ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 12

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1702A
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
12
U
V
RUN/SS
INFORMATION
V
OUT
U
0V
5V
0V
Figure 4. Soft-Start Operation in Start-Up and Current Limit
LTC1702A ENABLED
RUN/SS CONTROLS
W
START-UP
DUTY CYCLE
0.55V
1.0V
2V
U
COMP CONTROLS DUTY CYCLE
NORMAL OPERATION
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 I
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.
RUN/SS CONTROLS
2V
CURRENT LIMIT
DUTY CYCLE
1702A F04
1702afa
MAX
.