LTC3409
9
3409fc
to audio circuitry. When the converter is in Burst Mode
operation, the minimum peak current of the inductor is
set to approximately 200mA regardless of the output load.
Each burst event can last from a few cycles at light loads
to almost continuously cycling with short sleep intervals
at moderate loads. In between these burst events, the
power MOSFETs and any unneeded circuitry are turned
off, reducing the quiescent current to 65μA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifi er’s output rises above the sleep threshold signaling
the BURST comparator to trip and turn the top MOSFET
on. This process repeats at a rate that is dependent on
the load demand.
Short-Circuit Protection
When the output is shorted to ground the LTC3409 limits
the synchronous switch current to 1.5A. If this limit is
exceeded, the top power MOSFET is inhibited from turn-
ing on until the current in the synchronous switch falls
below 1.5A.
Dropout Operation
As the input supply voltage decreases to a value ap-
proaching the output voltage, the duty cycle increases
toward the maximum on-time. Further reduction of the
supply voltage forces the main switch to remain on for
more than one cycle.
Slope Compensation
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%.
User Controlled Switching Frequency
The internal oscillator of the LTC3409 can be synchronized
to a user-supplied external clock applied to the SYNC pin.
Alternately, when this pin is held at a fi xed High or Low
level for more than 30μs, the internal oscillator will revert
to fi xed-frequency operation; where the frequency may
be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC
High).
Internal Soft-Start
At start-up when the RUN pin is brought high, the internal
reference is linearly ramped from 0V to 0.613V in 1ms. The
regulated feedback voltage will follow this ramp resulting
in the output voltage ramping from 0% to 100% in 1ms.
The current in the inductor during soft-start will be defi ned
by the combination of the current needed to charge the
output capacitance and the current provided to the load
as the output voltage ramps up. The start-up waveform,
shown in the Typical Performance Characteristics, shows
the output voltage start-up from 0V to 1.5V with a 2.5Ω
load and V
IN
= 2.2V. The 2.5Ω load results in an output
of 600mA at 1.5V.
OPERATION
