LTC3522_15 Datasheet, PDF (9/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

LTC3522
OPERATION
The LTC3522 combines a synchronous buck DC/DC
converter and a 4-switch buck-boost DC/DC converter
in a single 3mm × 3mm QFN package. The buck-boost
converter utilizes a proprietary switching algorithm which
allows its output voltage to be regulated above, below or
equal to the input voltage. The buck converter provides a
high efficiency lower voltage output and supports 100%
duty cycle operation to extend battery life. In Burst Mode
operation, the combined quiescent current for both con-
verters is reduced to 25μA. Both converters operate from
the same internal 1MHz oscillator.
BUCK CONVERTER OPERATION
PWM Mode Operation
When the PWM pin is held high, the LTC3522 buck con-
verter uses a constant frequency, current mode control
architecture. Both the main (P-channel MOSFET) and
synchronous rectifier (N-channel MOSFET) switches are
internal. At the start of each oscillator cycle, the P-chan-
nel switch is turned on and remains on until the current
waveform with superimposed slope compensation ramp
exceeds the error amplifier output. At this point, the syn-
chronous rectifier is turned on and remains on until the
inductor current falls to zero or a new switching cycle is
initiated. As a result, the buck converter operates with
discontinuous inductor current at light loads which im-
proves efficiency. At extremely light loads, the minimum
on-time of the main switch will be reached and the buck
converter will begin turning off for multiple cycles in order
to maintain regulation.
Burst Mode Operation
When the PWM pin is forced low, the buck converter will
automatically transition between Burst Mode operation
at sufficiently light loads (below approximately 10mA)
and PWM mode at heavier loads. Burst Mode entry is
determined by the peak inductor current and therefore
the load current at which Burst Mode operation will be
entered 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. Under dropout
and near dropout conditions, Burst Mode operation will
not be entered.
Dropout Operation
As the input voltage decreases to a value approaching the
output regulation voltage, the duty cycle increases toward
the maximum on-time. Further reduction of the supply
voltage will force the main switch to remain on for more
than one cycle until 100% duty cycle operation is reached
where the main switch remains on continuously. In this
dropout state, the output voltage will be determined by
the input voltage less the resistive voltage drop across the
main switch and series resistance of the inductor.
Slope Compensation
Current mode control requires the use of slope compensa-
tion to prevent sub-harmonic oscillations in the inductor
current waveform at high duty cycle operation. This is ac-
complished internally on the LTC3522 through the addition
of a compensating ramp to the current sense signal. In
some current mode ICs, current limiting is performed by
clamping the error amplifier voltage to a fixed maximum.
This leads to a reduced output current capability at low
step-down ratios. In contrast, the LTC3522 performs cur-
rent limiting prior to addition of the slope compensation
ramp and therefore achieves a peak inductor current limit
that is independent of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the error amplifier
will saturate high and the P-channel MOSFET switch will
turn on at the start of each cycle and remain on until the
current limit trips. During this minimum on-time, the in-
ductor current will increase rapidly and will decrease very
slowly during the remainder of the period due to the very
small reverse voltage produced by a hard output short.
To eliminate the possibility of inductor current runaway
in this situation, the buck converter switching frequency
is reduced to approximately 250kHz when the voltage on
FB2 falls below 0.3V.
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