参数资料
型号: LTC3533EDE
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: 7 A SWITCHING REGULATOR, 2000 kHz SWITCHING FREQ-MAX, PDSO14
封装: 4 X 3 MM, 0.75 MM HEIGHT, PLASTIC, MO-229WGED-3, DFN-14
文件页数: 2/16页
文件大小: 313K
代理商: LTC3533EDE
LTC3533
10
3533f
OPERATION
For automatic operation a lter capacitor must also be
connected from BURST to ground. The equation for the
minimum capacitor value is:
C
CV
BURST MIN
OUT
()
,
60 000
where CBURST(MIN) and COUT are in F.
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 xed frequency mode and an internal pull-up will be
momentarily applied to BURST, rapidly charging CBURST.
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
xed frequency mode, BURST should be connected to
VIN. 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 xed 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 xed frequency mode,
raising VOUT. 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 VC is disconnected from the error amplier.
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 xed 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 VC at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
xed 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 lter. (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 amplier duty cycle is clamped
from VC. A detailed diagram of this function is shown in
Figure 3. The components RSS and CSS provide a slow
ramping voltage on RUN/SS to provide a soft-start function.
To ensure that VC is not being clamped, RUN/SS must be
raised above 1.6V.
VIN
VC
RUN/SS
3533 F03
Figure 3.
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