LTC3409A_15 Datasheet, PDF (9/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409A
OPERATION
Burst Mode Operation Near Dropout
With a light load the part will transition from Burst Mode
operation to 100% duty cycle as (V_IN to V_OUT) approaches
I_LOAD • (R_MAINSWITCH + R_INDUCTOR). When (V_IN to V_OUT)
results in near 100% duty cycle the inductor up slope will
be quite shallow compared to the inductor down slope,
with low peak currents.
The LTC3409A is a micropower part and the speed of the
comparator controlling the synchronous switch may allow
the inductor current to reverse in a low (V_IN to V_OUT) situa-
tion, resulting in CCM operation. Transition from CCM back
to Burst Mode operation will occur when (V_IN to V_OUT) is
sufficient for the average inductor current to exceed the
load current. This occurs when the average positive cur-
rent in the inductor exceeds the average reverse current
caused by synchronous switch propagation delay. The
CCM to Burst Mode operation re-entry transition point will
be a function of V_IN, V_OUT, I_LOAD, C_OUT and the inductor
used in the application.
Short-Circuit Protection
When the output is shorted to ground the LTC3409A 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 until it reaches 100% duty cycle.
The output voltage will then be determined by the input
voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
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 LTC3409A can be synchronized
to a user-supplied external clock applied to the SYNC pin.
Alternately, when this pin is held at a fixed high or low
level for more than 30μs, the internal oscillator will revert
to fixed-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 0.612V 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 defined
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 1.8V with a 1kΩ
load and V_IN = 3.6V. The 1kΩ load results in an output of
1.8mA at 1.8V.
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