Datasheet

LTC3533
11
3533fc
For more information www.linear.com/LTC3533
operation
Manual Burst Mode Operation
For manual control of Burst Mode operation, the RC
network connected to BURST can be eliminated. To force
fixed frequency mode, BURST should be connected to
V
IN
. To force Burst Mode operation, BURST should be
grounded. When commanding Burst Mode operation
manually, the circuit connected to BURST should be able
to sink up to 2mA.
For optimum transient response with large dynamic loads,
the operating mode should be controlled digitally by the
host. By commanding fixed frequency operation prior to a
sudden increase in load, output voltage droop can be min
-
imized. Note that if the load current applied during forced
Burst Mode operation (BURST pin is grounded) exceeds
the current that can be supplied, the output voltage will
start to droop and the L
TC3533 will automatically come out
of Burst Mode operation and enter fixed frequency mode,
raising V
OUT
. Once regulation is achieved, the LTC3533
will then enter Burst Mode operation once again (since the
user is still commanding this by grounding BURST), and
the cycle will repeat, resulting in about 4% output ripple.
Burst Mode Operation to Fixed Frequency Transient
Response
In Burst Mode operation, the compensation network is
not used and V
C
is disconnected from the error amplifier.
During long periods of Burst Mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fixed frequency mode of operation, even
at the same load current. To prevent this, the LTC3533
incorporates an active clamp circuit that holds the voltage
on V
C
at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fixed frequency mode operation. For optimum transient
response, Type 3 compensation is also recommended
to broad band the control loop and roll off past the two
pole response of the output LC filter. (See Closing the
Feedback Loop).
Soft-Start
The soft-start function is combined with shutdown. When
the RUN/SS pin is brought above 1V typical, the LTC3533
is enabled but the error amplifier duty cycle is clamped
from V
C
. A detailed diagram of this function is shown in
Figure 3. The components R
SS
and C
SS
provide a slow
ramping voltage on RUN/SS to provide a soft-start function.
To ensure that V
C
is not being clamped, RUN/SS must be
raised above 1.6V.
V
IN
V
C
RUN/SS
3533 F03
Figure 3.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.