LTC3103
9
3103fa
For more information www.linear.com/LTC3103
OPERATION
The LTC3103 step-down DC/DC converter is capable of
supplying 300mA to the load. The output voltage is adjust-
able over a broad range and can be set as low as 0.6V.
Both the power and the synchronous rectifier switches
are internal N-channel MOSFETs. The converter uses a
constant-frequency, current mode architecture and may
be configured using automatic Burst Mode operation for
highly efficient light load operation or configured for low
noise forced conduction continuous operation where the
converter is optimized to operate over a broad range of
step-down ratios without pulse skipping. With the auto-
matic Burst Mode feature enabled the typical DC supply
current drops to only 1.8µA with no load.
Main Control Loop
During normal operation, the internal top power MOSFET
is turned on at the beginning of each cycle and turned off
when the PWM current comparator trips. The peak induc-
tor current at which the comparator trips is controlled by
the voltage on the output of the error amplifier. The FB
pin allows the internally compensated error amplifier to
receive an output feedback voltage from an external resis-
tive divider from V
OUT
. When the load current increases,
the output begins to fall causing a slight decrease in the
feedback voltage relative to the 0.6V reference, this in turn
causes the control voltage to increase until the average
inductor current matches the new load current. While the
top MOSFET is off, the bottom MOSFET is turned on until
either the inductor current starts to reverse as indicated by
the current reversal comparator, I
ZERO
, or the beginning
of the next clock cycle. I
ZERO
is set to 40mA (typical) in
automatic Burst Mode operation and –110mA (typical) in
forced continuous mode.
Forced Continuous Mode
Grounding MODE enables forced continuous operation
and disables Burst Mode operation. At light loads, forced
continuous mode minimizes output voltage ripple and
noise but is less efficient than Burst Mode operation.
Forced continuous operation may be desirable for use in
applications that are sensitive to the Burst Mode output
voltage ripple or its harmonics. The LTC3103 offers a broad
range of possible step-down ratios without pulse skipping
but for very small step-down ratios, the minimum on-time
of the main switch will be reached and the converter will
begin turning off for multiple cycles in order to maintain
regulation.
Burst Mode Operation
Holding the MODE pin above 1.2V will enable automatic
Burst Mode operation and disable forced continuous
operation. As the load transitions current increases the
converter will automatically transition between Burst
Mode and PWM operation. Conversely the converter will
automatically transition from PWM operation to Burst
Mode operation as the load decreases. Between bursts
the converter is not active (i.e., both switches are off)
and most of the internal circuitry is disabled to reduce the
quiescent current to 1.8µA. Burst Mode entry and exit is
determined by the peak inductor current and therefore the
load current at which Burst Mode operation will be entered
or exited depends on the input voltage, the output voltage
and the inductor value. Typical curves for Burst Mode
entry threshold are provided in the Typical Performance
Characteristics section of this data sheet.
Soft-Start
The converter has an internal closed-loop soft-start circuit
with a nominal duration of 1.4ms. The converter remains
in regulation during soft-start and will therefore respond
to output load transients that occur during this time. In
addition, the output voltage rise time has minimal depen-
dency on the size of the output capacitor or load current.
Thermal Shutdown
If the die temperature exceeds 150°C (typical) the converter
will be disabled. All power devices will be turned off and
the switch node will be forced into a high impedance state.
The soft-start circuit is reset during thermal shutdown
to provide a smooth recovery once the overtemperature
condition is eliminated. If enabled, the converter will restart
when the die temperature drops to approximately 130°C.
