LTC3533 Datasheet, PDF (10/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
OPERATION
For automatic operation a filter capacitor must also be
connected from BURST to ground. The equation for the
minimum capacitor value is:
C_BURST(MIN)
≥
C_OUT • V_OUT
60,000
where C_BURST(MIN) and C_OUT are in µF.
In the event that a load transient causes FB to drop by
more than 4% from the regulation value while in Burst
Mode operation, the LTC3533 will immediately switch
to fixed frequency mode and an internal pull-up will be
momentarily applied to BURST, rapidly charging C_BURST.
This prevents the IC from immediately re-entering Burst
mode operation once the output achieves regulation.
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 mini-
mized. 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 LTC3533 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
RUN/SS
V_C
Figure 3.
3533 F03
3533f
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