Datasheet

LTC3530
9
3530fb
The effi ciency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation ef-
fi ciency is given by:
EFFICIENCY 
 •I
LOAD
40μA + I
LOAD
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 fi xed
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: I
BURST
=
8.8
R
BURST
Leave Burst Mode: I
BURST
=
11.2
R
BURST
where R
BURST
is in kΩ and I
BURST
is the load transition
current in Amps. Do not use values of R
BURST
greater
than 500kΩ.
For automatic operation, a fi lter 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 IC will immediately switch to fi xed 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
fi xed 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 manually by the
host. By commanding fi xed 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 fi xed frequency mode,
raising V
OUT
. 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 V
C
is disconnected from the error amplifi er.
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 fi xed 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 V
C
at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fi 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 fi lter. See Closing the Feedback
Loop under Applications Information.
OPERATION