LTC3442 Datasheet, PDF (11/20 Pages) Linear Integrated Systems – Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

U
OPERATIO
current in Amps. Do not use values of R_BURST greater than
250k.
For automatic operation, a filter capacitor should also be
connected from BURST to ground to prevent ripple on
BURST from causing the IC to oscillate in and out of Burst
Mode operation. 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 the feedback pin
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 the
BURST cap. 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
V_OUT. 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.
LTC3442
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 fre-
quency 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. Note that Burst Mode operation is inhibited during
soft-start.
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 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 LTC3442
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
fixed frequency mode operation. For optimum transient
3442f
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