Datasheet

LTC3528/LTC3528B
10
3528fd
OPERATION
(Refer to Block Diagram)
Burst Mode OPERATION
The LTC3528 will automatically enter Burst Mode opera-
tion at light load current and return to fixed frequency
PWM mode when the load increases. Refer to the Typical
Performance Characteristics to see the output load Burst
Mode threshold vs V
IN
. The load at which Burst Mode
operation is entered can be changed by adjusting the
inductor value. Raising the inductor value will lower the
load current at which Burst Mode operation is entered.
In Burst Mode operation, the LTC3528 continues switch-
ing at a fixed frequency of 1MHz, using the same error
amplifier and loop compensation for peak current mode
control. This control method minimizes output transients
when switching between modes. In Burst Mode opera-
tion, energy is delivered to the output until it reaches the
nominal regulated value, then the LTC3528 transitions to
sleep mode where the outputs are off and the LTC3528
consumes only 12µA of quiescent current from V
OUT
. Once
the output voltage has drooped slightly, switching resumes
again. This maximizes efficiency at very light loads by
minimizing switching and quiescent current losses. Burst
Mode output ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10µF
or greater).
As the load current increases, the LTC3528 automatically
leaves Burst Mode operation. Note that larger output ca-
pacitor values may cause this transition to occur at lighter
loads. The regulator will also leave Burst Mode operation if
a load transient occurs which causes the inductor current
to repeatedly reach current limit. Once the LTC3528 has left
Burst Mode operation and returned to normal operation,
it will remain there until the output load is reduced below
the Burst threshold.
Burst Mode operation is inhibited during start-up and
until soft-start is done and V
OUT
is at least 0.24V greater
than V
IN
.
The LTC3528B features continuous PWM operation at
1MHz. At very light loads, the LTC3528B will exhibit pulse-
skip operation.