LTC3530EMS Просмотр технического описания (PDF) - Linear Technology
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LTC3530EMS Datasheet PDF : 16 Pages
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LTC3530
OPERATION
The efficiency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation ef-
ficiency is given by:
EFFICIENCY
•ILOAD
40μA + ILOAD
where η is typically 90% during Burst Mode operation.
Automatic Burst Mode Operation Control
Burst Mode operation can be automatic or manually con-
trolled with a single pin. In automatic mode, the IC will
enter Burst Mode operation at light load and return to fixed
frequency operation at heavier loads. The load current at
which the mode transition occurs is programmed using a
single external resistor from BURST to ground, according
to the following equations:
Enter Burst Mode:
IBURST
=
8.8
RBURST
Leave Burst Mode:
IBURST
= 11.2
RBURST
where RBURST is in kΩ and IBURST is the load transition
current in Amps. Do not use values of RBURST greater
than 500kΩ.
For automatic operation, a filter capacitor must also be
connected from BURST to ground. The equation for the
minimum capacitor value is:
CBURST(MIN)
COUT • VOUT
60,000
where CBURST(MIN) and COUT 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 IC will immediately switch to fixed 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
fixed 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 manually by the
host. By commanding fixed frequency operation prior to
a sudden increase in load, output voltage droop can be
minimized. 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 IC will automatically come out
of Burst Mode operation and enter fixed frequency mode,
raising VOUT. Once regulation is achieved, the IC will then
enter Burst Mode operation once again, 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 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 LTC3530
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
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 under Applications Information.
3530fb
9
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