Datasheet

LTC3533
10
3533fc
For more information www.linear.com/LTC3533
operation
BURST MODE OPERATION
Burst Mode operation reduces the LTC3533’s quiescent
current consumption at light loads and improves overall
conversion efficiency, increasing battery life. During Burst
Mode operation the LTC3533 delivers energy to the out
-
put until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 40µA. In this mode the output ripple has a variable
frequency component that depends upon load current,
and will typically be about 2% peak-to-peak. Burst Mode
operation ripple can be reduced slightly by using more
output capacitance. Another method of reducing Burst
Mode operation ripple is to place a small feed-for
ward
capacitor across the upper resistor in the V
OUT
feedback
divider network (as in Type III compensation).
During the period where the device is delivering energy
to the output, the peak switch current will rise to 900mA
typical and the inductor current will terminate at zero
current for each cycle. In this mode, the typical maximum
average output currents are given by:
I
MAX(BURST)BUCK
≈ 450mA; V
OUT
< V
IN
I
MAX(BURST)BOOST
≈ 450mA • (V
IN
/V
OUT
); V
OUT
> V
IN
I
MAX(BURST)BUCK-BOOST
≈ 700mA; V
OUT
≈ V
IN
, since the input
and output are connected together for most of the cycle.
The efficiency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation effi
-
ciency is given by:
Efficiency ≅
η
• I
LOAD
40µA +I
LOAD
where h is typically 90% during Burst Mode operation
Programmable Automatic Burst Mode Operation
Burst Mode operation can be automatic or digitally con
-
trolled with a single pin. In automatic mode, the LTC3533
enters Burst Mode operation at the programmed threshold
and returns to fixed frequency operation when the load
demand increases. 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Operation:I
BURST
=
17
R
BURST
ExitBurstModeOperation:I
BURST
=
19
R
BURST
Where R
BURST
is in kW and I
BURST
is the load transition
current in Amps. Do not use values of R
BURST
greater
than 250kW.
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.
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