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.
12
VOUT
0V
5V
4.5V
VRUN/SS
0V
2.5V
1.0V
0.55V
LTC1702 ENABLED
RUN/SS CONTROLS
DUTY CYCLE
COMP CONTROLS DUTY CYCLE
MIN COMPARATOR ENABLED
START-UP
NORMAL OPERATION
2.5V
RUN/SS CONTROLS
DUTY CYCLE
CURRENT LIMIT 1702 F04
Figure 4. Soft-Start Operation in Start-Up and Current Limit
1702fa
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