LTC3520
11
3520fa
OPERATION
The LTC3520 combines a synchronous buck DC/DC
converter and a four-switch buck-boost DC/DC converter
in a single 4mm × 4mm 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 effi ciency lower voltage output and supports 100%
duty cycle operation to extend battery life. In Burst Mode
operation, the total quiescent current for both converters
is reduced to 55μA (typical). Both converters operate
synchronously from a common internal oscillator whose
frequency is programmed via an external resistor. In ad-
dition, the LTC3520 contains an uncommitted gain block
which can be confi gured as a comparator for low battery
detection or as a power-good indicator. Alternatively, the
gain block can be utilized in conjunction with an external
PNP to create an LDO, thereby allowing the LTC3520 to
generate a third low noise output voltage.
BUCK CONVERTER OPERATION
PWM Mode Operation
When the PWM2 pin is held high, the LTC3520 buck converter
uses a constant-frequency, current mode control architec-
ture. Both the main (P-channel MOSFET) and synchronous
rectifi er (N-channel MOSFET) switches are internal. At
the start of each oscillator cycle, the P-channel switch is
turned on and remains on until the current waveform with
superimposed slope compensation ramp exceeds the error
amplifi er output. At this point, the synchronous rectifi er 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 cur-
rent at light loads which improves effi ciency. 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
Burst Mode operation is enabled by either connecting
PWM2 to ground through a resistor, R
BURST
, or by shorting
PWM2 to ground. The buck converter will automatically
transition between PWM mode at high load current and
Burst Mode operation at light currents. Typical curves for
the Burst Mode entry threshold are provided in the Typical
Performance Characteristics section of this datasheet.
Under dropout and near dropout conditions, Burst Mode
operation will not be entered.
The value of R
BURST
controls the load current at which
Burst Mode operation will be entered. Larger resistor
values will cause Burst Mode operation to be entered at
lighter load currents. However, if the value of R
BURST
is
too large, then the buck converter will not enter Burst
Mode operation at any current, especially when operating
with V
IN
close to the buck output voltage. Conversely, if
R
BURST
is too small, the ripple in Burst Mode operation
may become objectionable, especially at high input volt-
ages. For most applications, choosing R
BURST
= 301k
represents a reasonable compromise.
The output voltage ripple in Burst Mode operation is de-
pendent upon the value of R
BURST
, the input voltage, the
output voltage, the inductor value and the output capaci-
tor. The Burst Mode operation output voltage ripple can
be reduced by increasing the size of the output capacitor,
increasing the value of the inductor or increasing the
value of R
BURST
.
Low 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 power P-channel MOSFET switch
to remain on for more than one cycle until 100% duty
cycle operation is reached and the power 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 subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is ac-
complished internally on the LTC3520 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 amplifi er voltage to a fi xed maximum.
